Subject: [HELP] Water Leakage at Home – Can’t Find the Source

[m k]

the expansion tank/bladder had to be reinflated regularly. I imagine a larger bladder would have been beneficial; don't know why there wasn't one.

So membrane(rubber?) is leaking?

First stand still water is pressing.
Then pump start pumping and pressure is not equal.

If pump can't generate enough pressure the top of piping has gas.
But lower routes are still generating some pre pump pressure.

What defines an automatic rinsing?

I've seen once some remnants of freely circulating central heating, three floors with logs.
No radiators were present, but plumbing was considerable, regular hardware of today can't cope.
Not exactly arm thick, but not much behind were those first pipes.
If I remember correctly the topmost part of the system was freely breathing expansion vessel.

[tggzzz]

the expansion tank/bladder had to be reinflated regularly. I imagine a larger bladder would have been beneficial; don't know why there wasn't one.

So membrane(rubber?) is leaking?

First stand still water is pressing.
Then pump start pumping and pressure is not equal.

If pump can't generate enough pressure the top of piping has gas.
But lower routes are still generating some pre pump pressure.

What defines an automatic rinsing?

I've seen once some remnants of freely circulating central heating, three floors with logs.
No radiators were present, but plumbing was considerable, regular hardware of today can't cope.
Not exactly arm thick, but not much behind were those first pipes.
If I remember correctly the topmost part of the system was freely breathing expansion vessel.

House was sold several years ago, and completely renovated

Everything was working normally; it was just the pressure reading that I cannot understand.

[SteveThackery]

IIRC the expansion tank/bladder had to be reinflated regularly. I imagine a larger bladder would have been beneficial; don't know why there wasn't one.

In that case it is probably faulty. If the membrane gets a hole in it, air will mix with the water, circulate, and get bled out. Alternatively the water might absorb the air.

If the membrane is OK, and the Schrader valve on the vessel is properly sealee, it should never need reinflating.

[SteveThackery]

Everything was working normally; it was just the pressure reading that I cannot understand.

As I mentioned earlier, I think the water column has to be open at the top for it to generate a head of pressure. If it's sealed at the top, like a central heating circuit, then no head of pressure will show on the gauge.

[tggzzz]

Everything was working normally; it was just the pressure reading that I cannot understand.

As I mentioned earlier, I think the water column has to be open at the top for it to generate a head of pressure. If it's sealed at the top, like a central heating circuit, then no head of pressure will show on the gauge.

If there is 32ft head of water, how can the pressure at the bottom be less than 1 bar?

If you heat the water and it is unsealed, the water level will rise a little due to water expansion, and that will increase the pressure slightly.
If you heat the water and it is sealed, then the pressure will increase a lot because there is no room for the water to expand. ^PV/_T=constant

[tggzzz]

IIRC the expansion tank/bladder had to be reinflated regularly. I imagine a larger bladder would have been beneficial; don't know why there wasn't one.

In that case it is probably faulty. If the membrane gets a hole in it, air will mix with the water, circulate, and get bled out. Alternatively the water might absorb the air.

If the membrane is OK, and the Schrader valve on the vessel is properly sealee, it should never need reinflating.

My recollection is that the membrane was either replaced or reinflated more than once. I presume that it was simply too small for the house.

[SteveThackery]

If there is 32ft head of water, how can the pressure at the bottom be less than 1 bar?

Because the water is "held up" by the vacuum that would have to appear at the top in order for water to flow into the pressure gauge.

It's easier to imagine a vertical fully sealed tube filled completely with water (radiators and stuff complicate matters). Make a small hole in the bottom of the tube: no water will flow out unless you let air in to displace it. Put your finger over the small hole - you will feel no pressure. The column of water is supported by the vacuum at the top that would have to appear for water to flow out. The water is "hanging" on the vacuum, if you like.  More correctly, air pressure from the environment is pushing the column of water upwards.

Sorry, @tggzzz, that's the best I can do!

[tggzzz]

If there is 32ft head of water, how can the pressure at the bottom be less than 1 bar?

Because the water is "held up" by the vacuum that would have to appear at the top in order for water to flow into the pressure gauge.

It's easier to imagine a vertical fully sealed tube filled completely with water (radiators and stuff complicate matters). Make a small hole in the bottom of the tube: no water will flow out unless you let air in to displace it. Put your finger over the small hole - you will feel no pressure. The column of water is supported by the vacuum at the top that would have to appear for water to flow out. The water is "hanging" on the vacuum, if you like.  More correctly, air pressure from the environment is pushing the column of water upwards.

Sorry, @tggzzz, that's the best I can do!

Such a vacuum could only support 32ft of water, but the house is much higher than that. Plus when the top vent is open there is no vacuum and I'm allowing more mains water in while bleeding the radiators.

No, I don't understand the pressure gauge either, nor do the heating technicians!

[Sorama]

If there is 32ft head of water, how can the pressure at the bottom be less than 1 bar?

Because the water is "held up" by the vacuum that would have to appear at the top in order for water to flow into the pressure gauge.

It's easier to imagine a vertical fully sealed tube filled completely with water (radiators and stuff complicate matters). Make a small hole in the bottom of the tube: no water will flow out unless you let air in to displace it. Put your finger over the small hole - you will feel no pressure. The column of water is supported by the vacuum at the top that would have to appear for water to flow out. The water is "hanging" on the vacuum, if you like.  More correctly, air pressure from the environment is pushing the column of water upwards.

Sorry, @tggzzz, that's the best I can do!

Such a vacuum could only support 32ft of water, but the house is much higher than that. Plus when the top vent is open there is no vacuum and I'm allowing more mains water in while bleeding the radiators.

No, I don't understand the pressure gauge either, nor do the heating technicians!

mains water is not connected to top vent (again, it's a closed circuit).

[tggzzz]

If there is 32ft head of water, how can the pressure at the bottom be less than 1 bar?

Because the water is "held up" by the vacuum that would have to appear at the top in order for water to flow into the pressure gauge.

It's easier to imagine a vertical fully sealed tube filled completely with water (radiators and stuff complicate matters). Make a small hole in the bottom of the tube: no water will flow out unless you let air in to displace it. Put your finger over the small hole - you will feel no pressure. The column of water is supported by the vacuum at the top that would have to appear for water to flow out. The water is "hanging" on the vacuum, if you like.  More correctly, air pressure from the environment is pushing the column of water upwards.

Sorry, @tggzzz, that's the best I can do!

Such a vacuum could only support 32ft of water, but the house is much higher than that. Plus when the top vent is open there is no vacuum and I'm allowing more mains water in while bleeding the radiators.

No, I don't understand the pressure gauge either, nor do the heating technicians!

mains water is not connected to top vent (again, it's a closed circuit).

It is when you are refilling radiators with water from the mains. When refilled you close the inlet valve to isolate it again.

That's SOP: see the boiler operation manual.

[Sorama]

If there is 32ft head of water, how can the pressure at the bottom be less than 1 bar?

Because the water is "held up" by the vacuum that would have to appear at the top in order for water to flow into the pressure gauge.

It's easier to imagine a vertical fully sealed tube filled completely with water (radiators and stuff complicate matters). Make a small hole in the bottom of the tube: no water will flow out unless you let air in to displace it. Put your finger over the small hole - you will feel no pressure. The column of water is supported by the vacuum at the top that would have to appear for water to flow out. The water is "hanging" on the vacuum, if you like.  More correctly, air pressure from the environment is pushing the column of water upwards.

Sorry, @tggzzz, that's the best I can do!

Such a vacuum could only support 32ft of water, but the house is much higher than that. Plus when the top vent is open there is no vacuum and I'm allowing more mains water in while bleeding the radiators.

No, I don't understand the pressure gauge either, nor do the heating technicians!

mains water is not connected to top vent (again, it's a closed circuit).

It is when you are refilling radiators with water from the mains. When refilled you close the inlet valve to isolate it again.

That's SOP: see the boiler operation manual.

yes, but why do you mention the top vent?
There is no vent at all wrt water mains.

[SteveThackery]

Such a vacuum could only support 32ft of water, but the house is much higher than that. Plus when the top vent is open there is no vacuum and I'm allowing more mains water in while bleeding the radiators.

No, I don't understand the pressure gauge either, nor do the heating technicians!

Well of course the technicians don't understand it - you must have met a few by now and arrived at a view of their intellectual prowess.

When the column of water is greater than 32ft then of course a vacuum will be drawn at the top, and then the pressure gauge shows the total head minus one atmosphere. However, when you are bleeding the radiators you have connected the mains water to the bottom of heating circuit and the atmosphere to the top. The mains water is at a higher pressure than the heating circuit (it must be, or it wouldn't bleed it).  That situation only exists whilst you are bleeding it.

Tell you what, @tggzzz, instead of hoping other people will explain it to you, and finding them lacking, how about you put the effort in yourself? You're a smart guy - smarter than the average heating tech - and you have a good knowledge of the relevant physics, so sit yourself down with a pencil, paper and pad and draw it out. Work through the physics and you will arrive at the answer.

[tggzzz]

Such a vacuum could only support 32ft of water, but the house is much higher than that. Plus when the top vent is open there is no vacuum and I'm allowing more mains water in while bleeding the radiators.

No, I don't understand the pressure gauge either, nor do the heating technicians!

Well of course the technicians don't understand it - you must have met a few by now and arrived at a view of their intellectual prowess.

When the column of water is greater than 32ft then of course a vacuum will be drawn at the top, and then the pressure gauge shows the total head minus one atmosphere. However, when you are bleeding the radiators you have connected the mains water to the bottom of heating circuit and the atmosphere to the top. The mains water is at a higher pressure than the heating circuit (it must be, or it wouldn't bleed it).  That situation only exists whilst you are bleeding it.

Tell you what, @tggzzz, instead of hoping other people will explain it to you, and finding them lacking, how about you put the effort in yourself? You're a smart guy - smarter than the average heating tech - and you have a good knowledge of the relevant physics, so sit yourself down with a pencil, paper and pad and draw it out. Work through the physics and you will arrive at the answer.

I've had a relatively cursory look for the answer, and not found sufficient information.

A schematic of a boiler would be a good start, but the manuals I've found are "which pipes to connect during installation" and "if you see this error code, do that".

I don't have the inclination to hunt for the equivalent of "how to design a PSU", and learn the characteristics of a BJT vs MOSFET.

[MrAl]

Hi everyone,

I know this forum is more about electronics, but I’ve seen a lot of practical and knowledgeable people here, so I hope it’s okay to ask for some off-topic help.

I’ve recently discovered a water leak somewhere in my home’s plumbing system, but I can’t figure out exactly where it’s coming from. There’s no obvious dripping or visible damage, but I can hear a faint sound of water flow even when all faucets are turned off. My water meter is also slowly spinning, which confirms there’s a leak.

Here’s what I’ve tried so far:

Turned off all water-using appliances and fixtures inside the house.

Checked under sinks, behind toilets, and around the water heater.

Walked around the house to listen for leaking sounds.

Monitored the water meter for signs of usage.

Unfortunately, nothing obvious is showing up, and I’m not sure whether the leak is under the floor, in the walls, or even outside underground.

Does anyone here have experience with this kind of issue or suggestions on how to locate a hidden leak? Any tips, tools, or diagnostic methods would be appreciated. I’m trying to avoid calling a plumber immediately if I can narrow it down a bit myself first.

Thanks in advance!

Best regards,
Eassssse

Hi,

Do you have hot water heating in your house?

Some, if not all, hot water systems have a direct connection to the cold water pipe in the home.  That means if the hot water pipes for the heating has a leak, it will draw more water from the cold-water pipe.  That would include radiator leaks where the cold water keeps flowing into the heating system.

Obviously if you have electric heating this would not happen.

No, heating system with radiators is not possible as it is a CLOSED system.

Are you sure about that?
I ask because hot water systems can loose water through various means so they often have a connection to the mains water line.  This is so new water can be added if there is a tiny leak or small loss somewhere.  If there is a larger leak, this can lead to a lot of water usage.

Here's a quote:
"Hot water heating systems, whether they are gas or electric, typically receive their water from the municipal water supply or a well. This cold water enters the water heater and is then heated by a burner or heating element. "

If yours really is closed then it is closed, but I wonder how you know that it is really completely closed.

you need to distinguish between hot water systems for consumption/washing and hot water for heating systems like radiators.

Heating systems are of course a closed loop because water does not disappear in the heating system (apart from some very minor evaporation).

Using (hot) water for consumption/washing is of course connected to the mains as the water that is used, is gone.

I don't think you have experience with every hot water heating system under the sun.  I say that because I know of two systems in use today that do have a cold water feed into the hot water heating system (this is not for hot tap water only for heating).  They use baseboard heater cores with copper pipes and aluminum fins.  Normally the hot water flows though the pipes and stays in the system, but even a tiny leak over time will drain any water system.  Connecting to the mains water feed means this won't happen, but with a large leak of course could cause water damage.

I'll repeat that I know of at least two systems in homes that work the way I said they do, they have a direct and constant connection to the mains water, the cold side not the hot side from the hot water heater which is a different system just for getting hot tap water.
I might even have a picture of one of those systems where you can see the cold water feed and there is a valve you can close if something goes wrong with the heater and has to be fixed.

[Sorama]

Hi everyone,

I know this forum is more about electronics, but I’ve seen a lot of practical and knowledgeable people here, so I hope it’s okay to ask for some off-topic help.

I’ve recently discovered a water leak somewhere in my home’s plumbing system, but I can’t figure out exactly where it’s coming from. There’s no obvious dripping or visible damage, but I can hear a faint sound of water flow even when all faucets are turned off. My water meter is also slowly spinning, which confirms there’s a leak.

Here’s what I’ve tried so far:

Turned off all water-using appliances and fixtures inside the house.

Checked under sinks, behind toilets, and around the water heater.

Walked around the house to listen for leaking sounds.

Monitored the water meter for signs of usage.

Unfortunately, nothing obvious is showing up, and I’m not sure whether the leak is under the floor, in the walls, or even outside underground.

Does anyone here have experience with this kind of issue or suggestions on how to locate a hidden leak? Any tips, tools, or diagnostic methods would be appreciated. I’m trying to avoid calling a plumber immediately if I can narrow it down a bit myself first.

Thanks in advance!

Best regards,
Eassssse

Hi,

Do you have hot water heating in your house?

Some, if not all, hot water systems have a direct connection to the cold water pipe in the home.  That means if the hot water pipes for the heating has a leak, it will draw more water from the cold-water pipe.  That would include radiator leaks where the cold water keeps flowing into the heating system.

Obviously if you have electric heating this would not happen.

No, heating system with radiators is not possible as it is a CLOSED system.

Are you sure about that?
I ask because hot water systems can loose water through various means so they often have a connection to the mains water line.  This is so new water can be added if there is a tiny leak or small loss somewhere.  If there is a larger leak, this can lead to a lot of water usage.

Here's a quote:
"Hot water heating systems, whether they are gas or electric, typically receive their water from the municipal water supply or a well. This cold water enters the water heater and is then heated by a burner or heating element. "

If yours really is closed then it is closed, but I wonder how you know that it is really completely closed.

you need to distinguish between hot water systems for consumption/washing and hot water for heating systems like radiators.

Heating systems are of course a closed loop because water does not disappear in the heating system (apart from some very minor evaporation).

Using (hot) water for consumption/washing is of course connected to the mains as the water that is used, is gone.

I don't think you have experience with every hot water heating system under the sun.  I say that because I know of two systems in use today that do have a cold water feed into the hot water heating system (this is not for hot tap water only for heating).  They use baseboard heater cores with copper pipes and aluminum fins.  Normally the hot water flows though the pipes and stays in the system, but even a tiny leak over time will drain any water system.  Connecting to the mains water feed means this won't happen, but with a large leak of course could cause water damage.

I'll repeat that I know of at least two systems in homes that work the way I said they do, they have a direct and constant connection to the mains water, the cold side not the hot side from the hot water heater which is a different system just for getting hot tap water.
I might even have a picture of one of those systems where you can see the cold water feed and there is a valve you can close if something goes wrong with the heater and has to be fixed.

You obviously don’t know a damn thing how these sun collectors work.
Please, don’t tell me how to do my job.

[SteveThackery]

I'll repeat that I know of at least two systems in homes that work the way I said they do, they have a direct and constant connection to the mains water, the cold side not the hot side from the hot water heater which is a different system just for getting hot tap water.

This surprises me, because here in the UK, at least, we use corrosion inhibitor in the sealed heating circuit. If there was a connection between the heating circuit and the cold water mains, it could result in corrosion inhibitor getting into the drinking water, and then the resultant corrosion in the radiators.

I might even have a picture of one of those systems where you can see the cold water feed and there is a valve you can close if something goes wrong with the heater and has to be fixed.

Ah, but that's different. All 'combi' boilers have a valved connection between the mains cold water and the heating circuit. It's called the 'filling loop'. However, it should be closed in normal use, not open. Are you sure the systems you have in mind are supposed to run with that valve open? I'd be really surprised.

[Sorama]

I'll repeat that I know of at least two systems in homes that work the way I said they do, they have a direct and constant connection to the mains water, the cold side not the hot side from the hot water heater which is a different system just for getting hot tap water.

This surprises me, because here in the UK, at least, we use corrosion inhibitor in the sealed heating circuit. If there was a connection between the heating circuit and the cold water mains, it could result in corrosion inhibitor getting into the drinking water, and then the resultant corrosion in the radiators.

I might even have a picture of one of those systems where you can see the cold water feed and there is a valve you can close if something goes wrong with the heater and has to be fixed.

Ah, but that's different. All 'combi' boilers have a valved connection between the mains cold water and the heating circuit. It's called the 'filling loop'. However, it should be closed in normal use, not open. Are you sure the systems you have in mind are supposed to run with that valve open? I'd be really surprised.

Exactly.

[MrAl]

Hi everyone,

I know this forum is more about electronics, but I’ve seen a lot of practical and knowledgeable people here, so I hope it’s okay to ask for some off-topic help.

I’ve recently discovered a water leak somewhere in my home’s plumbing system, but I can’t figure out exactly where it’s coming from. There’s no obvious dripping or visible damage, but I can hear a faint sound of water flow even when all faucets are turned off. My water meter is also slowly spinning, which confirms there’s a leak.

Here’s what I’ve tried so far:

Turned off all water-using appliances and fixtures inside the house.

Checked under sinks, behind toilets, and around the water heater.

Walked around the house to listen for leaking sounds.

Monitored the water meter for signs of usage.

Unfortunately, nothing obvious is showing up, and I’m not sure whether the leak is under the floor, in the walls, or even outside underground.

Does anyone here have experience with this kind of issue or suggestions on how to locate a hidden leak? Any tips, tools, or diagnostic methods would be appreciated. I’m trying to avoid calling a plumber immediately if I can narrow it down a bit myself first.

Thanks in advance!

Best regards,
Eassssse

Hi,

Do you have hot water heating in your house?

Some, if not all, hot water systems have a direct connection to the cold water pipe in the home.  That means if the hot water pipes for the heating has a leak, it will draw more water from the cold-water pipe.  That would include radiator leaks where the cold water keeps flowing into the heating system.

Obviously if you have electric heating this would not happen.

No, heating system with radiators is not possible as it is a CLOSED system.

Are you sure about that?
I ask because hot water systems can loose water through various means so they often have a connection to the mains water line.  This is so new water can be added if there is a tiny leak or small loss somewhere.  If there is a larger leak, this can lead to a lot of water usage.

Here's a quote:
"Hot water heating systems, whether they are gas or electric, typically receive their water from the municipal water supply or a well. This cold water enters the water heater and is then heated by a burner or heating element. "

If yours really is closed then it is closed, but I wonder how you know that it is really completely closed.

you need to distinguish between hot water systems for consumption/washing and hot water for heating systems like radiators.

Heating systems are of course a closed loop because water does not disappear in the heating system (apart from some very minor evaporation).

Using (hot) water for consumption/washing is of course connected to the mains as the water that is used, is gone.

I don't think you have experience with every hot water heating system under the sun.  I say that because I know of two systems in use today that do have a cold water feed into the hot water heating system (this is not for hot tap water only for heating).  They use baseboard heater cores with copper pipes and aluminum fins.  Normally the hot water flows though the pipes and stays in the system, but even a tiny leak over time will drain any water system.  Connecting to the mains water feed means this won't happen, but with a large leak of course could cause water damage.

I'll repeat that I know of at least two systems in homes that work the way I said they do, they have a direct and constant connection to the mains water, the cold side not the hot side from the hot water heater which is a different system just for getting hot tap water.
I might even have a picture of one of those systems where you can see the cold water feed and there is a valve you can close if something goes wrong with the heater and has to be fixed.

You obviously don’t know a damn thing how these sun collectors work.
Please, don’t tell me how to do my job.

You are taking this too personally and not gleaning the meaning of my intent.

I don't have to know how your sun collectors work because I am just telling you about a DIFFERENT system, and that should be obvious.
Also, I NEVER force anyone to do anything, especially not on the web and even less in a forum where there should be free expression.
You are under NO obligation to do anything I ask you to do, you have a free will so you are free to use it.  Suggestions are just suggestions, and reports are just reports.

If I told you to jump off a bridge you wouldn't do it, and I wouldn't blame you for not doing it
I would never tell you that though but I would tell you about other systems that I have experience with.  If they don't help you then you don't have to worry about it.  You have to do what YOU think is best.

[MrAl]

I'll repeat that I know of at least two systems in homes that work the way I said they do, they have a direct and constant connection to the mains water, the cold side not the hot side from the hot water heater which is a different system just for getting hot tap water.

This surprises me, because here in the UK, at least, we use corrosion inhibitor in the sealed heating circuit. If there was a connection between the heating circuit and the cold water mains, it could result in corrosion inhibitor getting into the drinking water, and then the resultant corrosion in the radiators.

I might even have a picture of one of those systems where you can see the cold water feed and there is a valve you can close if something goes wrong with the heater and has to be fixed.

Ah, but that's different. All 'combi' boilers have a valved connection between the mains cold water and the heating circuit. It's called the 'filling loop'. However, it should be closed in normal use, not open. Are you sure the systems you have in mind are supposed to run with that valve open? I'd be really surprised.

Hi there,

Well it is possible that they do some systems like that here too, but apparently not every system because the ones I have seen are as I described exactly.  One system was upgraded significantly too and the water mains stayed connected and the plumber left the valve open.  I'll try to dig up a picture, but it would just show a mains water line running into the pipes of the  hot water heater.

There is also the possibility of a check value to prevent back water flow.  I didn't think to check for that though because we never had a big problem with any of it.  I know we did not use any corrosion prevention stuff though.  I also lived in both the homes I described.

There is also the possibility that these are much older systems.  However, we can learn more by doing a search for "Why some hot water heating systems have a direct connection to the water mains".  Some keep the valve closed and rely on manual refilling, while others stay open.
This reminds me of an automobile cooling system.  It is closed, but sometimes needs water to be added.

I encourage anyone who doesn't like hearing this to just go on with whatever they believe and there's no problem with that. 

[tggzzz]

My mains supply is ~7 bar. If that was left connected to the boiler, the overpressure valve would open at ~3 bar!

[SteveThackery]

Well it is possible that they do some systems like that here too, but apparently not every system because the ones I have seen are as I described exactly.  One system was upgraded significantly too and the water mains stayed connected and the plumber left the valve open.  I'll try to dig up a picture, but it would just show a mains water line running into the pipes of the  hot water heater.

OK, but can you see my problem? If there is a permanent connection between the heating circuit and the mains water, then the water you drink might have been around the radiators a few times, picking up rust and scum. Surely you would agree that such a thing would be disgusting and seems most unlikely. Yes?

Could you address that specific point? I'm just trying to understand how the system you describe works.

[SteveThackery]

Also, respectfully, is it possible you are confusing the hot water system and the heating system?

In a normal combi boiler installation, the hot water is fed directly from the mains water, through a heat exchanger in the boiler, and out to the hot water taps. So obviously the hot water system has an open connection to the mains water.

But we are talking about the heating circuit, and this is the one I can't understand because a permanently open connection to the cold water mains is guaranteed to mix your drinking water with water that has been around the radiators.

[m k]

No, I don't understand the pressure gauge either, nor do the heating technicians!

Residual pressure?

When pressure goes up it should also go up inside expansion vessel's gas volume.

System has min - max pressure.
Expansion vessel is keeping values in check, pressure compensation.
If vessel is too small it can't cope with high pressure and over pressure valve releases some water.
Next when min pressure situation happens the actual pressure is too low and somewhere so low that bubbles start emerging.

After a pump is high pressure and before it low pressure.
(NPSH)

[MrAl]

Well it is possible that they do some systems like that here too, but apparently not every system because the ones I have seen are as I described exactly.  One system was upgraded significantly too and the water mains stayed connected and the plumber left the valve open.  I'll try to dig up a picture, but it would just show a mains water line running into the pipes of the  hot water heater.

OK, but can you see my problem? If there is a permanent connection between the heating circuit and the mains water, then the water you drink might have been around the radiators a few times, picking up rust and scum. Surely you would agree that such a thing would be disgusting and seems most unlikely. Yes?

Could you address that specific point? I'm just trying to understand how the system you describe works.

Yes I can see your problem, but can you see mine?  This whole thing has been questioned as if it could never happen in a million years, yet I have seen two systems firsthand and had direct experience with both systems and described those systems.  I explained that, but still there are questions about the validity.  The only explanation I can think of is that some people have not had any experience with these systems so they believe it could not be possible, even though someone states that they had direct experience with them.
Well, the other possibility is that we have learned that on the web a lot of information is not reliable at all.  I myself also question some things I don't think are possible, but if someone insists then I give them the room they deserve.  Still, it's an interesting subject I guess after all so still interesting to talk about.

That said, I am posting a picture of the system.  The pipe going straight up is the cold-water mains pipe that comes from the same main pipe as the cold water for sinks.  The blue handle valve is the valve that can shut off the mains water feed in case of a bad water leak.

As to the specific point, yes that would be gross ha ha and I'd hate to have that happen, and I actually did address this you may have missed it.  I understand though because sometimes these threads get long and involved.  We are not alone with our concerns too because apparently one of the plumbing codes is that there has to be a backflow preventer valve, the abbreviation is BBFP and that, from what I understand, stands for "Boiler Back Flow Preventer".  Some have TWO check valves inside, not just one.  The check valves only allow water flow in one direction ... a "water diode" as it were.

So I think this should set it to rest, but anyone is free to look up that abbreviation as: "BBFP valve" and see that there are several types.

Also, respectfully, is it possible you are confusing the hot water system and the heating system?

In a normal combi boiler installation, the hot water is fed directly from the mains water, through a heat exchanger in the boiler, and out to the hot water taps. So obviously the hot water system has an open connection to the mains water.

But we are talking about the heating circuit, and this is the one I can't understand because a permanently open connection to the cold water mains is guaranteed to mix your drinking water with water that has been around the radiators.

Also respectfully, there is no way I would ever do that because I've worked on these systems myself although not all by myself (someone else with me).  I am not a plumber though and do not profess to know everything about plumbing, but I have done some small work on these systems like replacing a hot water tank which is for the hot water feed not for the heater.

As far as being guaranteed to mix drinking water with water that has been in the radiator for 10 years, the BBFP prevents that and that is what those valves are for.  Also, I did not install the  new updated system a rather large plumbing company did the work.  There were three guys working on it ha ha.

So now that we should have this issue cleared up, did I happen to mention yet that the sewer pipe from the toilet connects directly to the cold-water pipe? Just in case the city turns off the mains water we can still get drinking water after the toilet flushes.
Now there is something that is completely unbelievable so I suggest that nobody should actually believe that one

[Eassssse]

Thanks so much for the advice! It really helped a lot! Unfortunately, I'm not an expert (and this is my first time dealing with something like this), so I ended up calling a plumber to come check and fix it (and he actually used the methods you suggested!). The inspection confirmed there’s definitely a leak, and now I’m thinking about bringing in someone more specialized to handle it. Thanks u again Sorama