General > General Technical Chat

Electric shower, anything I can do?

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soldar:

--- Quote from: Someone on July 25, 2019, 07:45:51 am ---This is all dependent on the specific heater, instantaneous/continuous/on demand gas water heaters are widely used in Australia but they have closed loop regulation at the heater. When used with a controller to set the desired output temperature for each use there are no problems with stability other than low flow rates.
--- End quote ---
Yes, they are widely used in Spain too. They are the most used solution here. Some drawbacks I see:

Compared to tanks they are complicated, expensive to buy, expensive to repair and prone to needing repairs. To do all that regulation they have complex electronic systems that can break down and if you are lucky you can find a replacement board very expensively. If you are unlucky you need to replace the whole heater.  OTOH, a water tank just sits there for decades and knows nothing about electronics or repairs.

On demand gas heaters can only modulate power within certain limits, say 35 or 40% at the lower limit and 100% at the upper limit. If it is going to actually modulate then you need your centerpoint to be, say between 50 and 80% of capacity, so you have range to move around. So, really, the modulation range is not that great.

Add to that the great variation in power needs and you can see an on-demand heater is just not suitable for many circumstances.

Assume in winter water arrives at 4ºC and you need 15 L/min at 74ºC.

Assume in summer water arrives at 24ºC and you need 3 L/min at 40ºC.

The ratio of power requirements is over 20 and no on-demand heater can do that or even come close while a tank can do that and more.

The combined heating and domestic water on-demand gas boilers we have here resort to different tricks and they kind of work but they are more inefficient than simpler solutions.

In summary, a complicated solution, expensive, prone to problems, inefficient, expensive to buy, expensive to run, expensive to maintain and repair.

These days we are having a heat wave in Europe and water is coming out of the faucet at over 25ºC. The minimum amount of heating my gas heater can provide is way over what I need for a shower so the only way to take a shower is to open another faucet and dump heated water into the drain while I shower. Not something I like to do. Maybe I can take a bath instead.

I understand the need for complicated solutions in a spacecraft where space is at a premium but I prefer not to spend hundreds or thousands of euros and endure crappy showers just to save a little space and have a nice heater hanging on the wall.

Yes, we have on demand gas heaters here. I don't like them. Personally, I prefer a tank any day but that's just me. Other people have other preferences.

richard.cs:

--- Quote from: soldar on July 25, 2019, 10:05:07 am ---Assume in winter water arrives at 4ºC and you need 15 L/min at 74ºC.

Assume in summer water arrives at 24ºC and you need 3 L/min at 40ºC.
...
These days we are having a heat wave in Europe and water is coming out of the faucet at over 25ºC.

--- End quote ---

Your incoming water temperature range is very wide, here we see much lesser variation though it is still problematic for on demand heating. In the UK our water temperature range is more like 10ºC winter to 20ºC summer.

Are your water distribution pipes run on the surface of the ground (I have seen this a lot in Italy but I have never been to Spain) or very shallow buried? In the UK we bury ours about 1 metre deep to prevent them freezing in the winter and I suspect that may be why we see less variation in temperature. As you said before the variation between how different countries do things is very interesting.

soldar:
Water pipes here are buried because we also get hard freezes here. I suppose it is fairly complicated to determine the factors in water temperature. First would be the temperature in the open reservoirs. I suppose there are huge pipes many km long that take the water to the city. I don't think those would add much. Then maybe big tanks and reservoirs and then thinner distribution pipes. I have no idea what the influence of each may be.

It may be that the last riser from the sidewalk to the apartments has most influence as it is the thinnest and most exposed. I don't know.

I am always measuring this kind of thing but it never occurred to me to ask other neighbors. They already think I'm weird enough and I don't need to make it worse.

Receiving the water in winter just above freezing seems logical to me. In the summer I would expect more variation because if you have thin pipes exposed to the Sun the water can get very hot. The good thing then you don't need heating it.

Someone:

--- Quote from: soldar on July 25, 2019, 10:05:07 am ---Compared to tanks they are complicated, expensive to buy, expensive to repair and prone to needing repairs. To do all that regulation they have complex electronic systems that can break down and if you are lucky you can find a replacement board very expensively. If you are unlucky you need to replace the whole heater.  OTOH, a water tank just sits there for decades and knows nothing about electronics or repairs.
--- End quote ---
In this country (before rebates or incentives) instantaneous heaters are around the same price or cheaper to buy and have been showing similar longevity to tank systems. But they cost significantly less to operate over their lifetime, so much so that they have a payback period of just 3 years if you replace a storage system. This will vary on your local energy pricing, tariffs, temperatures, etc.


--- Quote from: soldar on July 25, 2019, 10:05:07 am ---On demand gas heaters can only modulate power within certain limits, say 35 or 40% at the lower limit and 100% at the upper limit. If it is going to actually modulate then you need your centerpoint to be, say between 50 and 80% of capacity, so you have range to move around. So, really, the modulation range is not that great.

Add to that the great variation in power needs and you can see an on-demand heater is just not suitable for many circumstances.

Assume in winter water arrives at 4ºC and you need 15 L/min at 74ºC.

Assume in summer water arrives at 24ºC and you need 3 L/min at 40ºC.

The ratio of power requirements is over 20 and no on-demand heater can do that or even come close while a tank can do that and more.
--- End quote ---
All the units I'm familiar with do have power control with ratios over 20, even beyond a factor of 30 so based on your choice of numbers they work fine. This is going back 20 years so its nothing new or exclusive, there are multiple suppliers of products which meet those requirements.

Storage units can't deliver any temperature other than their current point, so most of the time they are being used with cold water mixed in at the load. You're keeping a big pile of hot water hot for the minority use case, its silly and outdated.


--- Quote from: paulca on July 25, 2019, 08:22:58 am ---
--- Quote from: Someone on July 25, 2019, 07:45:51 am ---
--- Quote from: soldar on July 24, 2019, 09:51:05 am ---On-demand systems intrinsically suck at maintaining water temperature because basically they dump a fixed amount of energy into the water and any change in any parameter will result in a change in temperature. So you need to be able to modulate the energy and have a feedback loop that measures output temperature and modulates energy accordingly. This is possible and is done but adds initial expense and complication and risk of problems and failures.
--- End quote ---
This is all dependent on the specific heater, instantaneous/continuous/on demand gas water heaters are widely used in Australia but they have closed loop regulation at the heater. When used with a controller to set the desired output temperature for each use there are no problems with stability other than low flow rates.

--- End quote ---

I think it depends on how you want to use the hot water.  I 'can' imagine that a shower could send a signal to the boiler to request a temperature so the shower only runs the hot water and varies it's signal to the boiler to achieve the temperature it wants.  However I can't see this as practical unless that boiler is specifically for the that shower alone.  If someone turns on the kitchen hot tap what temperature would the boiler deliver?
--- End quote ---
Its a simple system, the boiler unit has a well regulated output temperature which can be changed in a second or so. If you can't possibly wash you hands in the kitchen with 43 degree water while someone else is showering, or wait for them to finish their shower before running a washing tub at 60 degrees, then you've got some first world problems.

soldar:

--- Quote from: Someone on July 25, 2019, 11:25:15 am ---All the units I'm familiar with do have power control with ratios over 20, even beyond a factor of 30 so based on your choice of numbers they work fine. This is going back 20 years so its nothing new or exclusive, there are multiple suppliers of products which meet those requirements.
--- End quote ---

I am interested in this. I'd like to see a gas water heater that can modulate output from say 12 KW down to say 500 W. Sounds very interesting. 

Other than that there is no need to belittle my preference just as I don't belittle yours. You do whatever you want. I don't care. Just allow others their own choice.

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