Heat Pump Hot Water System vs Gas Hot Water

[coppice]

Most HPWH also have backup resistance heating and he should investigate adding that to dump additional power.

The heat pump water heaters I know of have a resistive heater, but its for defrosting, not actually heating the water. Where Dave lives I doubt such a heater would ever kick in.

[Seekonk]

If you are capable of exporting 4KW of power and the HPWH only takes 600W there is 3.3KW of power you are paid almost nothing for. If the HPWH continues after excess power period that power is at utility rates.  I have a HPWH and it runs for many hours each day.  It only makes sense to add resistive heating to reduce the run time and get faster recovery in the export period. I do supplemental PV water heating with my HPWH.

[EEVblog]

Most HPWH also have backup resistance heating and he should investigate adding that to dump additional power.

The heat pump water heaters I know of have a resistive heater, but its for defrosting, not actually heating the water. Where Dave lives I doubt such a heater would ever kick in.

The heat pump unit (not tank) has a defrost mode that kicks in 5C. That's will practically never be activated here in Sydney.

[EEVblog]

I don't think Dave will have much success in reducing his export problem with a short load of 600W. Most HPWH also have backup resistance heating and he should investigate adding that to dump additional power. End result will likely be a fail in export and increased electrical use.

It's not a "short load of 600W", it's 950W-1100W, and it latss for about 3 hours. Current figures are anywhere from 2.5-3kWh/day.
It will never use power outside of solar times, it's literally on a timer.
The only exception to that will be if for some reason it does not hit 59C during the day, the controller will do whatever it has to to hit 59C at least once per day.

[coppice]

Most HPWH also have backup resistance heating and he should investigate adding that to dump additional power.

The heat pump water heaters I know of have a resistive heater, but its for defrosting, not actually heating the water. Where Dave lives I doubt such a heater would ever kick in.

The heat pump unit (not tank) has a defrost mode that kicks in 5C. That's will practically never be activated here in Sydney.

5C? How can they work by temperature? They usually work by detecting the incorrect pressure when the radiator freezes over and the fan tries to move air?

[EEVblog]

Most HPWH also have backup resistance heating and he should investigate adding that to dump additional power.

The heat pump water heaters I know of have a resistive heater, but its for defrosting, not actually heating the water. Where Dave lives I doubt such a heater would ever kick in.

The heat pump unit (not tank) has a defrost mode that kicks in 5C. That's will practically never be activated here in Sydney.

5C? How can they work by temperature? They usually work by detecting the incorrect pressure when the radiator freezes over and the fan tries to move air?

Manua says this:

At ambient temperatures below 5.5°C, frosting may occur,
if this happens the heat pump will go into defrost mode in
order to protect the unit (i.e. this defrost functionality will
turn the heat pump on and will normally run for no more
than 15 minutes in duration). When the unit is in the
defrost function the pump and fan will stop and the
refrigerant heat will defrost the evaporator. As defrost
advances, the evaporator temperature will increase.

[coppice]

Manual says this:

At ambient temperatures below 5.5°C, frosting may occur,
if this happens the heat pump will go into defrost mode in
order to protect the unit (i.e. this defrost functionality will
turn the heat pump on and will normally run for no more
than 15 minutes in duration). When the unit is in the
defrost function the pump and fan will stop and the
refrigerant heat will defrost the evaporator. As defrost
advances, the evaporator temperature will increase.

A heat pump can freeze up at pretty much any temperature if you work it hard enough, and the humidity is high, or the airflow poor. The radiator can get damned cold. If they are trying to say it generally happens below a certain temperature, its odd for them to say something as specific as 5.5C

[EEVblog]

A heat pump can freeze up at pretty much any temperature if you work it hard enough, and the humidity is high, or the airflow poor. The radiator can get damned cold. If they are trying to say it generally happens below a certain temperature, its odd for them to say something as specific as 5.5C

I don't know, maybe it has something to do with using CO2?

[VK3DRB]

I looked into all of this a few years ago. I replaced the gas powered water storage heater with a gas instant on-demand water heater. It was by far the most cost effective solution at the time. We never run out of hot water. Most of the old gas storage heaters are inefficient because of heat loss through the leaky insulation and non-insulated parts of the water tank. It cost around $3K to install the new heater here, mostly due to a larger diameter copper gas pipe that had to be installed between the gas meter and the heater. Did you consider this alternative? Some newer houses already have the larger diameter pipe installed.

As for going green, if your solar panels are producing the energy for the heat pump, that is a big plus in going down the heat pump path. But if you were driving the electricity off the normal mains, that is not so good.

These days the gas producers are ripping us off, without question. When the Russians lose the war, and Australia eventually gets a government that is not weak, gas prices may well drop considerably. Another factor might be Australian will abandon using gas in their droves, reducing local demand. I can see a time not far away when gas appliances will not be allowed in new houses, not due to cost of gas but greenhouse gas emissions.

[Kleinstein]

The heat exchanger will be colder than the air by a certain temperature difference. How much depends on the construction of the heat pump / flow of the fan. The 5.5°C seem to be the point where with this heat pump the freezing point is about reached - at least as long as the heat exchanger is reasonably clean. So they can reasonably exclude formation of ice above that air temperature.
Humidity can also shift that point: very low humidity can avoid condensation even well below freezing. High humidity could help by gaining extra heat from condensation of water keeping the heat exchanger a little bit warmer. So the worst case is with some intermediate humidity.  With some 5 K below the environment this would be above some 70% RH where you can expect to see water dropping from the heat pump due to condensation.

The text does not tell how they detect ice formation. They just tell that you don't have to expect iceing above 5.5 C - so not a problem for Dave. Chanches are this would be more than just the air temperature, but more like a reduced air flow.
Even with ocasional defrosting the heat pump can still work better than a simple resistive heater. The resistive heater is just more reliable / less likely to have mechanical failure and it may make up for reduced power at lower temperature. With the rather warm climate and just for hot water there is not real need for this.

[EEVblog]

I looked into all of this a few years ago. I replaced the gas powered water storage heater with a gas instant on-demand water heater. Did you consider this alternative?

No. The point was to use excess solar and disconnect the gas.

[Seekonk]

Most HPWH also have backup resistance heating and he should investigate adding that to dump additional power.

The heat pump water heaters I know of have a resistive heater, but its for defrosting, not actually heating the water. Where Dave lives I doubt such a heater would ever kick in.

That makes us even. All the HPWH I know have built in high demand recovery heaters that everyone on solar boards is trying to figure out how to disconnect so the unit only works by compressor.  Seems some of these turn on occasionally to check that it is still connected and throw a fault if disconnected. These are all in one units common in US and a totally different animal. This board has an international audience, and the option of these heaters should be mentioned.

[BrianHG]

A heat pump can freeze up at pretty much any temperature if you work it hard enough, and the humidity is high, or the airflow poor. The radiator can get damned cold. If they are trying to say it generally happens below a certain temperature, its odd for them to say something as specific as 5.5C

I don't know, maybe it has something to do with using CO2?

If I remember correctly, at certain temperatures and rapid high pressure release to low pressure, doesn't CO2 phase change directly from a gas or liquid state to a solid state?  IE: dry ice...  If this happens, in my mind, this could kill a compressor.  In this case, the evaporator is the low pressure side.

[Kleinstein]

The compressor is more on the hot side. So if at all the CO2 would freeze before the compressor and thus no problem for the compressor itself from seeing a low pressure.
Dry ice is at some -55°C, so not a temperature one normally sees outside, unless you are in siberia or antarctica.

The freezing problem is just water condensation and that freezing at the air side of the heat exchanger. The condensation of water is actually helping, as it provides additional heat. The problem is if it freezes and than blocks the air flow. A suitable heat exchanger can still work under such conditions:  For a while it can work with a ice layer building up (but still air flow) and after a while the heat pump is turned off / reversed for a short time to heat the heat exchanger so that part of ice melt, so that is can drop down. It is not ideal, but many air based heat pumps for heating support that mode.

[kaz911]

I'm putting in heat pump in our Spanish apartment - replacing an old old resistive water-tank plus Under-Floor Heat system.  (Daikin Altherma 3)

Biggest problem is lead times... 1/2 the work is done just waiting for components from Daikin.

But our UK house is simply not good enough yet for heat pump. But I've applied for 10kW solar panels and looking at phase change water heater which both works on Heat-pump and our current gas boiler.

I have ordered about 35kWh of Pylontech batteries + Victron Inverter to be installed as well to manage load-shift.

That is very over dimensioned but plan is to keep battery >20% and not charge to 100% - but have enough excess capacity to be able to store excess solar - and have some leeway for the cloudy days.  (I'm developing some software to manage predicted solar delivery automatically through Victron/Myenergi API devices - so every night at close to midnight - the system will try to plan the next 24-48 hours charging strategy....)

I think for our future it is all about being energy flexible - right now gas is £0.10 / kWh and my electricity is £0.41 (with 4 hours of £0.10 night time rates)  - 7 years ago it was £0.0235/kWh for Gas and £0.095 / kWh for electricity all day. Already now without batteries we have migrated about 40-45% of consumption to night time rates. (Car, Dishwasher, Clothes wash/dry, charging of "non-essential" iPads/Computers etc)

But for heat pump to be financially viable to replace gas - it would have to be a COP of 4 during UK Nov/Dec/Jan/Feb - and that does not exists yet for high temperature output heat-pumps I can find.  Spain is 15c and COP stays > 3.7. All the ones with avg. COP => 4 have UK winter COP of about 2-2.5 (based on 6-8c average temperature outside and 55c hot water delivery for radiators and water tank - excl. once per week to 62c for water tank)

But the phase change water tank (SunAmp) is said to only drop about 0.5-0.8 kWh's worth of energy per day. And a 210L equivalent SunAmp stores about 11 kWh's of energy - so about 5-6% drop a day. Benefits are we don't need to worry about legionnaire - Zappi's brother Eddi can manage to divert excess solar OR cheap rate electricity into the "Tank" and no need for any fans outside. Eddi can also be used to "activate" other devices via the built in relay board you can buy.

[coppice]

But for heat pump to be financially viable to replace gas - it would have to be a COP of 4 during UK Nov/Dec/Jan/Feb - and that does not exists yet for high temperature output heat-pumps I can find.  Spain is 15c and COP stays > 3.7. All the ones with avg. COP => 4 have UK winter COP of about 2-2.5 (based on 6-8c average temperature outside and 55c hot water delivery for radiators and water tank - excl. once per week to 62c for water tank)

Well, there is your problem. Only heat to 55C for hot water. Heat to the low 20's for warming the house, and duct the warm air around. Heating to a high temperature for radiators ruins most of the benefits of heat pumps. In most UK houses its not easy to duct around the ground floor, unless you have a bungalow. Its easy to duct around the top floor, using the loft space, and the ducts can be very well insulated. Not only is your COP better, but heat pumps last longer with a smaller gradient for them to climb.

[kaz911]

But for heat pump to be financially viable to replace gas - it would have to be a COP of 4 during UK Nov/Dec/Jan/Feb - and that does not exists yet for high temperature output heat-pumps I can find.  Spain is 15c and COP stays > 3.7. All the ones with avg. COP => 4 have UK winter COP of about 2-2.5 (based on 6-8c average temperature outside and 55c hot water delivery for radiators and water tank - excl. once per week to 62c for water tank)

Well, there is your problem. Only heat to 55C for hot water. Heat to the low 20's for warming the house, and duct the warm air around. Heating to a high temperature for radiators ruins most of the benefits of heat pumps. In most UK houses its not easy to duct around the ground floor, unless you have a bungalow. Its easy to duct around the top floor, using the loft space, and the ducts can be very well insulated. Not only is your COP better, but heat pumps last longer with a smaller gradient for them to climb.

My old house is not capable to be heated with water circulating at <55c - 55c is the lower end of what is possible. Unless I completely re-insulate the house - ripping up floors and add internal or external wall insulation and add mega sized radiators. That would be a £100.000++ operation. And that is not in the plans for now.

I have a few friends around who live in similar 1920's/1930's buildings who are now quite angry they got "conned" (their words) into putting in heat-pumps when electricity was 10-15p/kWh - without major refurb - my Nat gas heating cost right now is < 1/2 of what they pay. But I have DYI insulated where possible but sadly a lot of things are not possible without major work due to lots of thermal bridges in the 1920's/1930's houses and lack of space for hollow wall insulation.

[coppice]

But for heat pump to be financially viable to replace gas - it would have to be a COP of 4 during UK Nov/Dec/Jan/Feb - and that does not exists yet for high temperature output heat-pumps I can find.  Spain is 15c and COP stays > 3.7. All the ones with avg. COP => 4 have UK winter COP of about 2-2.5 (based on 6-8c average temperature outside and 55c hot water delivery for radiators and water tank - excl. once per week to 62c for water tank)

Well, there is your problem. Only heat to 55C for hot water. Heat to the low 20's for warming the house, and duct the warm air around. Heating to a high temperature for radiators ruins most of the benefits of heat pumps. In most UK houses its not easy to duct around the ground floor, unless you have a bungalow. Its easy to duct around the top floor, using the loft space, and the ducts can be very well insulated. Not only is your COP better, but heat pumps last longer with a smaller gradient for them to climb.

My old house is not capable to be heated with water circulating at <55c - 55c is the lower end of what is possible. Unless I completely re-insulate the house - ripping up floors and add internal or external wall insulation and add mega sized radiators. That would be a £100.000++ operation. And that is not in the plans for now.

I have a few friends around who live in similar 1920's/1930's buildings who are now quite angry they got "conned" (their words) into putting in heat-pumps when electricity was 10-15p/kWh - without major refurb - my Nat gas heating cost right now is < 1/2 of what they pay. But I have DYI insulated where possible but sadly a lot of things are not possible without major work due to lots of thermal bridges in the 1920's/1930's houses and lack of space for hollow wall insulation.

I said don't circulate warm water. The UK seems locked into the idea of warm water to heat homes. Its will come back to bite you if we have warmer summers, and you can't use that heat pump for effective cooling. If you use the heat pump to gently warm and cool air, and duct that around the house, you can keep a pretty good COP through an entire UK year. Making good use of heat pumps is about minimising the temperature gradients. You can't just heat air to 20C and just pass that around the house, and get the house to 20C. The air needs to be a bit warmer, but not much. Certainly not the 50C to 60C you need to make radiators viable. I keep seeing people talk like there is a deep connection between good insulation and heat pumps. There isn't. Insulation bring the same benefits in cost reduction and more consistent temperatures from wall to wall however you heat your home.

[Marco]

You could add low speed fan boosters to the old radiators and add some FCUs on the wall for when it gets really cold. FCUs are pretty cheap ... in Italy. Can double as AC in the summer.

FCUs trade noise for size, you can get a lot of kW of low water temperature heating in a small size.

PS. a Dutch engineer made a controller for DIY radiator fan boosters BTW. A lot of discussion related to it here, in Dutch.

[kaz911]

I said don't circulate warm water. The UK seems locked into the idea of warm water to heat homes. Its will come back to bite you if we have warmer summers, and you can't use that heat pump for effective cooling. If you use the heat pump to gently warm and cool air, and duct that around the house, you can keep a pretty good COP through an entire UK year. Making good use of heat pumps is about minimising the temperature gradients. You can't just heat air to 20C and just pass that around the house, and get the house to 20C. The air needs to be a bit warmer, but not much. Certainly not the 50C to 60C you need to make radiators viable. I keep seeing people talk like there is a deep connection between good insulation and heat pumps. There isn't. Insulation bring the same benefits in cost reduction and more consistent temperatures from wall to wall however you heat your home.

having lived in both very hot and and very cold countries with a huge mix of heating and cooling systems - I will say I do by far prefer heat from indirect sources (like UFH or radiators) - UFH is the preference by a MILE.

Hot air heat is ok - but it has a tendency to dry out everything - skin gets dry - eyes get dry and itchy etc.  I had a unit in my old office that replaced my resistive storage heaters - and that even had a humidification option built in.

Cool air-condition has some of the same issues - but you get nothing but problems if you try to reverse water based UFH to UF cooling - as hot countries often are humid - and cold surface in humid conditions creates liquid water.

[coppice]

I said don't circulate warm water. The UK seems locked into the idea of warm water to heat homes. Its will come back to bite you if we have warmer summers, and you can't use that heat pump for effective cooling. If you use the heat pump to gently warm and cool air, and duct that around the house, you can keep a pretty good COP through an entire UK year. Making good use of heat pumps is about minimising the temperature gradients. You can't just heat air to 20C and just pass that around the house, and get the house to 20C. The air needs to be a bit warmer, but not much. Certainly not the 50C to 60C you need to make radiators viable. I keep seeing people talk like there is a deep connection between good insulation and heat pumps. There isn't. Insulation bring the same benefits in cost reduction and more consistent temperatures from wall to wall however you heat your home.

having lived in both very hot and and very cold countries with a huge mix of heating and cooling systems - I will say I do by far prefer heat from indirect sources (like UFH or radiators) - UFH is the preference by a MILE.

Hot air heat is ok - but it has a tendency to dry out everything - skin gets dry - eyes get dry and itchy etc.  I had a unit in my old office that replaced my resistive storage heaters - and that even had a humidification option built in.

Cool air-condition has some of the same issues - but you get nothing but problems if you try to reverse water based UFH to UF cooling - as hot countries often are humid - and cold surface in humid conditions creates liquid water.

I've also lived in both hot and cold, dry and super humid places. Forced air heating varies a lot. If they just blast air at a high temperature it can work OK, but if they blast slightly warm air they cause too much evaporation from the skin, and things feel cold. Well designed systems, like Daikin, allow for this very well. In cooling mode they blast out the air. In warming mode they are much more subdued, adapting to avoid too much of that cooling effect. Keeping the circulators well away from the people (e.g. high up on the least used side of the room) helps, too. I heated and cooled my last home with Daikin split air-cons, and the in room air circulators were very pleasant, and made very little noise.There are options which make the air circulators bigger and slower, which are extremely hard to hear, unless you are very close. These things are very efficient.

[Marco]

having lived in both very hot and and very cold countries with a huge mix of heating and cooling systems - I will say I do by far prefer heat from indirect sources (like UFH or radiators) - UFH is the preference by a MILE.

Unless you have and want extremely expensive flooring, ripping up a floor for a room and putting in a dry build floor heating system isn't big or expensive job, you can just lay a floating floor straight on top. If the gas prices persist, or you're still there in a couple decades putting UFH in just in the living room and FCUs everywhere else seems a good option, which should cost a lot less than 100k.

[EEVblog]

Update:

[BrianHG]

Here is a nice short docu/explanation on an electric hot water tank's dual heating element function while in use, filmed with an infrared camera.
It's explained and photographed why the hot water doesn't mix with the new cold water coming in.

[rthorntn]

Dave, did you disconnect your gas then, I still have a 5yo gas stove (do you have induction then) and so cant disconnect (making my decision more difficult because I cant get rid of the gas supply charge)?

In saying that I'm pretty sure I have the excess solar to be able do this.

The main problem for me seems to be the availability of the Reclaim stuff, a friend of mine owns a large plumbing company and he's drawing a blank.  How do you buy this and what is the street price in Sydney for a REHP-CO2-160 hot water heat pump installed to replace a Rheem?

It's hovering above the too hard basket for me atm.

Update: