Heat Pumps for water heating

[Geoff.D]

I just spent a week at a holiday destination, where each unit has a heat pump installed, feeding into a 150l geyser, with the electric element permanently disconnected. The unit is supposed to sleep 6 persons

There was never any hot water available. At best the water temperature barely reached 40 deg C.

Taking the matter up with management was a fruitless exercise, as no one could actually explain what needed to be done to fix the situation. They claimed that if the electric element was kept connected, the heat pump never switches on, hence why the geyser element is disconnected. Further, they said they cannot set the heat temperature above 45 degrees C on the heat pump. The heat pump is about 4 "pipe meters" from the geyser location installed outside and the pipes are all lagged. I would have thought setting the temperature control at maximum would be mandatory to try and meet the legionella standards.


No amount of explanation about the dangers of Legionella had any affect either. In the end, at my cost , an electrician was called out to disconnect the supply to the heat pump and re-connect the element.

Now that lead me to investigating why would the system be performing so badly? The Heat pump supplier is a reputable supplier. The heat pump is rated at 55 deg C. The installation "looks" correct, but the point at which the temperature control is connected seem to be incorrect on the hot water outlet pipe coming from the heat pump. How can that ever give one a true gauge regarding the water temperature in the geyser?

It was pretty clear, that the moment hot water is drawn from the system, the geyser fills up with cold water and then there is a sudden drop on the overall water temperature. There is a dual cold feed into the geyser and the heat pump system, with the "hot water connections coupled together at the outlet of the geyser.

Obviously, there is something really drastically wrong with the installation but what? Is it simply a malfunctioning heat pump, or is there a basic flaw in the overall installation design?

Anyone on the forum with experience with heat pump water heating system able to comment?

 

[Arthur]

Nothing to do with a heat pump. I've used one since 2010. Not an issue. Keep water at 62 degrees.

There are threads on heat pumps on MyBB (but watching cricket now).

 

[Geoff.D]

Nothing to do with a heat pump. I've used one since 2010. Not an issue. Keep water at 62 degrees.

There are threads on heat pumps on MyBB (but watching cricket now).

Thanks Arthur, found this post of yours very informative.

As you have had your system running for a good few years now, maybe you have some more insights to offer?

A few questions/comments.

(1) I see you opted for the inside the roof installation. This solution makes a lot of sense to me, but none of the contractors I have approached suggest or support this type of installation. All suggest rather that the outside against the wall installation is "better" without any explanation as to why it should be "better". My assessment is that in our climate, the inside the roof install is advantageous as this stabilises the heat source available to a large extent. The other benefit is that the pipe lengths are minimised, ensuring much better heat retention between the heat pump and the storage tank.
(2) I notice the flexible connections to the Heat pump -- presumably installed for ease of replacement and to reduce vibration transfer to the rest of the system?
(3) I also notice that the inlet and outlet into the heat pump are at the same level --- Any info as to why? Examples from contractors simply split the incoming cold water at the geyser inlet.
(4) What is your experience regarding changing the temperature settings on the heat pump controller? The SABS spec dealing with legionella suggest that the temperature should never be set below 50 deg C.
(5) Where is the temperature measured? On the outlet of the geyser? An inside measurement at the low point in the geyser ( basically where a geyser currently has the thermostat located is the right place for the temperature measurement to be done.)
(6) The geyser presumably has a back up element installed. Do you control that with a timer/interlock so that the element only comes on after heat pump stops operating?

Best
Geoff

 

[Arthur]

Thanks Arthur, found this post of yours very informative.

As you have had your system running for a good few years now, maybe you have some more insights to offer?

A few questions/comments.

(1) I see you opted for the inside the roof installation. This solution makes a lot of sense to me, but none of the contractors I have approached suggest or support this type of installation. All suggest rather that the outside against the wall installation is "better" without any explanation as to why it should be "better". My assessment is that in our climate, the inside the roof install is advantageous as this stabilises the heat source available to a large extent. The other benefit is that the pipe lengths are minimised, ensuring much better heat retention between the heat pump and the storage tank.
(2) I notice the flexible connections to the Heat pump -- presumably installed for ease of replacement and to reduce vibration transfer to the rest of the system?
(3) I also notice that the inlet and outlet into the heat pump are at the same level --- Any info as to why? Examples from contractors simply split the incoming cold water at the geyser inlet.
(4) What is your experience regarding changing the temperature settings on the heat pump controller? The SABS spec dealing with legionella suggest that the temperature should never be set below 50 deg C.
(5) Where is the temperature measured? On the outlet of the geyser? An inside measurement at the low point in the geyser ( basically where a geyser currently has the thermostat located is the right place for the temperature measurement to be done.)
(6) The geyser presumably has a back up element installed. Do you control that with a timer/interlock so that the element only comes on after heat pump stops operating?

Best
Geoff

Hi Geoff

1. The manufacturer recommended installation in the ceiling. It also means the heatpump and geyser are close together, minimising temp loss. Also, they geyser is directly above the main bathroom, so hot water gets to taps with minimal run.

2. The flexible hoses came with the system, so that's what the installer used. When I did this no-one had heard of heatpumps, so the plumber was completely inexperienced. I understand the vibration concern, but I never thought of that at the time. This unit runs very quietly.

3. The system wasn't installed correctly. About a year after that post the system developed a fault, so I contacted the manufacturer. It was quickly diagnosed as a silted-up water pump. It turns out the installer didn't add an inline water filter on the inlet. I sent them photos of the installation and they recommended some changes to the piping, which my plumber performed (as well as adding the filters). It turns out our town water is rather silted, and this can gum up the water pump if not filtered out. I clean the filter once a year - 5 minute job. The system has worked flawlessly since then.

4. The temp settings are very easy on the control panel. A few button presses and that's it.

5. The temp probe from the control panel is inserted about 20cm into the geyser. It's the black wire near the bottom front in the pic.

6. Yes, there's a backup element installed. Have only used it once, when the water pump silted up - was simply a matter of pressing one button on the control panel (for element on/off). I haven't looked at how the element can be controlled in case of heatpump failure.

As I mentioned in the referenced 2010 post, the geyser is not a standard off-the-shelf one. Because hot water comes from outside the geyser, the inlet pipe is pushed in much deeper than with standard ones. The Tecron factory did this without extra charge.

 

[agentrfr]

Thanks Arthur, found this post of yours very informative.

As you have had your system running for a good few years now, maybe you have some more insights to offer?

A few questions/comments.

(1) I see you opted for the inside the roof installation. This solution makes a lot of sense to me, but none of the contractors I have approached suggest or support this type of installation. All suggest rather that the outside against the wall installation is "better" without any explanation as to why it should be "better". My assessment is that in our climate, the inside the roof install is advantageous as this stabilises the heat source available to a large extent. The other benefit is that the pipe lengths are minimised, ensuring much better heat retention between the heat pump and the storage tank.
(2) I notice the flexible connections to the Heat pump -- presumably installed for ease of replacement and to reduce vibration transfer to the rest of the system?
(3) I also notice that the inlet and outlet into the heat pump are at the same level --- Any info as to why? Examples from contractors simply split the incoming cold water at the geyser inlet.
(4) What is your experience regarding changing the temperature settings on the heat pump controller? The SABS spec dealing with legionella suggest that the temperature should never be set below 50 deg C.
(5) Where is the temperature measured? On the outlet of the geyser? An inside measurement at the low point in the geyser ( basically where a geyser currently has the thermostat located is the right place for the temperature measurement to be done.)
(6) The geyser presumably has a back up element installed. Do you control that with a timer/interlock so that the element only comes on after heat pump stops operating?

Best
Geoff

I can try answer some of your questions quick. Industrial heat pumps can generate superheated steam quite easily, commercial ones should also be able to get near boiling without much issue. Remember a heatpump uses electrical energy to pump a large amount of heat from one place to another - much like how a tanker truck burns a little diesel to move fuel from the depot to the fuel station. We measure the ratio of heat energy moved to electrical expenditure as the Coefficient of Performance, or COP. Typically aircons and heatpumps will operate at a COP of about 8, as in you spend 1J of electricity pumping 8J of heat. For a geyser this is fantastic, as otherwise if you use a resistive element you spend 8J of electricity for 8J of heat for the water.

Most importantly you should be aware of is legionella have monod growth phase between 25-45C, so to me setting your geyser at 50C is cutting it close. I'd recommend at least 65C (UK Health dpt. recommends 60C http://www.hse.gov.uk/healthservices/legionella.htm)

1) Actually, it is more efficient to run the heat pump in direct contact with ambient air, as there inst enough heat stored within the air locked in the roof to heat the water respectably. Quickly the air inside the roof acts more as an insulator that a heat reservoir, and you are pulling heat in from ambient anyway, so its easier to do it just from the outside air. Quick numbers: 150L of water from 15C to 65C requires 31*395 kJ (Q=mCp(\Delta(T))) of energy, and if you have a 10m x 10m house with a 1.5m roof with air at 40C, that is 50m^3 of air or 61.25 kg of air, with a total enthalpy between absolute 0 and 40C of 19*267 kJ (Q=mCp(\Delta(T))) - which isnt enough. So heat needs to be pumped from atmosphere through the air in your roof to the heat pump, and air is a notoriously terrible heat conductor and is normally considered a very good insulator... So it makes sense to just do the heat pumping from outside rather than wasting electricity pumping the heat through the air in your roof... what's a few extra meters of copper tubing? You'll save much more money in electricity by not having to pump heat through the roof first then into the water.

2) Probably easier to install that way - noise shouldnt be an issue since it is closed loop.

3) Water pressure will push the water through the geyser, so it doesn't matter the height of the inlet/outlet. It's always cheaper to machine multiple holes on a single level as you dont need to waste time moving/rotating it into the right place during construction.

4)Yeah no 65C for me thanks.

5)Temperature should be measured from within the geyser - that's what you are trying to maintain at the correct temperature, right? You might get away with measuring the temperature of the outlet manifold as the metal is at roughly the same temp as the water inside - but it is really not that much effort to simply drill a hole and plug it with a probe. Sounds like a shoddy job.

6) Maybe the element is there just for convenience if the pump is due for maintenance? Can't imagine needing to use it regularly.

 

[Geoff.D]

Thank you Arthur for the additional insight provided. And to Agentrfr for the explanation as to why an in-roof installation is not necessarily better.

What started out as simply trying to work out why the performance at the holiday accommodation was/is so bad, has turned into a full research as to what I should do next time my geyser pops.

I am in correspondence with the holiday accommodation management company, and so far their responses are not encouraging at all. It seems it was a "board decision" to turn DOWN the settings on the heat pump to 45 deg C!. They claimed that to meet their obligations to Eskom to reduce electricity consumption by 20% (apparently the holiday accommodation industry was expected to cut consumption by 20%) they were "forced" to cut the temperature of geysers!. Then because this then meant with their installation, the back up elements would come on, those were disconnected! Talk about the law of unintended consequences!

So far rather silent on the legionella matter! I think I have them there!
So basically, it is only when someone like me arrives and bitches, that they then get the sucker to pay for an electrician to come and reconnect the element!

There is more to this than meets the eye I am sure.

[How many times do ill informed management decisions cripple technology solutions I wonder!!]

 

[agentrfr]

Thank you Arthur for the additional insight provided. And to Agentrfr for the explanation as to why an in-roof installation is not necessarily better.

What started out as simply trying to work out why the performance at the holiday accommodation was/is so bad, has turned into a full research as to what I should do next time my geyser pops.

I am in correspondence with the holiday accommodation management company, and so far their responses are not encouraging at all. It seems it was a "board decision" to turn DOWN the settings on the heat pump to 45 deg C!. They claimed that to meet their obligations to Eskom to reduce electricity consumption by 20% (apparently the holiday accommodation industry was expected to cut consumption by 20%) they were "forced" to cut the temperature of geysers!. Then because this then meant with their installation, the back up elements would come on, those were disconnected! Talk about the law of unintended consequences!

So far rather silent on the legionella matter! I think I have them there!
So basically, it is only when someone like me arrives and bitches, that they then get the sucker to pay for an electrician to come and reconnect the element!

There is more to this than meets the eye I am sure.

[How many times do ill informed management decisions cripple technology solutions I wonder!!]

45C is right smack bang in the middle of idea growth temperature for legionella. They're seriously going to either hurt or kill someone by doing that.

 

[Arthur]

I1) Actually, it is more efficient to run the heat pump in direct contact with ambient air, as there inst enough heat stored within the air locked in the roof to heat the water respectably. Quickly the air inside the roof acts more as an insulator that a heat reservoir, and you are pulling heat in from ambient anyway, so its easier to do it just from the outside air. Quick numbers: 150L of water from 15C to 65C requires 31*395 kJ (Q=mCp(\Delta(T))) of energy, and if you have a 10m x 10m house with a 1.5m roof with air at 40C, that is 50m^3 of air or 61.25 kg of air, with a total enthalpy between absolute 0 and 40C of 19*267 kJ (Q=mCp(\Delta(T))) - which isnt enough. So heat needs to be pumped from atmosphere through the air in your roof to the heat pump, and air is a notoriously terrible heat conductor and is normally considered a very good insulator... So it makes sense to just do the heat pumping from outside rather than wasting electricity pumping the heat through the air in your roof... what's a few extra meters of copper tubing? You'll save much more money in electricity by not having to pump heat through the roof first then into the water.

Thanks, agentrfr. Makes complete sense. Fortunately in my case the above-ceiling roof space is vast (nearly 4m above ceiling at high point along backbone), and here in the Little Karoo the ceiling space air is several degrees above ambient. I think there's enough volume to extract the required heat, but I can see that in most typical domestic ceilings there isn't. Also, the exiting cold air can get dammed around the heatpump, further reducing heat extraction.

 

[Geoff.D]

Thanks, agentrfr. Makes complete sense. Fortunately in my case the above-ceiling roof space is vast (nearly 4m above ceiling at high point along backbone), and here in the Little Karoo the ceiling space air is several degrees above ambient. I think there's enough volume to extract the required heat, but I can see that in most typical domestic ceilings there isn't. Also, the exiting cold air can get dammed around the heatpump, further reducing heat extraction.

Having done the calculations for my roof space, I also have enough volume in my place to consider placing the HP in the roof. It then occurred to me that with a relatively simple duct structure, I could exhaust the cool air into the house. I have approached Enerflow regarding the impact this idea might have on the fan in the heat pump. It all depends on if the drop in temperature of the air through the heat pump is significant enough to make any difference to the cooling inside the home.

 

[Sinbad]

Just an aside - some municipalities have regulations/bylaws on the maximum discharge temperature of a tap...

eg durban:
Maximum temperatures in hot water circulating systems
58. The temperature of the water which discharges from a terminal water fitting supplied from a hot water re-circulating system may not exceed 55°C .

67. (1) Heat pumps shall be so selected as to provide the total hot water demand. Heat pump settings are not to exceed 60°C .


So if you're setting your geyser above that, you need a thermostatic mixer in line as well....

 

[Sinbad]

Having done the calculations for my roof space, I also have enough volume in my place to consider placing the HP in the roof. It then occurred to me that with a relatively simple duct structure, I could exhaust the cool air into the house. I have approached Enerflow regarding the impact this idea might have on the fan in the heat pump. It all depends on if the drop in temperature of the air through the heat pump is significant enough to make any difference to the cooling inside the home.

Interesting idea

There's quite a lot of coolness generated - you might want to have some sort of selector to direct the cold air outside for cooler nights...

 

[Geoff.D]

Just an aside - some municipalities have regulations/bylaws on the maximum discharge temperature of a tap...

eg durban:
Maximum temperatures in hot water circulating systems
58. The temperature of the water which discharges from a terminal water fitting supplied from a hot water re-circulating system may not exceed 55°C .

67. (1) Heat pumps shall be so selected as to provide the total hot water demand. Heat pump settings are not to exceed 60°C .


So if you're setting your geyser above that, you need a thermostatic mixer in line as well....

Thanks for the info. I wonder if the same local bylaws have anything to say about water temperatures to combat Legionella.
I understand the idea of limiting the water temperature from a scalding safety point of view.

The relevant SABS documents on the subject are SANS 893-1 and 893-2.

The SANS specs state the following facts about combatting Legionella. (extracted from an Eskom document).
Cold water systems must maintain a temperature < 20 deg C.
Hot water systems must maintain a temperature > 50 deg C.

This translates into geyser temperatures higher than that to allow for heat losses, say 60 deg C.
Yet the SABS calls for 65 deg C to ensure effective treatment of possible Legionella.

 

[Geoff.D]

Interesting idea

There's quite a lot of coolness generated - you might want to have some sort of selector to direct the cold air outside for cooler nights...

At the moment in the heat wave we are experiencing, any cooling will be most welcome! .

I have just read a paper prepared by a university in the USA, dealing with heat pumps installed inside buildings, inside air conditioned areas. The overall effect is to reduce the energy savings possible with heat pumps by about 4 %.

 

[Arthur]

Btw, I went back and checked pics. The flexible hose has been replaced by copper. Below are Before and After the pipe routing changes and addition of inlet filter.

The ding in the geyser was put there by the builder when installing.

 

[richjdavies]

... reduce the energy savings possible with heat pumps by about 4 %.

Yup, it's basically all down to the temperature difference; if you let the cold-side get too cold, it won't work as well (it's got to push against a bigger temperature difference). Ultimately though, some of that loss of efficiency you get back because you wanted the inside colder...

Depends how you look at it I suppose. Is the Heat Pump heating up water and cooling the air; or is the air-con cooling down the air and then you put the hot-side in some water?

 

[Arthur]

... and get one of those drop-down trapdoor ladders to get into the ceiling. I got an Arnold Smith one. They're custom made for your space. One of the best things ever in a house. Makes getting into the ceiling painless.

 

[Geoff.D]

Yup, it's basically all down to the temperature difference; if you let the cold-side get too cold, it won't work as well (it's got to push against a bigger temperature difference). Ultimately though, some of that loss of efficiency you get back because you wanted the inside colder...

Depends how you look at it I suppose. Is the Heat Pump heating up water and cooling the air; or is the air-con cooling down the air and then you put the hot-side in some water?

Now that is another topic all together. Just found a novel idea. An aircon unit with an add-on module Heat Pump for heating water ....

So the idea is you cool the interior, take the heat generated to feed through the heat pump to heat water. Looking for someone that has combined both systems, i.e. not as an afterthought but fully integrated system.

 

[Geoff.D]

... and get one of those drop-down trapdoor ladders to get into the ceiling. I got an Arnold Smith one. They're custom made for your space. One of the best things ever in a house. Makes getting into the ceiling painless.

Fell though my ceiling a few years back. Thought no problem easy to fix! Turned out to be one of the most challenging DIY jobs I have ever tackled!

 

[Arthur]

Now that is another topic all together. Just found a novel idea. An aircon unit with an add-on module Heat Pump for heating water ....

So the idea is you cool the interior, take the heat generated to feed through the heat pump to heat water. Looking for someone that has combined both systems, i.e. not as an afterthought but fully integrated system.

Siemens and others make them for the German market, istr.