General HP Questions

[Jon Glanfield]

Hi All, my head has been working over time since we found this source of knowledge and would like to pose some general questions about heat pumps if that's ok.

Buffers

Whilst users on here and indeed most European manufacturers recommend buffers, whenever we discuss it with Mitsubishi, Daikin and their ilk, the view is that the heat loss from the buffers causes a drop off in efficiency due to standing losses. I have not got any data specifically on buffers but a Kingspan 300l cylinder is cited at losing 2.43kw per day (I am not sure of the measuring criteria though). Is it conversely true though that the lower number of compressor starts, constant flow rate, support for the space heating during defrost (if ASHP) and cyl reheat and therefore delta T either cancel out or outweigh the standing losses?

Instantaneous DHW

We have effectively employed thermal stores with large coils for the provision of constant DHW previously with GSHP's. However the manufs we have used for ASHP's are very reluctant to suggest this type of technology with their products. From researching Austrian products it looks like this is fairly common elsewhere, and I cannot see any reason why the methodology should be any different if an ASHP is used as opposed to a GSHP. As we migrate to other manufs we may find our thinking on this re-directed anyway.

Thermal Storage

We have also been talking to an IDM distributor this week and their methodology uses accumulators sized to hold a full charge of circa 1 days heating for the property. It is then recharged overnight via the HP on low tariff electicity. DHW is via the same store by means of a plate heat exchanger. Does anyone have any experience of doing things this way and how does it stack up efficiency wise against a more conventional set up? Our concern is the high heat requirement for the DHW, which is constantly reduced by the space heating draw off unless stratification is very good, which they claim it is.

HX Location

Conventionally with ASHP's the units we have used have been monoblocs but from reading posts on here and general logic, it makes sense to avoid these for many reasons, glycol, heat loss and so on. Is it better to use a unit that can be sited inside and ducted or is it better to employ the use of split, or is there no real difference so long as the HX is internal. Are people happy to use either depending on the project requirements?

Constant or Timed DHW

On our GSHP products the manuf instructions are always to have DHW set as priority, i.e. not timed, cyl reheats when the contents drop to a preset temp. However on our ASHPs the standing instruction is to time the DHW production to keep efficiencies high.

However if it is on a timed basis, there is a definite period when no space heating will occur, which then has to be recovered as the unit switches back. Is it therefore a trade off in energy efficiency or are there definite arguments for one over the other?

External Switching Stats

On the GSHP product we have been using the ext WC sensor also fulfills the function of bringing the HP on at a pre-set ambient at a low flow temp point in the slope, usually 18 (ambient) degrees to start banking heat into the property. Should we consider employing this functionality on our ASHP projects?

We are in the process of researching new products anyway that may use this functionality, but in the meantime are thinking of using it, to reduce user intervention as much as possible.

Sizing

There seem to be a whole host of approaches to sizing, ranging from 100% on the coldest design day with the immersion as bail out or extreme back-up, to 80% on the coldest design day and 20% on the immersion. We have tended to use different approaches depending upon the project and product, but generally aim for 100% coverage if at all possible. What approach do others use or are you guided mainly by manuf's recommendations?

SAP

We have had 1 issue with SAP where the co-efficient of heat loss plus a multiplier appears to be at odds with the heat loss calculated via other means (not incl any additional percentages added on for say an ASHP application to account for drop offs at lower ambients).

Yesterday in a meeting with a large UK housebuilder making it's first tentative steps into the renewables world, they advised that they too had experienced issues where SAP was coming up with inaccuracies, if compared with a more traditional heat loss calc tool. Apparently the Peabody Trust commisisoned a report to support this via Malcolm Bell at Leeds Met, I am trying to get hold of a copy.

I just wondered whether anyone else had experienced this or had a view and also what tools people use to calc heat loss.

Cheers

Jon

[mad fridgie]

As you have seen from the many threads, is would seem to appear that engineering is as fault, as in some of your points, however thats seem to be within the industry as whole a lack of understanding of the application and thus the implication on heat pumps in general, to much heating data is based upon electric and combustion.
Before I answer your questions, I have one for you what is IDM

[Bigfreeze]

They're an austrian Manufacturer of heat pumps MF. They developed HGL technology where they basicly use a desuperheater to remove energy to the hot water circuit while the heatpump is in heating mode. They make very good equipment.

[mad fridgie]

When did they develope this BF

[Bigfreeze]

Jon Glanfield;223225]Hi All, my head has been working over time since we found this source of knowledge and would like to pose some general questions about heat pumps if that's ok.

Buffers

Whilst users on here and indeed most European manufacturers recommend buffers, whenever we discuss it with Mitsubishi, Daikin and their ilk, the view is that the heat loss from the buffers causes a drop off in efficiency due to standing losses. I have not got any data specifically on buffers but a Kingspan 300l cylinder is cited at losing 2.43kw per day (I am not sure of the measuring criteria though). Is it conversely true though that the lower number of compressor starts, constant flow rate, support for the space heating during defrost (if ASHP) and cyl reheat and therefore delta T either cancel out or outweigh the standing losses?

As I said before, you should basicly ignore the advice given by Mits, Daikin etc. They really do not follow best practice when it comes to installation. Their focus is on the maximum turnover of equipment which means keeping costs down. A buffer is essential where flow will be restricted by closing of stats or where a defrost cycle occurs. Ignore the standing losses, 2kw when you're producing 100kw in a day is small change. Where is the heat being lost to... the house, so whats the problem?

Instantaneous DHW

We have effectively employed thermal stores with large coils for the provision of constant DHW previously with GSHP's. However the manufs we have used for ASHP's are very reluctant to suggest this type of technology with their products. From researching Austrian products it looks like this is fairly common elsewhere, and I cannot see any reason why the methodology should be any different if an ASHP is used as opposed to a GSHP. As we migrate to other manufs we may find our thinking on this re-directed anyway.

Instantaneous DHW is the way to go. All of our systems have it. It will cut your hot water production bills by 60% because you won't need any immersion back up. There is no difference in this respect from ASHP to GSHP. Your DHW tank should be sperate to your buffer if you have one. The buffer would be kept at as low a temp as possible and the DHW would be kept at about 50C

Thermal Storage

We have also been talking to an IDM distributor this week and their methodology uses accumulators sized to hold a full charge of circa 1 days heating for the property. It is then recharged overnight via the HP on low tariff electicity. DHW is via the same store by means of a plate heat exchanger. Does anyone have any experience of doing things this way and how does it stack up efficiency wise against a more conventional set up? Our concern is the high heat requirement for the DHW, which is constantly reduced by the space heating draw off unless stratification is very good, which they claim it is.

I don't agree with the IDM installer here. The buffer would need to be about 5000L in capacity in order to cover your average houses heat requirements over a day. I here installers here use this argument when installing a 500L buffer. Its basicly crap and a sales pitch. The ufh screed is your best buffer. That is what you should be aiming to charge. Below 12kw on a well insulated house I wouldn't even bother with night rate.

HX Location

Conventionally with ASHP's the units we have used have been monoblocs but from reading posts on here and general logic, it makes sense to avoid these for many reasons, glycol, heat loss and so on. Is it better to use a unit that can be sited inside and ducted or is it better to employ the use of split, or is there no real difference so long as the HX is internal. Are people happy to use either depending on the project requirements?

Ideally you should have the main body of the heat pump internal to the space being heated. Use a split if at all possible

Constant or Timed DHW

On our GSHP products the manuf instructions are always to have DHW set as priority, i.e. not timed, cyl reheats when the contents drop to a preset temp. However on our ASHPs the standing instruction is to time the DHW production to keep efficiencies high.

If you have a decent hot water tank this should not be a consideration. A good tank will only lose 1C over 24hrs. Ideally the tank would be on a 4-5C differential

However if it is on a timed basis, there is a definite period when no space heating will occur, which then has to be recovered as the unit switches back. Is it therefore a trade off in energy efficiency or are there definite arguments for one over the other?

External Switching Stats

On the GSHP product we have been using the ext WC sensor also fulfills the function of bringing the HP on at a pre-set ambient at a low flow temp point in the slope, usually 18 (ambient) degrees to start banking heat into the property. Should we consider employing this functionality on our ASHP projects?

We are in the process of researching new products anyway that may use this functionality, but in the meantime are thinking of using it, to reduce user intervention as much as possible.

WC is the way to go once you have a good wc control system. Don't just use an add on without fully testing it. It could impair the system if its not up to scratch
Sizing

There seem to be a whole host of approaches to sizing, ranging from 100% on the coldest design day with the immersion as bail out or extreme back-up, to 80% on the coldest design day and 20% on the immersion. We have tended to use different approaches depending upon the project and product, but generally aim for 100% coverage if at all possible. What approach do others use or are you guided mainly by manuf's recommendations?

You should aim to size your unit for 100% load at about -5/-8. At this size the unit will have enough running hrs left in the day to cover any shortfalls if it gets colder but be mostly on point for the majority of winter weather

SAP

We have had 1 issue with SAP where the co-efficient of heat loss plus a multiplier appears to be at odds with the heat loss calculated via other means (not incl any additional percentages added on for say an ASHP application to account for drop offs at lower ambients).

Yesterday in a meeting with a large UK housebuilder making it's first tentative steps into the renewables world, they advised that they too had experienced issues where SAP was coming up with inaccuracies, if compared with a more traditional heat loss calc tool. Apparently the Peabody Trust commisisoned a report to support this via Malcolm Bell at Leeds Met, I am trying to get hold of a copy.

I just wondered whether anyone else had experienced this or had a view and also what tools people use to calc heat loss.

Cheers

jon

[Bigfreeze]

About 8 yrs ago MF

[Bigfreeze]

http://www.idm-energie.at/en/hgl-technology.html

[mad fridgie]

Cheers BF, I have patent on something similar "SFT", but 100% potable water (did a similar application as shown about 15 years ago)
re thermal buffers, are we talking efficiency reduced power or reduced power price.
You either need a very massive tank and/or higher than normal temperatures. Most would opt for a combination a large tank with hot water in, great for an element, but why drive a heat pump to higher temp than need be.Plus the larger the surface area of your tank the greater the loss, how ever if this is tank is in your heated envelope, than the losses are not losses (same for all buffers)
The only way to look at thermal storage correct is uses of phase change materials (PCMs), and I would suggest that you look at different melting points for heating and DWH,
As far as I am concerned storage of DWH is to be seperated from the house heating. Having static coils in tank is just stupid, The water in the cylinder, and being passed through the heat pump should be the same ( We have to use vented double barrier heat exchangers to comply to regs)
Temperature control, and sizing does come down to the house, client requirement and expextations, and price limitations. (I would not use a rule of thumb here)
One point in sizing, if you are going to size a load correctly, then makes sure that your heat pump is sized correctly to meet your designed conditions. BF stated -5 to -8C, at these conditions your nominal duty, ASHP will have no relevence, you will need to allow for defrost (derating performance) and if temp compensated at these low ambient the heated water will likely be at it highest (exclude DHW at this point)

[MikeHolm]

This is a very good post as it condenses a lot of our previous conversation over many other posts down to the nub. Good one Jon.

An interesting question about the patents: If the HP is sold without the buffer, and the buffer is installed as an onsite assembly by a contractor along with all the other variations one house would have over another, will that infringe on either Mads or IDMs patents? I ask because, in Canada, the buffers are sold as pre-made tanks that are used as DHW buffers for boilers in commercial applications, not specifically made for the application. It may be a question for a patent lawyer but where does "packaged product" and "site assembly" meet?

Another question about PCMs. I was investigating the development of a unpressurized tank similar to Rotex or Latento with phase change materials so I am quite familiar with some of the products available (as of 4-5 years ago). Mad, have you actually used some of them and from whom did buy them? When I wanted to buy them at the time, either I went to the wrong places or I needed to purchase huge volumes to get them to listen to me.

[Jon Glanfield]

On our GSHP products the manuf instructions are always to have DHW set as priority, i.e. not timed, cyl reheats when the contents drop to a preset temp. However on our ASHPs the standing instruction is to time the DHW production to keep efficiencies high.

If you have a decent hot water tank this should not be a consideration. A good tank will only lose 1C over 24hrs. Ideally the tank would be on a 4-5C differential

What I am thinking about though are systems we have installed already and how to max efficiency for users retrospectively. I am unsure of whether a conventional UV cyl with oversized HP coil would be more efficient on a timed set up or left on priority demand, presumably by the 4-5c differential BF you mean between set point and HP reheat?

WC is the way to go once you have a good wc control system. Don't just use an add on without fully testing it. It could impair the system if its not up to scratch

All the systems we have installed employ on board WC, but again to enhance them I am thinking of linking the controls via an external stat, that switches power on when ambient is at 18c for example. Internals are left alone at desired internal set point and HP trickles over on its own WC to bank heat for colder drops.

Excuse my ignorance but what are PCM's?

Has anyone had any experience at all with CO2 probes or other less conventional extraction systems than traditional brine/glycol or indeed helix systems and radial drilling that do use brine/glycol mixes?

Jon

[Bigfreeze]

On our GSHP products the manuf instructions are always to have DHW set as priority, i.e. not timed, cyl reheats when the contents drop to a preset temp. However on our ASHPs the standing instruction is to time the DHW production to keep efficiencies high.

If you have a decent hot water tank this should not be a consideration. A good tank will only lose 1C over 24hrs. Ideally the tank would be on a 4-5C differential

What I am thinking about though are systems we have installed already and how to max efficiency for users retrospectively. I am unsure of whether a conventional UV cyl with oversized HP coil would be more efficient on a timed set up or left on priority demand, presumably by the 4-5c differential BF you mean between set point and HP reheat?

WC is the way to go once you have a good wc control system. Don't just use an add on without fully testing it. It could impair the system if its not up to scratch

All the systems we have installed employ on board WC, but again to enhance them I am thinking of linking the controls via an external stat, that switches power on when ambient is at 18c for example. Internals are left alone at desired internal set point and HP trickles over on its own WC to bank heat for colder drops.

Excuse my ignorance but what are PCM's?

Has anyone had any experience at all with CO2 probes or other less conventional extraction systems than traditional brine/glycol or indeed helix systems and radial drilling that do use brine/glycol mixes?

Jon

Yes the differential would be between set temp and cut in temp. Tbh unless you are running an immersion to back up the hot water temp it won't make alot of difference running it one way or the other. I wouldn't bother getting into the minutea of a design to that depth. The end result would be less than 5% per year in terms of running costs. Set up the system to suit the customers way of life.

Regarding the wc, unless the system is dictating a particular set point temp that depends on the actual outside temp at that time its not weather compensation. It should have a limit point, at which (outside temp), it would cease to heat the house.

We've installed CO2 probes, dx systems, glycol systems, air to water and water to water. Haven't worked on radial drilling yet, but there is little difference apart from alot more work connecting loops and balancing.

[MikeHolm]

Jon, PCM is phase change material. It is being used more and more in heat storage and works the same as any other refrigerant except that it comes in a solid form, typically small beads of wax which will go liquid at specific temperatures and thus absorb or release heat. The rule of thumb with available PCMs is that you can store 2.5-3 times the quantity of heat in a vessel over hot water. If memory serves 1m3 of water will store about 58kw of heat and therefore with a PCM you should get 120-170kw.

They are being used in walls where the solar gain will change the materials state and release it into the house at night, and many other applications.

[MikeHolm]

The solar industry is very interested in PCMs because we have a limited amount of sun in the winter and if we can store it effectively we can reduce or even eliminate the use of back-up heating. This is still some time away but they can be used quite effectively with HPs for storage.

[Bigfreeze]

Exremely expensive though, so I wouldn't be considering them as an option as a buffer for some time yet

[MikeHolm]

Have you checked out some of the companies who are using it such as Latento?

[MikeHolm]

For price, that is. Their tank with and without PCM?

[Bigfreeze]

Ya, Latento tanks are twice the price of a rotex tank in this part of the world, if not more

[MikeHolm]

For comparison you can buy the latento with and without PCM but I don't remember the price difference. That would be an accurate comparison.

[Bigfreeze]

That I don't know but with the PCM the price is extortionate and its only in the top one fifth of the tank.

There are other buffers that are full fill with PCM but they're a totally crazy price.

[MikeHolm]

Ya, I know but I was hoping it would have come down in price. It's only parafin and a few odds and sods

[mad fridgie]

PCMs, you can look at hydrated salts (the chemistry is not feild) I think the issue with price is about volume. I always like the idea of embedding PCM in the concrete.

[MikeHolm]

I know that BASF has made something in board form but I could never get any pricing on it. The hydrated salts sounds interesting so I will have to look into it. Putting it into a concrete slab, I suspect, is a bit tricky as it will be difficult to release the heat in a controlled fashion. I would rather find something to go in a tank and change phase at 35-40C.

[edgraff]

this is a very good thread, i have another question or point, is there a limit on TOO large of condenser or evaporator

[heat pump ac]

Can anyone advise on how to size an instantaneous hot water cylinder for an asap. Also in some cases there will not be the space for both a buffer tank and an instantaneous hot water cylinder, would you recommend combining the two into one cylinder?

[Bigfreeze]

With an instantaneous cylinder, the whole point is too only heat the water to 50C. Therefore the HP can do all the work and no backup heater is required. You will not be mixing as much cold water through the hot in this case so you will need a larger cylinder. A 500L cylinder will generally give you about 320L of "useable" hot water. This is about the size you would need for your average family home.

I wouldn't recommend combining the cylinder as this will reduce your COP quite a bit as you will be constantly heating to 50C. Some cylinders use a baffle to sort of seperate the two waters but I don't think they work all that well.

[heat pump ac]

Thanks Big Freeze for the info.
I have a GSHP project which i am designing at the moment.
The heat pump is a Stiebel Eltron 22KW, 500L DHW cylinder and an 700L buffer. They also do the combined cylinder/buffer with the baffle you mention and i was considering using this in 1000L size. I am now considering another option of a 700L buffer and a 800L instantaneous water cylinder would you agree or what would you suggest?

[Bigfreeze]

Thanks Big Freeze for the info.
I have a GSHP project which i am designing at the moment.
The heat pump is a Stiebel Eltron 22KW, 500L DHW cylinder and an 700L buffer. They also do the combined cylinder/buffer with the baffle you mention and i was considering using this in 1000L size. I am now considering another option of a 700L buffer and a 800L instantaneous water cylinder would you agree or what would you suggest?

It will very much depend on a number of factors

How many people in the house?

Any large jacuzzi baths, high output showers etc?

Is it UFH?

Are you using stats throughout the house?

[MikeHolm]

HP AC, just curious but how much is an 800L or 1000L buffer tank cost. We have regulations here in Canada that make them extremely expensive but I would like a comparison.

[heat pump ac]

The house is fully zoned UFH
4/5 bathrooms all with showers, i would allow for one large bath and one drencher style shower.
Its a4/5 bed house with occupancy of 4 adults and 2 kids

The 1000l buffer we buy for around £1500 + VAT

[MikeHolm]

It would be double that price here. Stupid regulations that haven't changed since the 20s


Thats a lot of potential water flow. Almost sounds like a B&B.

[edgraff]

any ideas, think i could stage compressors to work a rad and floor heat system

[Bigfreeze]

I would go with an 800L cylinder. Possibly your biggest concern will be the max full rate through the hx. Most setups are capable of between 22 - 28L per minute. In this case you may need to run a cascade or dual hx setup.

Regarding the buffer, 22kw x 30L = 660L, so I would probably go with a 800L cylinder here too. Is there no possibilty of removing the stats and running wc, so eliminating the buffer

[heat pump ac]

The client has agreed to go ahead with no stats, however he is now asking us to price for an ashp option as the test drilling he has done has indicated that the drilling will be very difficult.

At the moment the only offering we could give him is 2 x 16kw Daikin splits, buffer tank and instantaneous cylinder. Would you recommend another manufacturer and if so do you have any idea how they compare price wise with the daikin units?

[Bigfreeze]

You won't beat Daikin on price, they're built on that premise. You'll be looking at double the cost for a good system about 32kw. Does this guy realise the cost savings year on year a GSHP will have over an ASHP, especially Daikin, on that size of house. is he limited for space or why has a horizontal system been ruled out?
I'd exhaust every avenue before going ASHP.

If I was you I wouldn't even entertain the idea of heating a house that size with Daikin units. At 22kw its a fairly substantial building or its poorly insulated, but I'd assume the former. Try explaining to a guy like that next winter why his trophy house won't get past 18C.

[heat pump ac]

BG i appreciate your comments, ASHP is no longer an option! The client is going to consider a horizontal collector. Many Thanks