heat pump design

[jit]

i am electrical engineer , wanting to understand heat pump schematic and how system perform.

i am in the process off having my heat pump installed. i had 3 heat pump engineers turning up with all different design.

 

the house is fully insulated . 165m2 heating space and 7kw heat lost. 300l new cylinder (not 3m coil). ground floor UFH, 1st floor radiator circuit, 2nd floor radiator circuit (loft)

 

from my reseach and what i understand. i want to install 7kw vaillant heat pump, open loop and no buffer/volumiser.

engineer1 said. install bufffer. and new tank

engineer 2. install bufffer. and new tank + zone radiator circuits

engineer 3. suggest open loop system with no buffer and use existing cylinder with plate heat exchanger .

 

from my research and what i understand . i want to avoid buffer and have open loop installation with balancing valve to control flow. i am not sure if i should plate heat exchange on existing cylinder as it 10 people and 3 bath. and do i need volumizer as their is plenty of volume being 3 circuit.

 

i have boiler room outside and all the pipework run separate . the pump can be installed on same wall.

what would be the good and high efficiency set up?

[ProDave]

If you have 7kW heat loss then a 7kW heat pump will be no good.  At the coldest time (when the heat loss really is 7kW) the heat pump will have to be heating the house 24/7 and will not have time to heat any hot water.

 

So I would suggest minimum of 10kW heat pump.

[JohnMo]

2 hours ago, jit said:

10 people

Your 300L just isn't big enough for that at heat pump temperatures. With 10 people in the house DHW will be the dominant demand for 95% of the year. Are your quote companies aware you have 10 people in the house?

 

Ideally you would size radiators to flow same temperature as the UFH and have a single flow temp and no mixers.

2 hours ago, jit said:

with balancing valve to control flow

Not sure what this is?

[JamesPa]

Agree with @johnmos comments,  also (Im shocked he didn't say this), NO buffer, 2 port volumiser in the flow if you need it.  Operate space heating open loop, do not zone radiators.  Since only engineer 3 (who appears to be capable of thinking out of the box) said this, I would not trust either of the others, who are either grant chasers, outdated in their thinking, or boiler fitters who haven't bothered to understand heat pumps properly, or a combination of the above.  Sadly there appear still to be loads of these people around who between them risk being responsible for the destruction of the industry

 

@dilsue is also right in his general comment but (a) some '7kW' heat pumps will do more than 7kW and (b) based on what you say about the house Im doubtful heat loss is really as much as 7kW, whats the age and construction and (c) you don't want to go too high otherwise you will compromise efficiency.  

 

That said you do need to calculate and take into account your DHW consumption with that many people living in the house.  As @johnmo says you may well need >300l unless you space usage out.  its an unusual requirement so many installers wont take it into account; I would strongly recommend you do some sums on this and understand how its phased through the day,

 

Plate loading an existing (unvented?)  tank is certainly a sensible retrofit option if its large enough for your requirement.  Bear in mind with an ASHP you typically heat the DHW only to 50C to maximise efficiency.  That said, with a modern R290 heat pump there is nothing actually stopping you heating even to ~70C albeit COP for the DHW wont be as good (mine gets to 70 during the weekly legionella cycle which is done by the heat pump alone).  If you have a low tariff period you could do it then and top up later, it would still be cheap and the payback period for fitting a new, larger tank just so you can operate at a lower temperature is probably rather long.  Again this comes back to understanding your DHW requirement.

 

Engineer 3 may just be spot on with his/her proposal, particularly of the '7kW' pump he would fit is actually a bit more than 7kW under the conditions you will be operating at and you are qilling to be a bit flexible in your thinking, but I cant stress enough that you do need to work out your DHW requirements.

 

Edited Wednesday at 14:08 by JamesPa

[SteamyTea]

Using 125lt of 40°C DHW, for 10 people, that is 45 kWh.day^-1.

If the HP can deliver at a reliable 6 kW for most of the year, then it will be just heating water for 7.5 hours each day.

That, because of the relatively small size of the cylinder, will need to be recharged a few times each day, so very little chance of using a time of use electricity tariff.

 

165 m² is not a large house, and with 10 sweaty bodies in it, not much space heating will be needed.

If you are not too worried about looks and a slight bit of internal noise, then consider A2AHPs, they are the cheapest to fit, run, and give a fast response, and cooling if needed.

Then get a separate heat pump and more cylinders for DHW needed.

Make sure your incoming supply can run multiple heat pumps, and consider a bit of PV if suitable.

Edited Wednesday at 15:30 by SteamyTea

[JamesPa]

19 minutes ago, SteamyTea said:

Using 125lt of 40°C DHW, for 10 people,  that is 45 kWh.day^-1.

 

... and thats why OP needs to work out their DHW requirement properly.  125l/person is a lot, unless they each have long showers every day which of course is perfectly plausible but not necessarily the case!.  Does the existing 300l cylinder with the existing heating system suffice and if so how often does it get reheated?  This is definitely an 'edge case'!

 

 

Edited Wednesday at 15:47 by JamesPa

[JohnMo]

7 hours ago, jit said:

10 people

How do you stuff 10 people into 165m²? Are they all on camp beds?

[SteamyTea]

11 minutes ago, JamesPa said:

25l/person is a lot,

About what I use, but then I am self indulgent and have a bath every day.

Have I mentioned that the water and waste down here is the most expensive in the UK.

3 years ago, the water cost more than the heating if it, and if I had a heat pump, or a gas/diesel boiler, rather than E7, then it would still be more expensive.

[JamesPa]

3 minutes ago, SteamyTea said:

if I had a heat pump, or a gas/diesel boiler, rather than E7

If you had both a heat pump and E7 it would be cheaper than E7 alone, by a factor of 2-3.  Night time water heating (with E7 or similar) on a heat pump is nearly free, even if the COP isn't the best.

 

(PS I assume you mean 125l not 25l, otherwise its just a footbath)

Edited Wednesday at 15:58 by JamesPa

[SteamyTea]

5 minutes ago, JamesPa said:

PS I assume you mean 125l not 25l, otherwise its just a footbath

Yes.

Been steaming large posters off walks all morning and my fingers are still strange feeling.

125 lt is only a ten minute shower for many people.

[jit]

thank you for feedback everyone

 

currently 300l i have to run 1 and half cycle in morning and 1 cycle in afternoon is enough water for bathing and cooking.

 

165m2 is heating space . kitchen, hallway on 1st floor and loft is excluded as there no radiator and its in center of all the rooms. 200-210m2 is actual space. the house 200mm solid wall but has external wall insulation 100mm all around old walls. loft is 150mm solid insulation . all windows aur new double glassed and 3 glazed extension. i had heat loss done by 2 separate companies . room by room and editing exact material and size. they both come up with heat loss 6.98-7kw. 

 

my aim is to run 2 cycle in morning as night terrif cheaper (7p) and 1 cycle in afternoon .also i am only using half the energy my solar is producing atm. 

 

1 of the engineer made comment about heat pump not having enough time for heating if its working on hot water twice in morning. as 300l would take 90min estimate time heat up at 45degree. i don't mind installing 500l cylinder if it required but if plate heat exchanger resolve issue then i rather option for that as the cylinder is only 3month old .

 

 

most of the engineer suggesting new cylinder and their not much information out to compare 'new cylinder vs small coil cylinder with plate heat exchange. i can always get new cylinder and sale old cylinder to builder for 20-30% discounted price.

 

space is not issue and installation is very straight forward. i want to select engineer who know what they doing but it has to be in my budget. pipe work from cylinder to heat pump and consumer unit is only 2m max. no radiator replacement. still costing £10k. is this fair price?

[JohnMo]

24 minutes ago, jit said:

still costing £10k. is this fair price

If that price after grant, a rip off from what you say. 

 

25 minutes ago, jit said:

1 of the engineer made comment about heat pump not having enough time for heating if its working on hot water twice in morning

That is a fair comment - add another DHW heat in the afternoon, it will have lost the battle and you will have a cold house and large energy draw at the end.

 

You need to size ASHP based on how many hours you are heating DHW (3x 90 mins), so 4.5 hours. Take that away from 24 hrs. So you now have 19.5 hours to provide 24 hrs of heating. So (7kW x 24 hrs) / 19.5 hours, so heat pump size minimum is 8.6kW at your design outside temperature. So realistically you are looking at a 9 or 10kW heat pump.

[jit]

1 minute ago, JohnMo said:

If that price after grant, a rip off from what you say. 

 

£2500 after grant. when actual material cost is less then £5k as no new cylinder. 

 

if plate heat exchange is added to new cylinder. should this resolve issue? as plat heat exchange known to improve efficiency or i should get 500l tank . 7kw vaillant output 10kw at -2. this should still be ok or i should not consider output?

[-rick-]

Just thinking aloud, listen to others as to wether this is a good idea:

 

Given your DHW use might it be worth getting the second tank and keeping existing? The heat loss from the tanks will be a bit higher but it sounds like you'd use all the water quite quickly anyway.

 

If you were going with the PHE route for the existing tank, then the new tank could be a plain simple tank with no coil, these are much much cheaper. Alternatively, two tanks in parallel, the heating capacity of two coils might eliminate the need for the PHE.

 

Obviously only makes sense if you have the space.

[jit]

16 minutes ago, -rick- said:

Just thinking aloud, listen to others as to wether this is a good idea:

 

Given your DHW use might it be worth getting the second tank and keeping existing? The heat loss from the tanks will be a bit higher but it sounds like you'd use all the water quite quickly anyway.

 

If you were going with the PHE route for the existing tank, then the new tank could be a plain simple tank with no coil, these are much much cheaper. Alternatively, two tanks in parallel, the heating capacity of two coils might eliminate the need for the PHE.

 

Obviously only makes sense if you have the space.

no space for 2 tank. either i use exisiting 300ltr tank and PHE (hofuly reduce heating time and reduce cost.) or install 400-500lt new cylinder. and invest the PHE money on larger heat pump if 7kw does not meet heating and dhw load. (7kw to 10kw is £1000 more for heat pump cost) 

[JohnMo]

54 minutes ago, jit said:

plate heat exchange is added to new cylinder. should this resolve issue? as plat heat exchange known to improve efficiency or i should get 500l tank . 7kw vaillant output 10kw at -2. this should still be ok or i should not consider output?

You will now storing DHW at 50 or below, what do you currently store at? But plate heat exchanger would be able to heat cylinder, the current coil wouldn't be used any more.

 

You need to concider defrost cycling, this will bring down the 10kW quite a bit. There was some testing done on a Vaillant ASHP and the conclusion was the 7kW output in a UK setting was correct, the datasheet was ok for mid European location, where defrost isn't so much of an issue.

[JamesPa]

10 hours ago, jit said:

165m2 is heating space . kitchen, hallway on 1st floor and loft is excluded as there no radiator and its in center of all the rooms. 200-210m2 is actual space. the house 200mm solid wall but has external wall insulation 100mm all around old walls. loft is 150mm solid insulation . all windows aur new double glassed and 3 glazed extension. i had heat loss done by 2 separate companies . room by room and editing exact material and size. they both come up with heat loss 6.98-7kw. 

based on that additional info about the house I would withdraw my comment that your house may be (materially) less than 7kW.   It may well be a bit less but not enough to provide slack for your DHW at peak space heating load.  Even if you top the loft up to 300mm (which I definitely would) then still probably insufficient margin to run the DHW

 

10 hours ago, jit said:

my aim is to run 2 cycle in morning as night terrif cheaper (7p) and 1 cycle in afternoon .also i am only using half the energy my solar is producing atm. 

 

How much DHW do you currently use?

 

10 hours ago, jit said:

1 of the engineer made comment about heat pump not having enough time for heating if its working on hot water twice in morning. 

 

I agree given the additional info you have now provided about the house and your DHW intentions.  7kW Vaillant virtually certain to be too small to cope in the height of winter.  You have two options (1) go larger eg 10kW or (2) accept you use the Immersion when its (perhaps) 2C or below so the heat pump is used only for space heating at these times.  Even then it may be a bit marginal, but chances are you will get away with it unless there are several continuous days at -2 all day and night  (which did happen a couple of years ago).

 

re PHE vs new tank.  It really depends on the timing of your use.  If you can stand a reheat between uses then why not just add a PHE to existing tank.  If you cant tolerate a reheat then get a new cylinder, but as @-rick- says you could consider (if you go for new tank) plain tank plus PHE which may well be cheaper than a new large 'heat pump' cylinder.  Remember you can heat to >>50C if you want, but efficiency will suffer.  That said at 7p/kWh getting best efficiency doesn't really matter, in fact you are quite probably best maxing out the temperature you heat to in order to store more energy, thus reducing the consumption later in the day when leccy more expensive!

 

As to price - I got a 7kW Vaillant , 3 rads, 1 fancoil, 210l tank for 12.5k minus grant = 5k to me; South East England.     Two installers quoted that price, others were higher.  The two who quoted that price were the ones I would have chosen on quality grounds.

 

This excluded the following

 

• the wire (for the temp sensor) from the DHW tank to the heat pump, which I ran. 
• the power supply.  Second fix up to the external breaker was done in advance by a local sparks for <£200, after I had run the cable.
• exposing a void and making good, where the 28mm primaries were to run indoors - I did this myself

 

Edited 16 hours ago by JamesPa

[JamesPa]

One further thought to above is to consider a cylinder with integral heat pump.  They aren't that expensive and it would nicely separate the two systems allowing you to go for the 7kW for space heating.  I wouldn't normally suggest this but your extraordinary dhw requirement makes it worth considering imho.  

 

Not clear to me whether this is BUS compliant however.  It's clearly within the spirit but might require mental gymnastics to convince yourself it's within the letter!

Edited 9 hours ago by JamesPa

[JohnMo]

44 minutes ago, JamesPa said:

Not clear to me whether this is BUS compliant

It is, but heat pump cylinder or a normal cylinder has to sized per MCS guide lines. BUS just states heated by a heat pump, doesn't mean it can only be heated by one heat pump at the property. Only thing with integrated heat pump cylinders is they generally have a long recovery time, due to the dinky heat pump they use 

[torre]

I'd suggest installing a WWHRS to your showers if it's not too disruptive.

 

Waste water heat recovery saves about half the hot water needed to run the shower at the same temperature. That means your 300L tank will have an effective shower capacity of 450L and your running costs will also be lower as you won't be paying to heat a larger cylinder.

[jit]

9 hours ago, JamesPa said:

based on that additional info about the house I would withdraw my comment that your house may be (materially) less than 7kW.   It may well be a bit less but not enough to provide slack for your DHW at peak space heating load.  Even if you top the loft up to 300mm (which I definitely would) then still probably insufficient margin to run the DHW

its loft conversion . so 150mm pir insulation, warm deck 

9 hours ago, JamesPa said:

 

How much DHW do you currently use?

i predict 350-450ltr at estimate 55-65 degree temperature . it difficult to know exact volume as it store at different temperature and mixed with cold. but i know 1 full tank is not enough in morning so i run additional 30min

9 hours ago, JamesPa said:

 

I agree given the additional info you have now provided about the house and your DHW intentions.  7kW Vaillant virtually certain to be too small to cope in the height of winter.  You have two options (1) go larger eg 10kW or (2) accept you use the Immersion when its (perhaps) 2C or below so the heat pump is used only for space heating at these times.  Even then it may be a bit marginal, but chances are you will get away with it unless there are several continuous days at -2 all day and night  (which did happen a couple of years ago).

i am tilting toward 7kw as all the engineer suggest 7kw is sufficient . i also had doubt as well so i did my calculation through heat punk with heating and dhw load with heat lost and that suggested 7kw is 127%. and if it not sufficient it would only struggle for month max when temperature is 0 to -2 degree. i can always option to using immersion at 7p which is not high considering scop is low when it cold (i also have 10kw solar generation system) atm i can sell to grid but that might not be the case in future so i can potentially use extra generation in immersion . in result i can save £1000 for not going with 10kw pump. 

 

i think it might be good idea to go for larger cylinder. then decide to go heat pump ready or PHE . cost is similier to both as PHE is expensive .

 

if installation work out then i might not need to all the extra hack to meet demand. i am only turning heating on 45min in morning and 1hr in evening. it hold heating well.

 

 

[jit]

1 hour ago, JohnMo said:

It is, but heat pump cylinder or a normal cylinder has to sized per MCS guide lines. BUS just states heated by a heat pump, doesn't mean it can only be heated by one heat pump at the property. Only thing with integrated heat pump cylinders is they generally have a long recovery time, due to the dinky heat pump they use 

no point in investing into product if recovery time is slower.  every chance product to go faulty before recovery time and then i am having to spend the money for replacement 

[jit]

16 minutes ago, torre said:

I'd suggest installing a WWHRS to your showers if it's not too disruptive.

 

Waste water heat recovery saves about half the hot water needed to run the shower at the same temperature. That means your 300L tank will have an effective shower capacity of 450L and your running costs will also be lower as you won't be paying to heat a larger cylinder.

that gona require disruption. i just done full refurb last year. at the time i did not think about heat pump otherwise i would have done heating and plumbing according to that

[JohnMo]

2 hours ago, jit said:

might be good idea to go for larger cylinder. then decide to go heat pump ready or PHE . cost is similier to both as PHE is expensive .

If doing new cylinder, just do one with a big coil, less to go wrong, one less pump to run less for installer to do

[JamesPa]

3 hours ago, jit said:

tilting toward 7kw as all the engineer suggest 7kw is sufficient ... if it not sufficient it would only struggle for month max when temperature is 0 to -2 degree. i can always option to using immersion at 7p

TBH I would do the same but I am a bit of a taker of calculated risks and am prepared to work things out iteratively.  Many aren't.  This is not advice, just how I would approach it.

 

Just be aware 7hrs cheap rate on the immersion may not be enough so some may be at full price and full recovery is going to take a fair time.  It depends on how demanding your users are!  As you are an electrical engineer you can do the math without our help!

 

Just make sure you are not lumbered with buffer or unnecessary external controls.  Vaillant heat pumps come with all the controls you need other than manual (set and forget) balancing valves.  If your chosen engineer insists, choose another!

Edited 3 hours ago by JamesPa