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be capable of thinking out of the box) said this, I would not trust either of the others, who are either grant chasers, outdated in their thinking, or boiler fitters who haven't bothered to understand heat pumps properly, or a combination of the above.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 yesterday 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 yesterday 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 yesterday 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 yesterday 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.