Small ASHPs / Units primarily for DHW

[Conor]

Had previously convinced myself on the mains gas boiler route, but the inescapable truth is that it is a 100% fossile fueled combustion device...

 

As we're building passive, head demand is quite small (PHPP attached) and I'm only putting three UFH loops - just wet rooms on the three floors.

 

So our primary demand will be DHW. I've read briefly about ASHPs with COPs tailored to hot water production. 

 

Any suggestions and thought prices for supply? Only quote I've got was a Nilan compact P close to £10k.

 

I've previously been dismissive and a little ignorant of the tech... Please educate me!

 

[PeterW]

Don’t do it....

 

Go with E7, heat the tank to 45c with ASHP to maintain CoP above 2.5, and then top to 62c with E7 immersions. 

 

Time of the top up is about 75 mins at 3Kw for a 300 litre tank, so you’re 3.75KwH or 20p ish to get a lot of hot water. 

 

For the sort of input you need, a 4kw Mitsubishi would be change of £3.5k installed as you don’t need MCS at those levels for RHI. 

 

The Nilan unit is similar to the Genvex units and includes MVHR so not a good example. 

[Jeremy Harris]

What's your hot water demand likely to be?

 

If it's not too demanding, then one of the combined MVHR/water heating solutions might work.  The Genvex Combi 185 is a pretty good solution for a modest hot water demand: https://www.genvex.com/en/products/air-ventilation---air-heat-pump/combi-185-bp  @PeterStarck has one, and has found it works well for them, I believe.  It's expensive if purchased from the UK sales outlet, but can be purchased from a Danish company for a very much better price.

 

 

[Conor]

20 minutes ago, JSHarris said:

What's your hot water demand likely to be?

 

If it's not too demanding, then one of the combined MVHR/water heating solutions might work.  The Genvex Combi 185 is a pretty good solution for a modest hot water demand: https://www.genvex.com/en/products/air-ventilation---air-heat-pump/combi-185-bp  @PeterStarck has one, and has found it works well for them, I believe.  It's expensive if purchased from the UK sales outlet, but can be purchased from a Danish company for a very much better price.

 

 

 

HW demand will be low... Just two of us, I work away from home 3 days a week, partner often showers at the gym.

 

That's the kind of thing I'm thinking of... All in one, simple. Think the simplified install will make it cost effective compared to gas.

[redtop]

24 minutes ago, JSHarris said:

What's your hot water demand likely to be?

 

If it's not too demanding, then one of the combined MVHR/water heating solutions might work.  The Genvex Combi 185 is a pretty good solution for a modest hot water demand: https://www.genvex.com/en/products/air-ventilation---air-heat-pump/combi-185-bp  @PeterStarck has one, and has found it works well for them, I believe.  It's expensive if purchased from the UK sales outlet, but can be purchased from a Danish company for a very much better price.

 

 

Just looking at this product. so its getting the MVHR to do the hot water, leaving ASHP to just do UFH, is that right?

[Jeremy Harris]

Just now, redtop said:

Just looking at this product. so its getting the MVHR to do the hot water, leaving ASHP to just do UFH, is that right?

 

 

Yes, that's right.  Because the exhaust air heat pump in the Genvex unit (there are other manufacturers of these) uses warmer air that's been exhausted from the house, it can deliver a higher flow temperature than an ASHP that's using cooler outside air.  This means the COP is higher when delivering hot water, so saving electricity.

 

It also means that an ASHP for heating can run at a lower flow temperature, probably no higher than around 35° to 40°C, and so also operate with a higher COP than if it were asked to deliver hot water as well.

 

Both are pretty simple to install.  A monobloc ASHP is very easy to fit, just flow and return pipes to the heating, a power cable and a control cable.  Similarly, a combined MVHR and hot water unit is pretty simple, just the normal ventilation duct connections, plus cold water in, hot water out, a drain for the pressure relief valve and power and control cables.

[redtop]

thanks, so that answers my question; its a more efficient use of the ASHP / MVHR combo. Is adding cooling to the ASHP an efficient approach (I realise cooling is power hungry but you know what i mean ? ) and is it simple or are we getting into 'plant room' territory

[Jeremy Harris]

11 minutes ago, redtop said:

thanks, so that answers my question; its a more efficient use of the ASHP / MVHR combo. Is adding cooling to the ASHP an efficient approach (I realise cooling is power hungry but you know what i mean ? ) and is it simple or are we getting into 'plant room' territory

 

We find that floor cooling, just by switching the ASHP to cooling mode and turning on the UFH, works very well where we have UFH, which is downstairs only.  It is a bit slow to react, as it takes a few hours to cool the slab down, but the advantage of that is that we can pretty much get away with cooling the slab only when there's enough sunshine to make doing so free.

 

I wish I'd fitted a couple of fan coil units to the UFH manifold, as that would have given us air cooling upstairs and saved having to fit the air conditioning unit.  Retrofitting fan coil units to our system now would have been a great deal more work than fitting the stand alone air conditioning unit.  It's one of those things I overlooked when modelling the house systems, as I just failed to appreciate how important hot weather cooling was going to be.

[Gone West]

1 hour ago, Conor said:

 

HW demand will be low... Just two of us, I work away from home 3 days a week, partner often showers at the gym.

 

That's the kind of thing I'm thinking of... All in one, simple. Think the simplified install will make it cost effective compared to gas.

We use the Genvex Combi 185LS to heat the DHW to 45C which seems to be very efficient. There are only two of us but being retired we are at home most of the time. As Jeremy said we bought ours from Denmark and IIRC saved over £3000 and still reclaimed the higher VAT rate. The only downside to that is,  if there is a fault with it, the supplier wants it returned to Denmark for repair. Of course there maybe the Brexit effect after October.

[ragg987]

On 25/07/2019 at 10:29, Conor said:

As we're building passive, head demand is quite small (PHPP attached) and I'm only putting three UFH loops - just wet rooms on the three floors

I may be misunderstanding this - are you saying that the rest of the floorspace (apart from your wetrooms) will have no UFH? How are you going to heat the house? Your PHPP shows you still need the ability to input 2.7kW of heat (11W/m2 x 242m2).

 

And do check your assumptions about primary demand being DHW. That may well be the case in warmer months, but in winter your primary demand may be space heating. The problem with average values is you miss the reality.

 

I would keep it simple and get an ASHP for both DHW and space heating (with UFH - helps with floor cooling too). Minimise your capital spend as your fuel costs are going to be relatively small, so a small saving on a relatively small cost is easily swallowed up by excessive capital (read complex) installation.

[ragg987]

On 25/07/2019 at 10:39, PeterW said:

heat the tank to 45c with ASHP to maintain CoP above 2.5

We heat our DHW to 50C using ASHP. Our average COP (DHW and space heating) is around 4. I don't bother with electrical heating to 60C, the immersun will divert solar PV to provide a boost. Admittedly it means we hit over 60C in spring, summer, autumn but may not in winter.

[Conor]

4 hours ago, ragg987 said:

I may be misunderstanding this - are you saying that the rest of the floorspace (apart from your wetrooms) will have no UFH? How are you going to heat the house? Your PHPP shows you still need the ability to input 2.7kW of heat (11W/m2 x 242m2).

 

And do check your assumptions about primary demand being DHW. That may well be the case in warmer months, but in winter your primary demand may be space heating. The problem with average values is you miss the reality.

 

I would keep it simple and get an ASHP for both DHW and space heating (with UFH - helps with floor cooling too). Minimise your capital spend as your fuel costs are going to be relatively small, so a small saving on a relatively small cost is easily swallowed up by excessive capital (read complex) installation.

2.7kw is tiny is it not? That's a couple electric heaters. We've a total of 4 bathrooms, wardrobe room and a plant/laundry room that would have ufh. . Other half is also (despite objections) insisting on a wood burning stove!

 

I'm sorely tempted to make the house "ASHP ready" (UFH loops) and just rely on E7 and solar for hot water and the stove and a couple storage heaters for heating for the first year. A gas boiler installation will be at least £2500 and an ASHP will be double. Heard so many stories from people with passive houses installing heating systems and never using them. We're on a tight budget and £5k saving is significant.

[ProDave]

8 minutes ago, Conor said:

2.7kw is tiny is it not? That's a couple electric heaters. We've a total of 4 bathrooms, wardrobe room and a plant/laundry room that would have ufh. . Other half is also (despite objections) insisting on a wood burning stove!

 

I'm sorely tempted to make the house "ASHP ready" (UFH loops) and just rely on E7 and solar for hot water and the stove and a couple storage heaters for heating for the first year. A gas boiler installation will be at least £2500 and an ASHP will be double. Heard so many stories from people with passive houses installing heating systems and never using them. We're on a tight budget and £5k saving is significant.

Our total space heating requirement is a little over 2KW when +20 inside and -10 outside.

 

Heating bill for last winter £234 for heating via a 5KW ASHP and UFH only downstairs.  If I had done that just with an electric fire, the cost would have been more like £750 so the ASHP is saving us £500 per year in heating bills. Considering it cost less than £1000 I would say it is well worth it.  

[PeterW]

40 minutes ago, Conor said:

2.7kw is tiny is it not? That's a couple electric heaters. We've a total of 4 bathrooms, wardrobe room and a plant/laundry room that would have ufh. . Other half is also (despite objections) insisting on a wood burning stove!

 

I'm sorely tempted to make the house "ASHP ready" (UFH loops) and just rely on E7 and solar for hot water and the stove and a couple storage heaters for heating for the first year. A gas boiler installation will be at least £2500 and an ASHP will be double. Heard so many stories from people with passive houses installing heating systems and never using them. We're on a tight budget and £5k saving is significant.

 

So use the Willis heater solution that @TerryE has - less than £100 in bits and dead easy to install. 

[ragg987]

2 hours ago, Conor said:

2.7kw is tiny is it not?

Yes, but not abnormally so for a PH standard build, and you still need to provide heat input else your room temperature will drop. As long as you have allowed for that you should be fine. Ufh in bathrooms will not redistribute the heat to the rest of the house in any meaningful way.

 

2.7kW X 24hrs is over 60kWh, costing £9 per day of direct electric heating. Yes this is on a cold day and will be less on average during the heating periods, but are you also heating water with electric direct?

 

You have 2 options, direct electric heating or gas boiler / ASHP. Low capital + high running costs vs. high capital + low running costs. If direct electric you can save money by omitting the ufh and fitting electric emitters or heating element in mvhr.

 

You have to do the maths, I'm afraid, and decide based on your priority.

[joe90]

The other thing is, house value if you sell/have to sell, the majority of people would balk at a house with no heating, at least get UFH pipes in if nothing else, worth the cost, then you can instal a Willis heater or ASHP at a later date if you require it.

[joth]

On 25/07/2019 at 10:39, PeterW said:

 

For the sort of input you need, a 4kw Mitsubishi would be change of £3.5k installed as you don’t need MCS at those levels for RHI. 

 

Be aware that an MCS installation is (now?) a requirement of installing heat pumps under permitted development rights

https://www.planningportal.co.uk/info/200130/common_projects/27/heat_pumps/2

 

If you already have an ASHP on your PP then nothing to see here. 

[Conor]

Thanks everybody. I think a 4-5kw ASHP and 180l direct hot water cylinder is the way to go. Run it off PV during summer and E7 in winter.

 

Slightly related, I've been browsing for PV and battery systems... I've noticed prices have dropped significantly since I last looked a few months ago. Is this because of the end of the feed in tariffs in England?

[SteamyTea]

17 minutes ago, joth said:

Be aware that an MCS installation is (now?) a requirement of installing heat pumps under permitted development rights

Or can you just follow there rules and claim to be a competent person.

12 minutes ago, Conor said:

end of the feed in tariffs in England?

More probably global prices dropping.

[joth]

2 hours ago, SteamyTea said:

Or can you just follow there rules and claim to be a competent person.

 

oh yes, thank you for this.  Yes that's the distinction - this one does say the installation needs to comply with the MCS planning standards, not that it needs to have been done by an MCS certified installer.

The standard mostly seem to be measuring and documenting the compliance with noise limits.

[SteamyTea]

Long time since I had to do MCS RHI compliance, but there used to be a rule that you had to work out total heat loss and make sure the heat pump could supply that 99% of the time.

Was not difficult as you have all the info anyway.

[TerryE]

@Conor, as @ragg987 says, the heating costs do mount up.  We have a reasonably large 3-storey 4-bedroom (5 really because we have one room in the loft which is a general store room / drying room / dressing room for my son.) and as @PeterW says we heat this with a single Wills heater which runs in the E7 slot, though I do top this up with a little 1kW oil rad in my office on the first floor running in an E7 slot for the coldest month or so.  This means that all of our heating is at E7 cheap rate.  We will be getting an ASHP some time (we've got all the piping in place) but probably next summer, but this will be a self install as I can't make the payback numbers work if I pay someone to do it.

 

I just don't see the point of MCS myself.  The upfront markup you pay to for an MCS certificated installer is difficult to recover, IMO.

 

One last thing: Don't bother with the woodstove.  They make it a pig to seal the house and unbalance the MVHR and a passive compliant one costs a fortune.  You'll never use it.  Ask you wife when you last lit a fire between June-Aug, because if you've designed your house right that's how your house feels all the year round. 

 

We have enough problems with 6 people sitting socialising in one room as these warm bodies are enough for the temperature to start to rise, and we couldn't have the 1kW heater on with the room door closed for the same reason.  The minimum that a ticking over woodstove will emit is 2 kW and some go up to 6kW!  There have been a couple of topics on ethanol burners which give the aesthetic effect of a real woodstove without cooking the occupants.

[T M]

Regarding the discusion around ventilation heat pumps specifically the Genvex Combi 185.

 

I was initially quite enthousiastic about the compact simple solution of a ventilation heat pump.

After modelling the energy consumption it seems to be less efficient than I originally hoped (see attached summary)

 

The house I am building is an apartment for two people with a floor area of 70m2 in the Netherlands (similar climate to England) 

Insulation is relatively high: Floors: Rc 4,8 m2K/W   Walls: Rc 6,1 m2K/W   Roof: Rc 8,2 m2K/W   Windows: U-value   <1.0 [W/m2K]  

After a heat loss calculation was made by a professional it was determined the annual heat consumption would be approximately 5280kWh/year

Heat transmission loss: 28W/m2     Ventilation loss: 16W/m2    Absorption loss: 14W/m2    

 

Essentially the conclusion was the ventilation heat pump would rely on the back up immersion heater which has a COP of 1 far too much in the cold months to heat the air and hotwater cylinder. 

This basically translates into an operational cost of about 2-3 times higher than a split air to air or air to water heat pump.

If your electricity cost is reletively low then this is less of an issue, if it is high such as where I live (around 50 cents/kWh) then the operational cost is an important factor to consider. 

 

Attached is the annalysis that I made of the Genvex Combi 185 that perhaps someone considering a ventialtion heat pump might find useful.

Similar values would also apply to the Nibe F470 or Nilan Compact S as they all similar in performance and function. 

 

Installation conclusion based on total investment and operating cost (assuming you have space for a split outdoor unit) :

Economical option:  heating and cooling with a 5kWh mini split (Mitsubishi) and using a 120L electric water boiler for domestic hot water combined with a shower drain heat exchanger powered with 4 x 400wp solar panels. 

Comfortable option: (about twice the cost) was to use a Air to water split heat pump and integrated 190L water boiler (such as the Vaillant aroTHERM + unitower heat pump) combined with low temperature floor heating, shower drain heat exchanger and 4 x 400wp solar panels. 

 

Edited July 20, 2023 by T M

[Beelbeebub]

I thought CO2 heatpumps were supposed to be good for situations where the majority of the demand is DHW.

 

I believe they have been used for quite a while in Asia where there is no heating demand.

 

AFAIK the only unit readily available in europe is a mitsubishi one

 

https://les.mitsubishielectric.co.uk/products/residential-heating/outdoor/ecodan-quhz-monobloc-air-source-heat-pump

[T M]

Hi thanks for the link @Beelbeebub, It looks like a good system. 

 

As mentioned above my advisor specified the Vaillant Arotherm plus which is very simialr to the Mitsubishi Ecodan you referenced.

Both are good options and both cost roughly the same price and are similar in performance and noise levels. 

https://www.vaillant.co.uk/homeowners/products/the-arotherm-plus-air-to-water-heat-pump-71360.html

 

I was also trying to compare & consider the investment and operating cost of each system to make the comparison more clear to someone new to this topic.

 

1x Cost = Air to air split heat pump + Electric water heating cylinder  + Solar panels  would have a slightly higher operating for normal hot water usage down side is it has indoor & outdoor units.

2x Cost = Air to water split heat pump + floor heating is more comfortable and only a slightly lower operating cost for noramal hot water usage the down side is it has an outdoor unit

2x Cost = Air to Air ventilation heat pump + floor heating the operating cost would be about 2.5 times the cost of a similar air to water split system but has the benefit of no outdoor unit.

3x Cost = Water to water ground source + floor heating would be the ultimate solution as it is completely silent, has the lowest operating cost and the benefit of no outdoor unit. 

 

What surprised me was the much higher operating cost of the ventilation heat pump due to the lower COP efficiency. 

 

I would say the air to water split heat pump like the one you mentioned (or the Arotherm) is probably the best value / comfort for money. 

The air to air could also be a very practial solution if you are on a tight budget.