Heat Pump Help

[Piers]

Our energy consultant (EC) has finally written his report and is recommending we install a 16kW ASHP.

 

Peak heat loss has been calculated at 10.41kW and the EC writes 

 

Quote

This scheme relies upon an air source heat pump delivering 11kW of heat at -2.4°C. Heat pumps are often listed with the output at +7oC, because of the reduced output at lower temperatures we would usually recommend a larger heat pump, in this case 16kW, to ensure it meets the output at -2.4oC external temperatures, without the requirement for backup heating.

He's also recommending we pair it with 7.4kW of solar PV and 2 x Zehnder Q350 MVHR systems (Two due to issues running ducting).

 

We're planning to install fan coil units for active cooling because we're terrified of building a house that overheats in the uncreasingly hot summers.

 

400 litre HW tank

 

Planning to build in Isotex ICF if that makes any difference. Heating with UFH and intermediate floors will be built using Comfloor (i.e. concrete and screed)

 

Armed with this information I'm unsure of what the next steps are in actually selecting a heat pump and how the various elements tie in with one another (or not). Before I start engaging with potential installers/contractors I feel I need to know more so I can make informed decisions.

 

I'm open to general guidance and a bit of technical advice (preferably couched in layman's terms!). TIA

 

 

Edited March 29, 2023 by Piers

[Nickfromwales]

5 minutes ago, Piers said:

and 2 x Zehnder Q350 MVHR systems (Two due to issues running ducting).

Is this because the system designer has defaulted to series-run 125mm ducting ( or larger ) everywhere?

 

I've recently installed a heatpump for an Insulhub client in Oxford, and that is a 7kW unit in a medium sized 3 bedroom dwelling. It very rarely gets above 4-5kW output and that was in the utter depth of winter. To hear you will need this size of HP must mean you're building a much larger property or not going fully airtight etc? Has your EC dictated any of these target values to justify the sizing?

I am assuming you are going with UFH in an insulated raft foundation, as Jamie and the gang did in Oxford. That slab is performing ridiculously well btw.

[Piers]

4 minutes ago, Nickfromwales said:

Is this because the system designer has defaulted to series-run 125mm ducting ( or larger ) everywhere?

Yes, the layout of the house, position of steels and vaulted ceilings makes the ducting runs quite tricky. The advice we've been given is to split the MVHR into 2 zones.

 

6 minutes ago, Nickfromwales said:

To hear you will need this size of HP must mean you're building a much larger property or not going fully airtight etc? Has your EC dictated any of these target values to justify the sizing?

It's a 5 bed house, ~420m2 with some larger volume areas (double height). The EC hasn't really gone into any detail on airtightness and that's something I need to follow up on. We're certainly aiming for an airtight house, though not entirely sure what that entails on a design level.

 

10 minutes ago, Nickfromwales said:

I am assuming you are going with UFH in an insulated raft foundation

Kind of. The foundations are stepped. The basement area will be an insulated slab with retaining walls. There's also a section of rasied GF which will be built using steel and comflor. This PDF shows the design(2)010a - Long Section Proposed.pdf

[joth]

29 minutes ago, Piers said:

It's a 5 bed house, ~420m2 with some larger volume areas (double height). The EC hasn't really gone into any detail on airtightness and that's something I need to follow up on. We're certainly aiming for an airtight house, though not entirely sure what that entails on a design level.

 

It's unclear what stage your build is at, but if you haven't set an airtightness goal and figured out how (hint: written into contracts) you'll get the designer and builder to work to achieve it, then I think doing that is probably a higher priority than choosing the specific heat pump make model

 

1 hour ago, Piers said:

We're planning to install fan coil units for active cooling because we're terrified of building a house that overheats in the uncreasingly hot summers.

 

This is a good idea, but learn from my mistakes:

- MCS tradesmen (and builders in general) are not trained / experienced in FCU install so you need to find a unicorn, or do a lot of the design and install detailing yourself.

- remember the whole system will need insulating against condensation if you run cold water for FCU through it. This includes all pumps, filters, valves, etc

- typically a FCU will be much lower kW output than a UFH circuit on a house your size, and you often don't want all FCU running at once (e.g. if some bedrooms are overheating but others aren't you don't want to turn them into a refrigerator) so you'll almost certainly need a buffer tank / volumizer on the FCU circuit even if you don't have one (and it's more efficient not to) on the UFH circuit.

- installing ducts and plenums for hidden FCU as retrofit is a miserable job so get them put in in the build.

 

 

[Nickfromwales]

38 minutes ago, Piers said:

Yes, the layout of the house, position of steels and vaulted ceilings makes the ducting runs quite tricky. The advice we've been given is to split the MVHR into 2 zones.

Have you run this again, using a different supplier and 92mm radial ductwork? Punching smaller ( sub 100mm ) holes in steels before they're delivered to site is a doddle, and I do it for most of my M&E clients tbh. For 120mm holes ( penetrations ) you can still do this and just plate / reinforce the steels by design from your SE. Would save 2x plant rooms for MVHR ( if that bothers you or not of course? ).

 

38 minutes ago, Piers said:

It's a 5 bed house, ~420m2 with some larger volume areas (double height). The EC hasn't really gone into any detail on airtightness and that's something I need to follow up on. 

Your EC should be screaming "fabric first" from the rooftops.

 

38 minutes ago, Piers said:

We're certainly aiming for an airtight house, though not entirely sure what that entails on a design level.

See above.

 

38 minutes ago, Piers said:

Kind of. The foundations are stepped. The basement area will be an insulated slab with retaining walls. There's also a section of rasied GF which will be built using steel and comflor. This PDF shows the design(2)010a - Long Section Proposed.pdf

Nice! Defo a good candidate for ICF too. The insulated slab will likely need a lamination atop, of UFH in screed? If PH ( or as near as damnit ) then I'd say that it would be pointless to insulate > UFH > screed, and I'd just allow the heat to 'soak' into the insulated concrete as a bity of a heat capacitor.

[Nickfromwales]

What's the U value of the Isotex wall? I can't remember off hand.

[Piers]

25 minutes ago, joth said:

It's unclear what stage your build is at, but if you haven't set an airtightness goal and figured out how (hint: written into contracts) you'll get the designer and builder to work to achieve it

We're trying to get architects to finish building regs drawings and SE to complete structural package. The airtightness goal from the EC is 5m3/h.m2@50Pa. Should we be aiming lower? What are the things to consider to achieve this with an ICF build?

29 minutes ago, joth said:

This is a good idea, but learn from my mistakes:

Thank you this is v helpful. Can you run the FCUs from the ASHP and get it to do HW in the summer? No need for a separate unit?

 

31 minutes ago, Nickfromwales said:

Have you run this again, using a different supplier and 92mm radial ductwork?

No, we've only just got the report from the EC who's recommending 2 systems. I think we'll want to speak to a MVHR designer to get another opinion. My EC is more academic than pragmatic..

 

34 minutes ago, Nickfromwales said:

The insulated slab will likely need a lamination atop, of UFH in screed?

Exactly. We'll put the insulation under the slab and then UFH on top in screed.

 

35 minutes ago, Nickfromwales said:

Your EC should be screaming "fabric first" from the rooftops.

I wish my EC had any sort of passion TBH. He's taken 4 months to write his report and I don't feel much better informed having read it. It's heavy on theory and light on practical recommendations.

[Piers]

37 minutes ago, Nickfromwales said:

What's the U value of the Isotex wall? I can't remember off hand.

They go as low as 0.12. Think their standard block is 0.15 and they also have a 0.19 option.

 

In our EC's report he's assumed walls are 0.18 (I know not why)

 

We're getting quotes for both 0.12 and 0.15 versions

[Bitpipe]

1 hour ago, Piers said:

We're trying to get architects to finish building regs drawings and SE to complete structural package. The airtightness goal from the EC is 5m3/h.m2@50Pa. Should we be aiming lower? What are the things to consider to achieve this with an ICF build?

 

 

With our passive timber frame on cast in situ basement was 0.6 ACH/h - not sure how those numbers translate. I believe MVHR does not work well above 2ACH/h. High airtightness does not mean you need to live in a sealed box, open doors and windows when you want but if you've paid to heat or cool the internal air then makes sense to stop it from leaving (which is where MVHR comes in).

 

As concrete is inherently airtight you just need to pay attention to windows, doors, roof interface and any penetrations.

 

I assume you are speccing airtight doors and windows (no trickle vents or letter boxes, cat flaps etc)? If so then you need a decent seal between unit and wall (compriband is excellent) and air tightness tape from frame to wall.

 

Junction between roof and ICF will need taping and roof will need to be of an airtight design.

 

Penetrations (e.g. incoming/outgoing cables & piping) are easy to do - either put duct in before pouring or drill later and seal the inside of the penetration with tape, fill duct void with silicone or a flexible putty.

 

 

 

1 hour ago, Piers said:

Thank you this is v helpful. Can you run the FCUs from the ASHP and get it to do HW in the summer? No need for a separate unit?

 

No, we've only just got the report from the EC who's recommending 2 systems. I think we'll want to speak to a MVHR designer to get another opinion. My EC is more academic than pragmatic..

 

2 systems should be avoidable  - we have a large house (400m2) and have one Sentinel Kinetic +. Did not make provision for ducts in steel design so some runs are elongated but it all balanced well enough in the end.

 

1 hour ago, Piers said:

Exactly. We'll put the insulation under the slab and then UFH on top in screed.

 

Why? If slab is insulated, just clip the UFH pipes to the steel mesh and save expense of screed.

 

1 hour ago, Piers said:

I wish my EC had any sort of passion TBH. He's taken 4 months to write his report and I don't feel much better informed having read it. It's heavy on theory and light on practical recommendations.

 

[Nickfromwales]

1 hour ago, Piers said:

In our EC's report he's assumed walls are 0.18 (I know not why)

It's called "covering ones arse so one cannot possibly fail".

 

1 hour ago, Piers said:

I wish my EC had any sort of passion TBH. He's taken 4 months to write his report and I don't feel much better informed having read it. It's heavy on theory and light on practical recommendations.

I just reworked a similar sad ( near pathetic ) M&E 'proposal' for another member on here. The company then agreed to pay my fees to correct their shart attempt. I'm sure half or more of these clowns just hope nobody will ever challenge them .

Keeps me off the streets anyways  

 

16 minutes ago, Bitpipe said:

Why? If slab is insulated, just clip the UFH pipes to the steel mesh and save expense of screed.

+1, ( if the slab is good enough to tile or accept your final finishes, directly? ).

[Bitpipe]

40 minutes ago, Nickfromwales said:

It's called "covering ones arse so one cannot possibly fail".

 

I just reworked a similar sad ( near pathetic ) M&E 'proposal' for another member on here. The company then agreed to pay my fees to correct their shart attempt. I'm sure half or more of these clowns just hope nobody will ever challenge them .

Keeps me off the streets anyways  

 

+1, ( if the slab is good enough to tile or accept your final finishes, directly? ).

 

Our basement slab was just tamped level and a few bags of latex made it flat as a pancake for flooring. However it ranged from 10 to 2mm  - you'd need a lot more for UFH!

[JohnMo]

I would question if the consultant is just taking your money, he sounds to be adding zero value here.  Recommending 5m3/m2 and MVHR is just rubbish, wasting your time and more importantly your money. If you are not below 3m3/m2 on a new build with ICF walls something is well a miss with the build and as the consultant he should know that.  He should also know that MVHR with a leakage rate above 3 is not the smartest move and not recommended for optimal energy usage.

 

Making rubbish assumptions just makes heating systems bigger than they need to be.

 

Also the ASHP sounds well oversized.

4 hours ago, Piers said:

Heat pumps are often listed with the output at +7oC, because of the reduced output at lower temperatures we would usually recommend a larger heat pump

That is again just bone idle, no leg work just throw away rubbish statements.  

[Piers]

This is the first energy report I've ever read and I reckon that 90%+ is generic filler content like "What is a heat pump".

 

Does anyone have a copy of a good energy report that I can look at for comparison?

 

I've attached my EC's report if anyone wants to have a look Grey Cottage Renewable Energy Feasibility Study v1 .pdf

[SteamyTea]

19 minutes ago, Piers said:

I've attached my EC's report if anyone wants to have a look

How accurate do you think this is?

[Piers]

2 minutes ago, SteamyTea said:

How accurate do you think this is?

If that's a question for me: I have no idea and wouldn't even know where to start.

 

[SteamyTea]

20 minutes ago, Piers said:

If that's a question for me: I have no idea and wouldn't even know where to start.

Yes it is.

 

Working out heat losses is not that difficult, but it is very boring.

Basically a case of finding out all the wall, floor, window, doors and ceiling/roof thermal properties (the W.m^-2.K^-1), all the areas, and an assumption of the weather extremes, as well as any temperature differences between rooms.  Then the power loss from the air changes has to be added, and adjusted for the MVHR.

Then the water usage is added, but you can probably work that out pretty well from current usage.

There is a spreadsheet floating about that is pretty accurate.  Search for @Jeremy Harris as he created the spreadsheet.

 

[Lofty718]

Make sure the heat pump installer knows what they are doing

 

I'd suggest looking for an installer on the heat geek website so you don't end up with a cold house or sky high electricity bills (or both)

[Kelvin]

1 hour ago, SteamyTea said:

M

There is a spreadsheet floating about that is pretty accurate.  Search for @Jeremy Harris as he created the spreadsheet.

 

@Piers If you can’t find it let me know and I’ll send it to you 

Edited March 29, 2023 by Kelvin

[Piers]

14 hours ago, Kelvin said:

@Piers If you can’t find it let me know and I’ll send it to you 

@Kelvin please can you send to me. I did a trawl through @Jeremy Harrisposts but there are ~600 pages of them!

[Kelvin]

Sent it to you. Ask on here if you get stuck. 

[beebee]

Hi, I also would really appreciate a copy of the spreadsheet! A similar trawl has been fruitless so far! Thanks!

[Dunc]

For future reference/linking I'm guessing it's this one?

Which looks incredibly valuable! Thanks to the original author!

[markocosic]

On 29/03/2023 at 12:47, Piers said:

Our energy consultant (EC) has finally written his report and is recommending we install a 16kW ASHP

 

They're a muppet.

 

They're all rated differently.

 

Some rate at air +7/water 35

Others rate at air -7/water 55

 

And the output from the same unit at air -2 will be very different if your heating system runs at 35 Vs 45 Vs 55C 

 

(e.g. a "7kW vaillant unit running a 45C flow temperature at -2C air actually provides 9 kW output)

 

Oversizing results in a cluster fudge in operation. 

 

You need one that'll meet your design heat load at your design flow temperature.

 

Given the "chuck in 16 because it might be rated at 7C" shows such a lack of understanding or at least attention to detail that I wouldn't trust a word they say on heat pump selection OR indeed the heat loss calculation/heating system design to suit the heat pump sorry.

 

Better to ask a professional rather than somebody out of their depth before they make some expensive bad commitments on your behalf.

 

 

[Redbeard]

We got under 2.5m3/m2/hr in a refurb of a 19th century house with all sorts of issues and I was a bit disappointed that we did not get better. I would be really disappointed with 5 on a refurb, let alone a new-build.

 

Depending on what the consultant knows about your knowledge I'd have expected simple things like explaining what a solar diverter is, but also looking more broadly at air-tightness, explaining what can be achieved and why you want it, and so on.

 

Does 'finally written...' imply a long wait?

 

 

Edited April 8, 2023 by Redbeard
Read OP properly!

[IanR]

On 29/03/2023 at 10:47, Piers said:

Our energy consultant (EC) has finally written his report and is recommending we install a 16kW ASHP.

~~~

400 litre HW tank

~~~

Planning to build in Isotex ICF if that makes any difference.

 

On 29/03/2023 at 11:17, Piers said:

It's a 5 bed house, ~420m2 with some larger volume areas (double height).

 

On 29/03/2023 at 12:35, Piers said:

 The airtightness goal from the EC is 5m3/h.m2@50Pa. Should we be aiming lower?

 

On 29/03/2023 at 12:42, Piers said:

In our EC's report he's assumed walls are 0.18 (I know not why)

 

We're getting quotes for both 0.12 and 0.15 versions

 

Based on slightly better than Building Regs level performance data, the proposed 16kW ASHP seems about right for a 420m² house.

 

Of note is the caveat on your "Building fabric summary" image in your first post "*These costs are based on an air source heat pump providing the space heating.". So the 6332kWh annual figure quoted is the electricity to run the heat pump, equates to 21,044kWh of annual energy loss through the fabric and ventilation.

You therefore have a 50.1kWh/m² annual energy loss.

 

Is this planned to be a long term home? If so you are saddled with these energy losses for the time you own the home. I personally would be looking for cost effective ways of reducing this figure. 15kWh/m² annual energy loss is an achievable target.

 

Going for the 0.12 U Value wall option for instance.

 

The stated air tightness target of 5m³/h.m²@50Pa is really quite poor. Your EC shouldn't be suggesting an MVHR system at that level as it will bring no benefit, just cost a load to Install, run and maintain. MVHR's come into their own at 3m³/h.m²@50Pa, and are best at 1m³/h.m²@50Pa or lower. 

 

If this is your long term home I'd suggest targeting 1m³/h.m²@50Pa. You describe the property as having some large double volume areas, in those circumstances you'll find that good air tightness considerably lowers your energy losses.

 

If you are to target 1m³/h.m²@50Pa air tightness, then I'd research your chosen ICF. I've not looked into ICF much but the feedback I've picked up from this site, on wood-concrete ICF blocks, is that there is a lot of work involved in getting good airtightness from them. Maybe there are alternate ICF solutions that will make the job easier.

 

If you can target better energy performance from the fabric of the building, then rather than the space heating requirement driving ASHP it will become DHW. You've specified a 400l UVC, I'd say that's the minimum size you should go with for the size of property, or perhaps a 500l. If you are heating the hot water via an ASHP you need to be able to store the water at 50°C or less to keep the ASHP efficiency at a reasonable level, so you need a greater volume of 50°C water than if you were heating it to 65°C.  With large UVC's, you then need to watch your re-heat times. ie. you may get your space heating down to just a 6kW requirement, but you may still want to specify a 9kW - 12kW ASHP for better UVC reheat times.

 

My own property is a similar size to yours, my heat loss is calculated at 15kWh/m² per annum, but has come in at around 12.75kWh/m². At today's prices that's around £535 per year for space heating which is a £1,700 per annum saving on your current performance targets. Index linked over 20 years gives a reasonable budget for performance improvements at the build stage.

Edited April 9, 2023 by IanR