Radiators needed for ASHP and UFH?

[YorkieSelfBuild]

Hi, I'm building a new low energy house, now at ASHP planning stage. Installer's given me the design docs and main issue is insisting on radiators to get the 7.5k grant. I was expecting to have no rads, not even towel heaters in bathrooms as I don't like hot towels.

 

600 x 800mm on 1F bedrooms

600 x 1600mm(!) on 2F bedroom

Small one in bathroom (might insist in 2F ensuite too, late addition)

One each on 2 landings

 

Floor temps

Wet UFH (GF only) up to 35C

DHW 55C

Rads 41-45C

 

1. I was told rads were needed but thought they'd be smaller, especially 2F bedroom. Do they have to be that big?

2. Are rads needed on landings? Only told about bedrooms.

3. Are the floor temps correct? I thought DHW could be lower, 50C or less. I think the UFH and rads sound about right.

 

Shout if you need more info.

 

Any advice is appreciated, thanks

Edited Friday at 14:49 by YorkieSelfBuild

[JohnMo]

41 minutes ago, YorkieSelfBuild said:

insisting on radiators to get the 7.5k grant

Well that is just nonsense moved to a different installer. Or go umbrella scheme, such as cool energy.

 

DHW 48 is generally a fine compromise.

UFH sounds fine.

 

Depending on heat loss, you can just do ground floor UFH, design so it will give off enough energy to heat whole house. 

 

Low energy house is a bit vague, ground floor area?

Heat loss at designed temps?

Have you done an air test? Is installer using that figure to get infiltration rate?

MVHR?

[nod]

Get yourself a new installer 

If he’s lying to you this early in the project It doesn’t look good for aftercare 

[Beelbeebub]

Am I right in thinking you are effectively having no heating in thr upstairs and relying on heat transferring from the ground floor and not having particularly warm bedrooms?

 

I think the issue be the MCS rules state you have to have sufficient heating in each room to meet that rooms heat loss and they also set a temp for bedrooms at the design temps (migbt be 21c, can't remember)

 

Those rads aren't that expensive (assume I assume k22's). Fit them then remove and cap off afterwards.  Alternatively spec a higher flow temp to get smaller rads, then run at the lower flow temp (just make sure your UFH is spec'd to run at the lower flow temps, which it should be)

[joth]

Yes best to find another installer. 

Most likely they're also not applying your target airtightness so having a much higher heat loss per room. Do you have a PHPP of other heat loss model you can share?

 

If you wanted you could consider UFH on additional floors, it is handy for a bit of active cooling. But we didn't put any heating upstairs and never regretted it (enerphit passive house retrofit)

 

 

 

[JamesPa]

1 hour ago, Beelbeebub said:

I think the issue be the MCS rules state you have to have sufficient heating in each room to meet that rooms heat loss and they also set a temp for bedrooms at the design temps (migbt be 21c, can't remember)

 

 

I cant find this requirement in the definitive MCS document which is here.

 

The BUS rules say that the system must be capable of providing 100% of the space heating requirements, but say nothing about rooms

 

You might reasonably argue that the guidance (but not the actual requirement) of Part L of the building regs requires separate heating of each room so you might want to take a careful look at this (note that most of the document is guidance, the actual requirements are just a couple of paragraphs.)

 

DHW is frequently heated to 48 with an ASHP

If you heat rads to 41-45 then your flow temperature will have to be be 41-45 with mixing down for UFH, so you lose the efficiency advantages of UFH.  You want a design where the space heating is one flow temperature only (with weather compensation obviously). 

The towel rail in my bathroom is hopelessly undercapacity according to the calcs (because it wasnt upgraded from the DT50 design).  The bathroom is nice and warm from the adjacent hall and, when its in use, steam.

 

I would go with those who say find another installer/designer.

Edited Friday at 17:20 by JamesPa

[YorkieSelfBuild]

2 hours ago, JohnMo said:

w energy house is a bit vague, ground floor area?

Heat loss at designed temps?

Have you done an air test? Is installer using that figure to get infiltration rate?

MVHR?

Not PassivHaus but quite close. MVHR, PV, SIPs, insulated, triple glazing, etc

House still being built so assume no air test.

[YorkieSelfBuild]

I've already signed a contract but still within the 14 day cancellation period (lose £500 design fee).

 

The architect/designer is talking to installer, the email chain says the installer used MCS software that doesn't model heat flow between floors or rooms.

 

It's a new installer as they had trouble with previous installer on previous development plot.

 

I'm in Sheffield, if anybody can recommend local installers?

[JamesPa]

Why are they suggesting a 9.5kW heat pump for a 3kW house?

 

Unless 'peak power demand' means electrical power, but it cant, surely mean this.  There is no way a 120sq m recently built house is 9kW.  My house, 200sq m,  was built in the 1930s with a couple of modest 1980s extensions, only partial internal wall insulation (a minority of the walls are solid brick) and no floor insulation and its 7kW

 

I hope they arent putting in a buffer tank/LLH further to clobber your performance (but fear they may be)

 

Do yourself a favour and find another installer.

Edited Friday at 17:55 by JamesPa

[sharpener]

6 minutes ago, YorkieSelfBuild said:

It's a new installer as they had trouble with previous installer on previous development plot.

 

Your architect/designer is going to get just as much trouble with this one! Send him the link to this thread.

 

22 minutes ago, YorkieSelfBuild said:

Not PassivHaus but quite close. MVHR, PV, SIPs, insulated, triple glazing, etc

 

25W per sq m is very generous for a house like this. You will save more than the lost £500 design fee by finding someone who will put in a smaller HP than a 9.5 kW Riello.

[YorkieSelfBuild]

2 hours ago, JamesPa said:

I hope they arent putting in a buffer tank/LLH further to clobber your performance (but fear they may be)

 

Do yourself a favour and find another installer.

I don't think so, according to contract:

 

Our proposal includes the design, supply, installation, commissioning and handover of the following to full

Microgeneration Certification Scheme (MCS) standards:

• Heat Pump: Riello NXHM004 4kW 230V

• Controller: Homely

• Hot Water Cylinder: 200 litres Pre- Plumbed

• Buffer Cylinder: NA

• Antifreeze Valves

• Internet Expansion Card

Installation includes appropriate expansion vessels and temperature / pressure relief valves to comply with Part G3 of the Building Regulations guidance. Please see inclusions / exclusions.

Your Heat Pump system has been designed to meet 100% of space heating and domestic hot water demand at

MCS design temperature

[YorkieSelfBuild]

I found the airtightness score, 1.3, so higher than 0.6 PassivHaus figure

[YorkieSelfBuild]

Installer says they're certified Heat Geeks

[marshian]

12 minutes ago, YorkieSelfBuild said:

Installer says they're certified Heat Geeks

looks like they are trying to comply with MCS grant rules with what they are suggesting

 

ping an email to Adam he’d probably respond with some advice

[sharpener]

17 minutes ago, YorkieSelfBuild said:

Heat Pump: Riello NXHM004 4kW 230V

 

That makes more sense now. Can't see from this spec what output is at yr -3.4 OAT but probably enough. What is the 9.5 kW figure though? Why in 2025 aren't they recommending a unit with R290 refrigerant?

 

Heat Geeks pride themselves on being top drawer (and correspondingly expensive) so why can't they access s/w that will account for internal heat transfers if that is what you want. You could even model it yourself in HeatPunk.

 

Convenient that the running costs of an alternative gas boiler ar £1 p.a. more than the HP.

[JamesPa]

53 minutes ago, YorkieSelfBuild said:

I don't think so, according to contract:

 

Our proposal includes the design, supply, installation, commissioning and handover of the following to full

Microgeneration Certification Scheme (MCS) standards:

• Heat Pump: Riello NXHM004 4kW 230V

• Controller: Homely

• Hot Water Cylinder: 200 litres Pre- Plumbed

• Buffer Cylinder: NA

• Antifreeze Valves

• Internet Expansion Card

Installation includes appropriate expansion vessels and temperature / pressure relief valves to comply with Part G3 of the Building Regulations guidance. Please see inclusions / exclusions.

Your Heat Pump system has been designed to meet 100% of space heating and domestic hot water demand at

MCS design temperature

Ok that's makes more sense.  The homely controller will help get the best out of the heat pump.  So in this case, if price is ok, question some detail.  For example the bed 2 rad could presumably be physically smaller if they made it a type 22/K2.  What ft is it designed for and how does the ufh ft relate to the rad ft.  Some things still don't quite stack up but may be ok in fact.

[YorkieSelfBuild]

45 minutes ago, marshian said:

looks like they are trying to comply with MCS grant rules with what they are suggesting

 

ping an email to Adam he’d probably respond with some advice

Who's Adam? I'm still pretty new to the forum

[YorkieSelfBuild]

48 minutes ago, sharpener said:

 

That makes more sense now. Can't see from this spec what output is at yr -3.4 OAT but probably enough. What is the 9.5 kW figure though? Why in 2025 aren't they recommending a unit with R290 refrigerant?

The 9.5 is the outdoor design temperature DOT -3.4C (from their website https://www.imsheatpumps.co.uk/blog/understanding-your-heat-curve/)

 

Will ask about R290 refrigerant, thanks

[Nickfromwales]

3 hours ago, JamesPa said:

I hope they arent putting in a buffer tank/LLH further to clobber your performance (but fear they may be)

Easy tiger, I fitted a LLH to an ASHP (doing UFH only in a passive raft) so the manifold pump was constant flow, to promote a steady rate of circulation through the loops 24/7, which then allowed the modulating pump in the heat pump to be hydraulically separated so that could yo-yo up and down without any issues. Worked a charm.

 

3rd party service agent visited site last week to service it, and reported that the defrost immersion had never once come on (since commissioned in late 2022).

 

Things can be installed with thought, vs just plonked in to tick a box  

[Nickfromwales]

17 minutes ago, YorkieSelfBuild said:

Who's Adam? I'm still pretty new to the forum

@marshian

 

"Speak now, or forever hold my piece"

[YorkieSelfBuild]

19 minutes ago, JamesPa said:

Ok that's makes more sense.  The homely controller will help get the best out of the heat pump.  So in this case, if price is ok, question some detail.  For example the bed 2 rad could presumably be physically smaller if they made it a type 22/K2.  What ft is it designed for and how does the ufh ft relate to the rad ft.  Some things still don't quite stack up but may be ok in fact.

They already spec K2 rads in bedrooms

[YorkieSelfBuild]

Here's the UFH power figures

[YorkieSelfBuild]

Thanks for all the replies so far.

 

I've contacted several installers around Sheffield, let's see if what they come up with is drastically different.

[Chanmenie]

My MCS installer said I needed rads upstairs, similar size to yours @YorkieSelfBuild.

I have my flow temp lower though, with ufh and rads the highest it’s been is 28 degs when it was -5 outside, I was surprised that although the upstairs rads didn’t feel warm ( I measured them at 24deg)  they raised the temp in the rooms up to 19 quite quickly.

I had a heating supplies guy tell me I’d need 35 degs flow temp but that was rubbish most of Dec, Jan and Feb it averaged 25-26 and maintained the indoor temp throughout the ground floor at 19-20.

I run it on weather comp.

The flow temps in the spreadsheets above are rubbish, you can’t have rads at 45 and UFH at 35, it will be the same for both, 

 

I have 4KW pump and with the low flow temps the COP is  5.3 

Edited Friday at 21:30 by Chanmenie

[JohnMo]

2 minutes ago, Nickfromwales said:

I fitted a LLH to an ASHP (doing UFH only in a passive raft) so the manifold pump was constant flow, to promote a steady rate of circulation through the loops 24/7

Do exactly the same without buffer or LLH. All done via heat pump circulation pump. So no real need for additional stuff. Currently heat pump off and circulation rate is 14L/min, through my 7 loops, and a fan coil, when heat pump starts it ramps up to 19.5L/min.

 

6 minutes ago, Nickfromwales said:

defrost immersion

Never needed one of those either, in fact never had one.