MCS heat pump sizing calc is 7589W but 12kW pump is being recommended...

[ChrisInKent]

I hope someone can help me make sense of this. Let me know if I'm not providing the right data to make a judgement.

 

We were planning on a 10kW ASHP for a 4 bed 210SQM house over two floors, highly insulated, MVHR, UFH, air tight, triple glazed. I thought this was already oversized but the company has come back recommending a 12kW ASHP. Just two of us living there to start with.

 

The overall MCS calc is 7589W for building heat load but because we have a large open sitting area (landing) on the first floor which they are calculating a 405W shortfall at -3 degrees C they calculate a requirement of 1136W just for this area for the heat pump and suggest upsizing.

 

The landing is shown as '6' on the Heat loss and outputs attachment. Their estimate is the landing (which has a vaulted ceiling) would reach just 13 degrees C at an outside temp of -3 degrees (UFH Output calc is 684 / 63% coverage). This doesn't take account of any heat rising from the hallway down stairs (2117 UFH Output and 174% at -3).  

 

They also don't factor in the MVHR at all in the calculations. They acknowledge it would improve the efficiency of the 10kW ASHP but then say if it malfunctions or is out of action then 10kW wouldn't be enough. 

 

So my questions are:

Is it right to suggest a 12kW pump is needed?

Should we be getting them to factor in the MVHR and towel rails?

Would we be better off fitting out for towel rails now and an electric rad if needed in future rather than upsizing to a 12kW ASHP?

 

Thanks.

[MikeSharp01]

Eeeek scream. Your house looks like it has been built to, or close to,   Passive house standard so something odd is going on somewhere, I don't get why the double height space needs special attention and MVHR must have an impact as it recovers 80%+ of the heat normally lost in air changes. 

[JohnMo]

9 hours ago, ChrisInKent said:

Just two of us living there to start with.

Hi welcome

Number of people make no real difference to heat loss calcs.

 

Do you own heat loss calculations. All those numbers sound plain stupid. Our house from your description is similar to ours and our heat loss is 3kW. So sometimes well off.

 

Are you a new build? What does you SAP report state?

[JohnMo]

9 hours ago, ChrisInKent said:

Would we be better off fitting out for towel rails now and an electric rad if needed in future rather than upsizing to a 12kW ASHP?

Electric towel rads, I would fit those anyway.

 

Do as many on here have done

Do your calcs first

Buy and install the heat pump yourself.

 

Forget to mention in first post

Our living room is 6m tall, has one wall is all glass, all other room circa 3 to 3.5m all with 5m² of glass except wet rooms.

[PNAmble]

9 hours ago, ChrisInKent said:

We were planning on a 10kW ASHP for a 4 bed 210SQM house over two floors, highly insulated, MVHR, UFH, air tight, triple glazed

sounds massive,  we are 210sqm, only difference is Rads passive std insulation and airtightness.  We have a 5Kw Ecodan.   We also have a large open plan diner, kitchen, living room but no vaulted ceiling.

 

It’s an MCS install in our self build,  his calcs matched my own. 

[ReedRichards]

I know nothing about well-insulated houses but it looks to me as if the "shortfall" occurs because your installers think your UFH will not be sufficient to heat this one area of your house.  In my house, which is entirely heated by radiators, this issue was fixed by adding an extra radiator.  I suppose it could have been fixed by running the existing radiators at a higher temperature, which might then have required a more powerful heat pump but that is entirely the wrong way to go.  So if the calculations are correct you would need an extra heat source.  Is there any scope to plumb-in a wet radiator if there really is a problem?  

[Gary68]

Just another point for reference, we have a not very well built or insulated 190sqm house from 2003, heat loss was calculated at 7kw and have an 8.5kw Ecodan and it was fine at -6C last winter.

[ProDave]

Definitely something odd here.  But we don't know the details of your "well insulated" house.

 

For another comparison my 150 square metre house has a worst case heat loss of just over 2kW for the whole house when it's +20 inside and -10 outside (a realistic winter temperature here) and is heated fine with a 5kW ASHP.

 

There is a spreadsheet available on this forum that if you care to try it and input all the details of the house will give a very accurate idea of heat loss.  I found even the detailed full SAP with all those same details over estimated the heat loss by some margin.

 

Also if you do have a heat loss of say 7kW, then you will need more than a 7kW ASHP otherwise it would have to spend all it's time 24/7 heating the house and would have no off time and no time to heat your hot water.

[saveasteading]

So called specialist installers can be surprisingly ignorant technically. 

What if the mvhr breaks down? We don't design for that.

A serious talk with the plumber or a change of.

[nod]

2 hours ago, JohnMo said:

Electric towel rads, I would fit those anyway.

 

Do as many on here have done

Do your calcs first

Buy and install the heat pump yourself.

 

Forget to mention in first post

Our living room is 6m tall, has one wall is all glass, all other room circa 3 to 3.5m all with 5m² of glass except wet rooms.

Can you really self install a 12k echo den and tanks pumps etc for 5k ?

[ProDave]

9 minutes ago, nod said:

Can you really self install a 12k echo den and tanks pumps etc for 5k ?

On the 3 heat pump installs I have done for others, I recon 2 days for the plumber and electrician so calculate that at your local rate, plus parts.

 

Assuming you are going to have wet UFH anyway and a hot water tank, it really comes down to difference in cost between a system boiler and tank, or an ASHP and tank.  There is not much difference to install in a new build.

 

Or will bur BUS grant subsidise installation of UFH on a new build as well as the ASHP?

[nod]

I think I would have been pushing to buy the gear for 5k Last time 6 years ago we paid 1700 just for a SS tank 

[JohnMo]

1 hour ago, nod said:

Can you really self install a 12k echo den and tanks pumps etc for 5k ?

You can pick up an UVC for £1k incl VAT.  Doubt they would need 12k ASHP, something very wrong if they need that.  I would if buying again just go Panasonic monobloc R32, they seem easy to tune and run well out the box just about silent - they are around £2.5 to £3k. May cost a little more than £5k, but you will have a heat pump of the correct size instead of some monster.

 

UFH on 200mm centres, on ground floor and upstairs wet rooms (100mm centres). Provide provision for fan coils later if you want them for cooling, run at same temp as UFH. You need two zones at least, so use electrical towel rads in bathrooms to do this. Heat pump system simple wins the day, less cost to install, more efficient - no buffers or volumisers, no manifold pumps or mixers, no actuators or zone valves, just a diverter. If you end up at around 6kW and the pipe runs are short you only need 22mm or if you really want 28mm piping.

[ChrisInKent]

9 hours ago, JohnMo said:

Hi welcome

Number of people make no real difference to heat loss calcs.

 

Do you own heat loss calculations. All those numbers sound plain stupid. Our house from your description is similar to ours and our heat loss is 3kW. So sometimes well off.

 

Are you a new build? What does you SAP report state?

We are new build, just about to break ground. Will have a look at the SAP to see if that helps…

[haythorn_1]

18 hours ago, ChrisInKent said:

They also don't factor in the MVHR at all in the calculations. They acknowledge it would improve the efficiency of the 10kW ASHP but then say if it malfunctions or is out of action then 10kW wouldn't be enough. 

Bizarre, what happens if the ASHP malfunctions? Maybe they should factor installing 2 for this scenario.

[JohnMo]

33 minutes ago, ChrisInKent said:

We are new build, just about to break ground. Will have a look at the SAP to see if that helps…

Just had a look at my SAP report, "as built" and looked at the "target emission rate", which is bog standard building regs, no MVHR, double glazed, min insulation, air tightness of 7. The target build adds around 35% to the as built house. That had a heat loss rate of about 4kW. We are 195m² single storey long thin building and lots of glass. Worst design for heat efficiency.

 

So your 12kW is about 4x oversized for well insulated 3g glass MVHR etc - walk away from that quote.

 

 

[Conor]

I can highly recommend the "source yourself", and "supervise competent trades" model of ASHP installation. 

 

Fyi 9kW heat pump in 275m² net house, with open areas and vaulted areas. No bother.

Edited August 6 by Conor

[sharpener]

20 hours ago, ChrisInKent said:

They also don't factor in the MVHR at all in the calculations. They acknowledge it would improve the efficiency of the 10kW ASHP but then say if it malfunctions or is out of action then 10kW wouldn't be enough. 

Compared with a heat pump there is virtually nothing to go wrong with MVHR so a bizarre objection. IIRC there is no proper way of accounting for it in MCS calcs, my installer reduced the target room temps to 18C throughout as a rough correction for it.

[ProDave]

5 minutes ago, sharpener said:

IIRC there is no proper way of accounting for it in MCS calcs, my installer reduced the target room temps to 18C throughout as a rough correction for it.

If you do a proper heat loss calculation, e.g use Jeremy's spreadsheet available on this forum, it calculates the ventilation heat loss properly.

 

If the MCS calculation does not do that, then it is flawed.  Why do we tolerate a system that specifies a heat pump size without calculating it properly?

[JamesPa]

4 minutes ago, ProDave said:

 

 

If the MCS calculation does not do that, then it is flawed.  Why do we tolerate a system that specifies a heat pump size without calculating it properly?

Because the purpose of the MCS calculation and the MCS standards is to protect installers, not consumers.  Simples!

[AliG]

My parents have a similar sized house 0.14 U-Value, less than 1x air changes, MVHR, triple glazing and get by just fine with an 8kW heat pump.

 

We actually upsized the heat pump to heat the hot eater faster, 5kW was good enough for the heat loss calculations.

 

However, they have a double height hall with a glass wall. It accounts for a very high percentage of the heat loss in the whole house (I think around 1/3 from memory). Even with the UFH flow at 50C it struggles to heat the hall to 21C when it is below 0 outside. The flow could be much lower for all other spaces in the house, I turned it up on the weather compensation curve for this reason.

 

When I calculated the output from the floor area, I discovered it was not enough relative to the heat loss of a double height room. We have asked for an extra radiator to be attached to the manifold and put on the half landing.

 

That seems to be what they are getting at, but that would be fixed by adding a radiator, a larger ASHP would have no impact on this issue at all.

[ReedRichards]

41 minutes ago, AliG said:

My parents have a similar sized house 0.14 U-Value, less than 1x air changes, MVHR, triple glazing and get by just fine with an 8kW heat pump.

 

We actually upsized the heat pump to heat the hot eater faster, 5kW was good enough for the heat loss calculations.

 

However, they have a double height hall with a glass wall. It accounts for a very high percentage of the heat loss in the whole house (I think around 1/3 from memory). Even with the UFH flow at 50C it struggles to heat the hall to 21C when it is below 0 outside. The flow could be much lower for all other spaces in the house, I turned it up on the weather compensation curve for this reason.

 

When I calculated the output from the floor area, I discovered it was not enough relative to the heat loss of a double height room. We have asked for an extra radiator to be attached to the manifold and put on the half landing.

 

That seems to be what they are getting at, but that would be fixed by adding a radiator, a larger ASHP would have no impact on this issue at all.

 

I think this is a very useful comment.  Don't be distracted by the size of heat pump you need but first focus on whether you have enough heat output for your double height area.   When you have sorted that out, then you can get back to recalculating the heat pump size.  

[JohnMo]

36 minutes ago, AliG said:

very high percentage of the heat loss in the whole house (I think around 1/3 from memory)

That sounds about right our lounge is near to 1/3 our total heat loss.

 

Do they run on a timer, on/off thermostat or on all the time?

 

37 minutes ago, AliG said:

Even with the UFH flow at 50C it struggles to heat the hall to 21C when it is below 0 outside

Do they have thick carpet? Have increased the flow rate in those loops to give more energy out. Run those loop 50% higher, so you get a lower dT (differential temperature).

 

41 minutes ago, AliG said:

turned it up on the weather compensation curve for this reason

Then move the curve back down to lower flow temp.

 

I have never been above 35 and we have UFH at 300mm centres.

 

39 minutes ago, AliG said:

heat the hot eater faster,

What does that mean?

 

50 minutes ago, AliG said:

That seems to be what they are getting at

Heat loss isn't affected by height, just the external areas. Halls aren't normally designed for 21, and normally rob or borrow heat from every room attached to it.

[mk1_man]

Hi Chris,  we are in final stages of build / retrofit of 250m2 house, aiming for as near passive house as we can with attention to air tightness, triple glazing throughout, removed chimney and have mvhr.  Also underfloor ground and first floors with 28mm primaries etc. I approached a few MCS suppliers and was being pushed towards 11 - 12kw units.  This didn't match my own calcs or how the house behaved when on gas boiler.  The route I have taken is DIY install via an MCS Umbrella company. I am doing all my own plumbing and electrics. Headline costs have been £4k for 7Kw Vaillant Arotherm, £1k for Vaillant 250l cylinder and approx £1600 for Umbrella costs.  ( All inclusive of vat )  This way you can still install via BUS grant but are not tied to crazy prices or forced into a heat pump that you don't want or need.

[MikeSharp01]

10 minutes ago, mk1_man said:

MCS Umbrella company

This is actually an MCS allowed scheme as far as I can see.  https://mcscertified.com/umbrella-schemes/