New build in SE - MCS engineer says 22 kWh ASHP required....

[Navron]

Hello,

We are building a new(ish) house in the SE - the existing building was mostly demolished but foundations and some walls remained and are being reused as they were post-1986 so fairly decent: insulation under slab and insulated brick and block cavity walls. This is all being built to meet or exceed pre-June 2022 building regs, due to the date of the planning application.

 

This is not a passivhaus but will feature MVHR and ASHP CH and UFH to GF with radiators above.

 

The house is fairly large at 378sqm overall, with 5 bedrooms. 2850mm ceiling height to the GF and vaulted ceilings to the 1F with 2150mm to 3200mm ceiling height. We are having toilet cisterns plumbed in independently of the rest of the CW pipes for a potential future water harvesting effort.

 

The thing that has really surprised me is the MCS engineer's heat loss report:

 

Energy required to heat property: 42,552 kWh

Energy required for hot water: 6,550 kWh

Flow temperature: 50c

MCS SCOP Heating: 3.67

MCS SCOP Hot Water: 1.75

 

He says we need 2 x Mitsubishi Ecodan VAA 11.2kW R32 ASHPs.

 

I did my own rough Ventilation and Fabric heat loss calcs and mine came out 30%-60% lower.

Also I asked another installer to take a look (at the (seemingly high), heat loss calcs and he said a 16kW ASHP would be sufficient.

 

This seems like it's pretty over-specced. What do you think?

 

 

[Thorfun]

Put your data into the spreadsheet that is available on this forum (a search for ‘heat loss spreadsheet jeremy harris’ should find it) and see what that says. 

Edited May 2, 2023 by Thorfun

[MikeSharp01]

Hi - and Welcome to THE forum for people like us....

This may be another one of those occasions where the designer has not understood your build ups correctly. To get an accurate calculation do what @Thorfun suggests. The spreadsheet is here: https://forum.buildhub.org.uk/applications/core/interface/file/attachment.php?id=62325&key=a181232df44762993cdfe81c623c7c47 I think..

 

[Navron]

Thank you, I've done the calculations and I get the following output:

Largest Total heat loss power (W) is 8627 (for 30c delta)

Largest Total daily heat loss power for average OAT (W) is 4641

Largest Total daily heat loss power for minimum OAT (W) is 5441

 

The MCS engineer said he based his calculations on no MVHR, because he is not allowed to take it into account, so if I set MVHR efficiency to 0% I get 12639, 6727 and 7985 for those figures.

 

I have kept Outside air temp at -10, Room temperature at 20 and Under slab soil temp at 8

 

These figures all look a lot less than 22,000 kWh....

[SuperJohnG]

Pretty much all suppliers and installers are scared specifying ASHP and end up with them oversized. 

 

Vailliant wanted me to get 2-off 7kW units cascaded based on 45w/M2

 

I only need an actual 5kW unit to meet my heat demand but putting in a 7kW this week so my reheat times are better. 

[IanR]

  On 02/05/2023 at 21:40, Navron said:

The MCS engineer said he based his calculations on no MVHR, because he is not allowed to take it into account, so if I set MVHR efficiency to 0% I get 12639, 6727 and 7985 for those figures.

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What infiltration (uncontrolled/natural ventilation) rate have you used in the calcs, and provided to the heat pump supplier.

Jeremy's spreadsheet defaults infiltration to 0.6 ACH@50Pa, which is the Passivhaus target. If you've not provided the heat pump supplier a target infiltration rate they may be using the standard building regs rate of 8m³/m².h@50Pa (circa 6 - 7 ACH)

 

Very roughly, if you just scraped building regs U Values and infiltration rate your annual space heating energy requirement shouldn't be more than 28,000kWh (based on an approximate 75kWh/m².a worst case)

 

  On 02/05/2023 at 21:40, Navron said:

Largest Total heat loss power (W) is 8627 (for 30c delta)

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That's relatively low for a 30°C ΔT, coming in at around 20W/m²

[ProDave]

Did the people specifying the size of the ASHP have ALL the details of the build, individual wall U values etc?

 

Jeremy's spreadsheet proved very accurate and real as built heat loss closely follows what that predicted in my case, far more accurate than anything else including the SAP calculations.

 

My only comment is who specified an UFH system with a flow temperature of 50 degrees?  You should be running at a lot lower than that.

 

It is a big house but if the calculations are saying 8627W for delta t 30 that is pretty good for such a large house.  and it won't often be -10 in the SE and not for long (unlike here when -10 for a week is common in winter)