Heat Loss Calcs - Interpretation (to help size an ASHP)

[F113tch]

Hi All,

 

I have completed my heat loss calcs for my new-build using Jeremy's spreadsheet (many thanks Jeremy!) however I would like a little help in interpreting the results please.

 

From the spreadsheet I am hoping to size an ASHP, which will be the only source of heating for the house expect 2 x log-burners.  (Note: there is no mains gas to the property).

 

Any help in interpreting the results as well as a sense-check on the figures used would be gratefully received.

 

Kind Regards,

Steve.

 

 

Heat loss calculator - Hereford House.xlsx

[ProDave]

I would work on the figure in G 65, heat loss at 25 degrees delta t.  assuming you want the house at 20 degrees and it rarely gets below -5 where you are, that will be the worst case heat load on the coldest day you are likely to get.

 

So that's a figure of 9883W

 

Now you won't want the ASHP running 24/7 and in any case it will spend some time heating DHW so I would be looking at a minimum of a 15kW ASHP

 

Now go and tweak U values and see how much better you can get that figure.

 

For a comparison, I used the figure in H 65 as -10 outside is not uncommon here and my heat loss there is just under 2,5kW

[F113tch]

28 minutes ago, ProDave said:

I would work on the figure in G 65, heat loss at 25 degrees delta t.  assuming you want the house at 20 degrees and it rarely gets below -5 where you are, that will be the worst case heat load on the coldest day you are likely to get.

 

So that's a figure of 9883W

 

Now you won't want the ASHP running 24/7 and in any case it will spend some time heating DHW so I would be looking at a minimum of a 15kW ASHP

 

Now go and tweak U values and see how much better you can get that figure.

 

For a comparison, I used the figure in H 65 as -10 outside is not uncommon here and my heat loss there is just under 2,5kW

 

Thanks for your reply.

 

There were a couple of parameters that made a big difference to the heat loss, those being; air changes per hour and MVHR efficiency.  I set the ACH to 3 as I think this refers to the leakage through the building fabric?  If this is correct, I believe 3 ACH is achievable, and maybe I can improve on this through some of the advice I have recently read on this forum.  Having spoken with the guys that will be completing my air-testing they have advised that a wet plaster will certainly help with leakage so this is a route I will take.

 

Unfortunately the insulation is already fitted in the floors and in the walls, however I am trying to improve the U-values for the roof as the insulation is yet to be fitted here!

[Temp]

1 hour ago, F113tch said:

 I set the ACH to 3 as I think this refers to the leakage through the building fabric? 

 

Think you might be confusing units...

 

I think the recommended level of air tightness on a pressure test is ideally below 3 m3/(m2*h) measured at some pressure which I forget.

 

The ACH figure is what your MVHR is designed and set to deliver. I think its normally less than 1ACH. Passive houses 0.7 ACH I think? So I think you have it set too high in the spreadsheet.

 

Someone else can correct me if I have those numbers wrong.

 

[F113tch]

10 hours ago, Temp said:

 

Think you might be confusing units...

 

I think the recommended level of air tightness on a pressure test is ideally below 3 m3/(m2*h) measured at some pressure which I forget.

 

The ACH figure is what your MVHR is designed and set to deliver. I think its normally less than 1ACH. Passive houses 0.7 ACH I think? So I think you have it set too high in the spreadsheet.

 

Someone else can correct me if I have those numbers wrong.

 

 

Thanks for the reply.

 

I will adjust as ACH to <1, but where do I enter the “leakiness” of my property please?