Interpreting Heat Loss calculations and designing systems for a new build

[Don D]

Hi,

We're considering a self-build and looking at how to get to a point where we have a reasonable idea for the space and water heating systems, ahead of engaging/paying for any professional advice.

 

Our current build approach is based on the MBC insulated raft and timber frame package with triple glazed windows, they're local to us and we know the product well.

We THINK a correctly sized ASHP combined with UFH, an oversized pv array with battery capacity and a hw tank are likely to be our main system components and it's this we need to work on.

 

We know the u-values for the key fabric elements and we have the volumes, we even still have a copy of Jeremy's Fabric and Ventilation Heat heat loss calculator (from 2015)… the problem is, I am not sure which are the key output figures from it and how to use those to help size the heating requirement.

 

So there's a few things there, but if anyone has used a calculator like this and can advise or equally is there another way of getting to the same outcome, we'd be interested to know.

 

Cheers

 

 

[JohnMo]

1 hour ago, Don D said:

We THINK a correctly sized ASHP combined with UFH, an oversized pv array with battery capacity and a hw tank are likely to be our main system components and it's this we need to work on.

PV and battery are separate to heating requirements, so not really relevant to question asked.

 

There are some requirements for an ASHP often over looked and that is defrost - UK is pretty bad for it depending on exact location.  Inland being better than coastal or near a big body of water. So generally it wise to oversize for the typical -3 design in the UK.  Second is DHW generation, a small heat pump at design temp can take a while to heat a big cylinder as in a couple of hours.  So you are better to look at a 6kW heat pump but make sure it has good modulation so it runs well for long periods of times for best CoP.

 

The heat calculation, needs your ventilation system air changes per hour I would assume 0.5 to 0.3, and with MVHR efficiency circa 85 to 90%.  You need to find average temp in your area for the monthly heating needs. You need window and door sizes, general roof, floor and wall areas and thats about it, with U values

[Conor]

A better way to do it is to aim for a total heat demand / load per m² and make sure your build hits that through design choices and build quality. E.g. PH standard of 10W m² and take if from there. Decide what you want from your house, how you want it to feel and perform, and design accordingly.

[Don D]

4 hours ago, JohnMo said:

PV and battery are separate to heating requirements, so not really relevant to question asked.

 

There are some requirements for an ASHP often over looked and that is defrost - UK is pretty bad for it depending on exact location.  Inland being better than coastal or near a big body of water. So generally it wise to oversize for the typical -3 design in the UK.  Second is DHW generation, a small heat pump at design temp can take a while to heat a big cylinder as in a couple of hours.  So you are better to look at a 6kW heat pump but make sure it has good modulation so it runs well for long periods of times for best CoP.

 

The heat calculation, needs your ventilation system air changes per hour I would assume 0.5 to 0.3, and with MVHR efficiency circa 85 to 90%.  You need to find average temp in your area for the monthly heating needs. You need window and door sizes, general roof, floor and wall areas and thats about it, with U values

Thanks for the response, especially on the heat pump.

 

With the heat calc, I think I've input everything that's needed but it's understanding the result the spreadsheet is giving I need advice on. I've uploaded it here if you want to have a look.

Fabric_and_ventilation_heat_loss_AVETTE.xls

[SBMS]

Easy: build yourself using ChatGPT. 
easier: use jeremy spreadsheet. 
easiest: https://heatpunk.co.uk

 

check each. They were all within a few percentage of each other (which is good enough for heat pump sizing to size to nearest pump). 

[Gus Potter]

6 hours ago, Don D said:

We're considering a self-build and looking at how to get to a point where we have a reasonable idea for the space and water heating systems, ahead of engaging/paying for any professional advice.

Hi Don.

 

Admire the effort you are putting in, you will be rewarded. 

 

Jeremy Harris is an icon on BH, I've learnt a lot from his posts. His spread sheet is a good aid, a great tool to use as a sense check. 

 

I've opened up the spread sheet to see what inputs you are using. I do a quick sense check, mainly to see if the numbers look ok and where you are pushing the U values to think.. how much is this going to cost, get a supplier and so on. Your inputs are screen shotted below:

 

 

So to compare with the Scottish target U values, which are quite stringent but will serve you well if living further down south. The objective of my comment is to get you to think.. how much is this going to cost us to build, get local contractors interested in competing for you job and, can we spend our money in a better way. 

 

 

I wonder, where did you get the values you are using for your inputs?  and can you post some details on how you are going to construct the basic elements of the fabric. 

 

The U values for the glazing look a bit optomistic. Best to be pragmatic and conservative at this stage to avoid disappointment later. 

 

 

[Don D]

Thanks Gus. The floor, wall and roof values are direct from the MBC passive frame & foundation, the ACH similarly are what they will regularly achieve in the blower test. They are probably unique in the UK in offering the foundation and frame, it's one of the main selling points for us, reducing/removing the heat bridging issues and the risk of using separate contractors.

 

The windows are from Rationel, their Auraplus series and the advertised values. 

 

 

[JohnMo]

Cell B2, will be -3 in England, -5 Scotland.

Cell B4 use 6 degs if well insulated 

Cell B2 is ground floor area only

Cell B45 will give you your heat loss 

[JohnMo]

1 minute ago, Don D said:

ACH similarly are what they will regularly achieve in the blower test

The figure has nothing to do with that, it's your ventilation through put. Spreadsheet already assume very airtight build.

[Gus Potter]

16 minutes ago, Don D said:

Thanks Gus. The floor, wall and roof values are direct from the MBC passive frame & foundation, the ACH similarly are what they will regularly achieve in the blower test. They are probably unique in the UK in offering the foundation and frame, it's one of the main selling points for us, reducing/removing the heat bridging issues and the risk of using separate contractors.

 

The windows are from Rationel, their Auraplus series and the advertised values.

Ok what I can tell you is that I have designed as an SE a few insulated rafts over the years, once you get down to the engineering there is no ideal world, there is a trade off between engineering cost and satisfying the window suppliers " best performance". Building houses is an art where you have to make informed compromises. 

 

 

You have to recognise that if you go for the best value it will dictate everything else you do and that can come at and often nasty hidden cost. 

 

If you pair everything down now when doing you basic costing it will come back to bite you. 

 

The best advice I can give is to think about who might build this for you and sound out MBC on their caveats on the ground conditions, if you need steels and so on, point loads from the structure etc. 

MBC are by all accounts good at what they do but they will charge you for every extra! be that funny loads or funny geometry of you house. 

 

Also, sound out local Contractors, they may tell you, keep it simple and stupid, spec stuff we can source easily, and we will deliver an air tight structure. 

 

Oh and on a light note. If you are a doggy person then leave an unheated bit in the floor so your dog goes there for respite! UFH is not good for dogs!