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Heat loss based on gas usage

 
Air Source Heat Pumps (ASHPs)
Last Post by Bash 4 weeks ago
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 Bash
(@bash)
Reputable Member Member
Joined: 4 months ago
Posts: 112
Topic starter 11/01/2026 8:26 pm  

We are 2 weeks away from our HP install.

In October I dropped the flow rate down on our Vaillant gas boiler to 45c, to match our HP design temperature.

We are upgrading all the radiators as some are way to small and others need increasing to meet the design temp of 45c.

I set the thermostat to 21c with a setback to 19.5c between 9pm and 5am.

Surprisingly this has significantly improved the comfort of the house, without increasing gas usage.

We have a warmer house.

I upped the flow to 50c during the cold snap of the last few weeks.

The house has stayed warm 24/7, which bodes well for the HP.

The gas boiler has run nearly 24/7 over this period.

Our heat loss was calculated at 6.2kw @ -2 45c.

Below is our gas usage for the coldest day Tuesday 6th Jan, which ranged between -8c and +4c

This looks like our heat loss calculation was bang on (to me).

Have I interpreted the gas usage right from the bright app?

Thanks.

Screenshot 20260111 201524

 

Screenshot 20260111 203025

 


This topic was modified 4 weeks ago 3 times by Bash

   
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JamesPa
 JamesPa
(@jamespa)
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Joined: 3 years ago
Posts: 4164
11/01/2026 8:59 pm  

Posted by: @bash
↑

Have I interpreted the gas usage right from the bright app?

In a word yes.

 

Measurements  based on a single day should be treated with extreme caution, houses take a long time to react.  However as a general indication this is likely to be good.

 

Like you I turned down my gas boiler before getting a heat pump and like you it improved comfort.  Unfortunately the way boilers are typically run in the UK increases cost by about 10% and reduces comfort.  Madness!


This post was modified 4 weeks ago by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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 Bash
(@bash)
Reputable Member Member
Joined: 4 months ago
Posts: 112
Topic starter 11/01/2026 9:12 pm  

@jamespa 

Thanks.

The numbers posted above were just for the coldest day over the last few weeks.

Looking through the bright app it's similar across many days over the last few weeks suggesting it's not a one off.

Last winter we had our flow temperature at 65c and set the thermostat to 19.5c during the day and 17c overnight.

This resulted in the heating coming on for about 1.5-2hrs and switch off for 2-3 hours.

The house was significantly colder and less comfortable.

Comparing gas usage we don't appear to be consuming any more so far than last winter (which wasn't particularly cold).



   
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