Heatloss Interpretation from emoncms

Installed Heatmeters to existing boiler setup before removal to fit HP. Its a two zone setup (s plan) with a terribly oversized boiler that cycles.

From this 24hr window (24 hrs just past ) , can I roughly estimate the heatloss of the property is 2164+2234 = 4.398 kW . Can I also roughly expect a total radiator capacity need for ground floor of 2164 W and upstairs 2234W

Calculation looks sound. You can also add together the Wh column and divide by 24 to get average power, comes out the same.

A more accurate estimate can be done by looking at daily heat energy (kWh) vs. average temperature.

Required radiator capacity will depend on what flow temperature they’ll be running at, as they are usually rated at 50°C above room temperature. At lower temperatures, they will output less heat, so you’ll need to increase the total radiator size.

For example, 4 kW output at 40°C flow temperature, radiators will output a third their rated heat, so you’ll need to make them 3 times bigger.

If the radiators aren’t bigger enough, they’ll need to run at a higher temperature to output enough heat.


You probably want to survey your existing radiators and add up their rated output.

There’s sites like heatpunk.co.uk that provide tools for calculating room-by-room heat loss, and the radiator upgrades to match them.


Thanks @Timbones . I forgot to mention that I have already adjusted the Flow / Return to MWT around 36C to get a close as possible match to the HP. Lovely article from Michael de Podesta . Thanks for sharing .

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I would be a little cautious using data from a single day to make this calculation, though I notice many others have much cleaner straight line relationships between indoor - outdoor temp and space heat requirement than I seem to get…

Here’s my average daily space heating only heat output vs indoor - outdoor temperature. The spread of values has always surprised me but probably reflect the high thermal mass of our solid stone mid-terrace house?

All in all this plot for every day in this heating season so far suggests a space heating demand of 3.3 kW ± 0.5kW? at 23 DT for our house:

Thanks for the advice and makes sense. Did you manually fit that line or is there some in emoncms ?


Yes that was Libre office calc…

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Looks to be intersecting at 2.5° so your heat loss coefficient comes out at 164 W/K plus an additional 410 W from appliances and humans (which is why you don’t need heating on when it’s only slightly cooler outside).

Mine looks like this (includes residual heat of ~880 W), so HLC comes to 260 W/K.
So, at -2°C I’d only need 5 kW from the 11.2 kW heat pump {grumbles}.

(data from last winter)

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Ooo very nice and a great spread of values there as well … it’s not just me then :slight_smile:

It would be great to be able to do this analysis on heatpumpmonitor.org wouldnt it!? and populate a meta field for easy comparison with assessment heat loss…


Indeed. There are some subtleties for getting it accurate, like considering other electrical usage, discounting hot water use, etc, but I guess if we apply the same basic formula to every system they will at least be equally inaccurate.

For example:

  • compute heat kWh / demand temp for each day
  • where demand temp = inside - 4° - outside
    (heating typically only required when outside is below 15-16°C)
  • ignore days where demand temp < 1
  • take the average over year

Caveat: this is a formula I’ve just made up, but I think it’s sound.