@jamespa I used an assumed indoor temperature of 18C. Is this what you mean by nominal heating temperature? If i reduce the indoor temperature to 15.5C for the calculation, then the estimated peak heat loss drops to 6.7kW

Posted by: @ad3628

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@jamespa I used an assumed indoor temperature of 18C. Is this what you mean by nominal heating temperature?

Thats probably fair enough.  The degree-days calculation (on which quite a few heating formula are based) uses the formula

sum (max (Tbase-OAT ,0))

where Tbase is usually set at 15.5 (not sure why this figure is chosen).  This basically ignores completely any time periods where the OAT is above Tbase.  In the height of the season, when the OAT never rises above Tbase, thats the same as  Tbase - average (OAT), so degree days (which are easily downloaded for a load of whether stations) can be used as a proxy for OAT - which is what I did in my graph.

Another way to look at is that your house benefits from waste heat from your electrical appliances.  Given that the average house uses about 3.5MWh/year, and you can reasonably assume all of that ends up as heat, this is 3500kWh/365/24 = 400W.  You can add 100W per person to that, so 600-800W in total which is not insignificant.

Personally I preferred simply to plot gas consumption vs average OAT (for the reason set out above you can alternatively use degree days instead of average OAT).  This then automatically takes into account heat sources other than the heating system, and allows you directly to read off the figure at any desired design OAT.  If your water is heated by gas this also includes your water heating, so no need to add that in (that's probably true in your calculation also).

In principle you need, in this calculation, to account for the efficiency of the gas boiler which could be as high as 110%* if its actually condensing, or perhaps as low as 75% if its very old.  Very few boilers are set up to condense because installers set the flow temperature too high and we, stupidly, don't generally use weather compensation for boilers in the UK, so the return temperature is above the condense temperature.  This means that even modern boilers are most likely operating at max 95% efficiency, so measured gas consumption, provided most of the house is warm most of the day and night, really could be considered an upper limit on your house heating demand.

from what you have said I think there is a fighting chance 8kW will suffice, particularly if you accept that a little supplemental heating may be needed if there is an extended very cold patch.  Obviously its your call and also depends on whether you can persuade an installer to take any notice of measured consumption (some will, others wont)

Going back to the low loss header matter and noting that this fixed your neighbours system, please note that its actually the secondary pump that does the job, because it provides enough oomph to push the water through the narrow pipes.  The LLH decouples this from the primary, isolating the installer from any potential warranty issues with the boiler.  A better solution is simply to have a sufficiently powerful pump in the first place, but of course most boilers and many ashps come with a pump included (which is normally powerful enough).  This is another matter for your designer to get right - I would be looking for a solution where there was a single pump of sufficient power not two pumps and an LLH, and if that means restricting your choice to an ASHP that does not have a bundled pump then so be it.  Alternatively it may be that changing a few selected lengths of pipework will suffice, again this does depend on the layout and crucially on the actual demand, which is why, in your particular case, a methodical approach by a knowledgeable designer is required.  If you can deduce the pipe layout and in particular when pipes change sizes it will likely help.

Hope that helps.

* of course boilers aren't really 110% efficient, that would be impossible.  the declared calorific value of gas doesn't include the latent heat of condensation which is recovered in condensing boilers, so is less than the energy that theoretically can be extracted.