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@old_scientist i had a look at this, all my gas consumption for New Years Eve was 85kw. Thats the highest day for two years. Winters have been mild though. -3 was as low as it got, and even then not all day. Even then allowing 6 for hot water and cooking, leaves about 3.33kw an hour.
My boiler is a reasonably odern condensing combi boiler, installed in 2012, but se3rvices once a year and still seems ok, no major problems, so for my calcs i assume around 85 to 88%
Thanks for your feedback.
85kWh of gas usage, divided by 24 gives 3.54kW and adjusted for 85% boiler efficiency gives a heat loss of 4.16kWh. Allowing for DHW and cooking brings the heat loss down to 3.9kW and your calculated figure is right in the middle at 4kW.
I think that's very consistent, which should give you high conviction in your calculated heat loss.
@jamespa Also this coming winter I am going to run my combi system at as low as it will modulate and see how we get on.
This is exactly what we did before getting our heat pump. We had an oil boiler, so unfortunately didn't have accurate gas usage data like you do, so it was more of a finger in the air for us, but we turned the flow temps down to 50C on a really cold day over winter to get a sense check that our radiators (at their present sizes) could still output the required amounts of heat at lower flow temps, and they were fine.
We ended up replacing all our radiators with the largest sizes we could reasonably accommodate in each room which allows us to run at lower flow temps, and now we rarely raise the flow temp above 32-33C all winter (35C if it gets really cold). If you are replacing radiators anyway, fit the biggest you can accommodate in the space available. Don't worry if the heat output doesn't exactly match the heat loss on a room by room basis as heat will flow throughout the house - better to get the flow temps down as low as possible.
85kWh of gas usage, divided by 24 gives 3.54kW and adjusted for 85% boiler efficiency gives a heat loss of 4.16kWh. Allowing for DHW and cooking brings the heat loss down to 3.9kW and your calculated figure is right in the middle at 4kW.
I think this calculation goes the other way. 3.54kW gas is used of which 85% went to the house (the rest up the flue). So the house needed 3kW.
We ended up replacing all our radiators with the largest sizes we could reasonably accommodate in each room which allows us to run at lower flow temps, and now we rarely raise the flow temp above 32-33C all winter (35C if it gets really cold). If you are replacing radiators anyway, fit the biggest you can accommodate in the space available. Don't worry if the heat output doesn't exactly match the heat loss on a room by room basis as heat will flow throughout the house - better to get the flow temps down as low as possible.
Thats a reasonable algorithm, some people worry unduly about oversizing radiators which is really not a concern other than visually and in cost, you can always turn them down on the LSV but you cant turn them up.
That said its worth going specific focus on the most demanding (difficult) room which will almost certainly determine the flow temperature. In OPs case it is very likely the living room which accounts for 40% of the total energy. To a limited extent undersizing of the most demanded room may be compensated by oversizing elsewhere, but I wouldn't rely on it. We thought long and hard about our 'index room' which is also where we spend most of the time as there was no satisfactory solution with a conventional radiator. We ended up taking the plunge on a fancoil and now wish we had done that it a couple of other places where we had difficult to fit radiators.
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.
85kWh of gas usage, divided by 24 gives 3.54kW and adjusted for 85% boiler efficiency gives a heat loss of 4.16kWh. Allowing for DHW and cooking brings the heat loss down to 3.9kW and your calculated figure is right in the middle at 4kW.
I think this calculation goes the other way. 3.54kW gas is used of which 85% went to the house (the rest up the flue). So the house needed 3kW.
D'oh - of course. I shouldn't post before my brain is fully awake!
Samsung 12kW gen6 ASHP with 50L volumiser and large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW). Net Zero
@kilgorexv I think you are missing something in your heat loss calculations - the effect of thermal bridging. Thermal bridging is when you have joins between walls and floor, walls and roof, wall and windows etc. and these joints in the construction lose a lot of heat compared to the theoretical U-values of the individual building elements.
rdSAP10 (section 5.15) has a table of values (Table 21) to use to take account of thermal bridging depending on the age band of the house.
For age bands A to I the factor is 0.15, for band J its 0.11, and for bands K,L, and M its 0.08.
All you need do to take account of thermal bridging is to add this factor to every one of your U-values.
For my reasonably well insulated 1970's house, by my calculation, thermal bridging accounts for just under 20% of the heat loss, so it makes quite a big difference.
@rob-of-york Thank you for that, whilst familiar with the concept of bridging in terms of damp, never thought of it in terms of heating. i have been on the website and i have calculated i need to add something between 0.7kw and 0.9kw. (with the assistance of AI). That leaves me at just below or just above 4kw which is still pretty good for a 1969 1970 bungalow.
There is a simple rule of thumb by Michael De Podesta that is:
Heat loss = Annual gas usage x Design temperature difference / 57.3
For your 9300 kWh gas usage and a design temperature difference of 24, that gives 3895W, which is close to the values that you are getting from your calculations.
If you simply increase all your U-values in column D under building information in your spreadsheet by 0.15, then you will have the same overall accounting for thermal bridging as is used in rdSAP 10.
Finally, MCS is changing / has changed to using ACH = 0.5 for habitable rooms for properties of all ages (and 0.0 for non-habitable rooms).
You might also like to try out a way of calculating heat loss using the heat energy going into the house in a cold month and the temperature increase over the outside that creates. See my post for details and a simple spreadsheet:
Thank you all, i have made several adjustments based upon the advice given including bridging and going with the new MCs figure of 0.5 ACH. Also had a go with some of the alternate fomulae. My next step is to get an EPC done, although the last one i had doe when selling a house seemed like a complete waste of time. Afterwards will try a winter cool run on my combi boiler, although it turns out it wont modulate below 55 degrees. Afterwards. come the spring i will start getting quotes and assessments done, and pray my boiler starts playing up to get over the final major hurdle of a change conservative wife. i am also considerintg a larger inverter for my sigenergy system plus more solar, plus more batteries, but thats just becasue i like toys!
From experience, I think an 8kW inverter is the smallest worth having depending on household. In winter, the ASHP might pull 3kW or so, an oven heating up another 3kW, someone puts on a kettle to make hot water and you have yet another 3kW and so on. Most of those things are short term peaks and usually don't happen simultaneously but you get the drift. I also recommend being careful with battery sizing as you can overdo it. We have a 16kW stack and it wouldn't last a day but it would last long enough that over winter we used £108 of peak rate electricity which is an extra £75 over what it would cost at our off-peak rate (i.e. if the battery had lasted.) To totally cover that £75 so there was no peak rate consumption at all would cost us approximately £5k in extra battery capacity. I actually think you can have too much battery and the money is better invested elsewhere. And if you don't have an elsewhere go on holiday 😀
It's been interesting following this thread. We used 9000kWh of gas in the year prior to getting an ASHP, indicating a 5kW would be sufficient but have ended up with a 9kW ASHP because, well, assumptions I suppose. Interestingly, two installers came to within 200w of each other for heat loss (8.5kW) although one did subsequently re-survey and decided it was only 7.5kW. I wish I'd had a bit more nouse at the time to do what you have done.
Just discovered a major flaw... been using the 99% temperature, not the 99.6!!!!! So instead of a -3 design temp, it should be -6.1. i have attached my latest version in case anyone is anal enough to want to go through it again!
@andrewj we already have a 6kw inverter, 8.55kw of south facing panels, and 16.12 kw of signergy batteries. i am pretty sure its not cost effective to add more, i just want more solar and for that i need more MPPT and a higher export allowance. i also thing better to ask the DNO now as with the reintroduction of VAT in April on installs, and the tax increases on solar panels in China i think we could see solar panels 30% plus higher in the spring.
Just discovered a major flaw... been using the 99% temperature, not the 99.6!!!!! So instead of a -3 design temp, it should be -6.1. i have attached my latest version in case anyone is anal enough to want to go through it again!
Do you really see days where the average temperature is -6.1C over the whole day? You may see a minimum temperature of -6.1C, but I doubt the average is -6.1C
Samsung 12kW gen6 ASHP with 50L volumiser and large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW). Net Zero
