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Hi to the boffins

Big Jimbo

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Good afternoon to all the smart people. I have completed the J.Harris heat loss spreadsheet, and it has given me loads of info, none of which i understand. I know/hope that when i have my pre construction EPC done It will give me actual information that somebody else might be able to use for things like working out what size heating, and hot water generating system i will need for the house ?

Because i am still waiting for my proper building regulation drawings i can't as yet have this done. I need these because they should show how i intend to improve many of the thermal leaky areas, such as around windows, door bottoms, roof eaves etc. My method of improving these elements have been drawn up by me and wshould be shown on my building reg drawings.

In the meantime i am costing things up. For instance i know that i am going to have 4 en-suite bathrooms, and that equates to 1 bath, 3 showers, 1 shower over bath, and 4 sinks. The house will also have 3 further sinks downstairs. (W.C. kitchen, and utility) I want to potentially have the ability for all 4 showers to be used at the same time in the morning, and because of this i am trying to work out what size hot water storage i might need to make this possible.

I was also hoping that using the heat loss calculations that i have obtained from the spreadsheet, i would be able to work out aprox what size of ASHP i might need to provide the hot water, and the heating. 

Any idea how i might be able to extract the heating requirement from the spreadsheet, and find out the hot water requirement with reference to the above and use both the figures to size an ASHP ?

If it ain't simple, than i ain't gonna be able to do it. If it can be done, it would help me at least get some idea of a heat pump and tank size required. Is anybody else going to be able to calculate those things for me prior to getting my Building regulation drawings done, and my pre built EPC ?

Thanks in advance for your thoughts.


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8 minutes ago, Big Jimbo said:

Good afternoon to all the smart people.

That’s me out then…… I did no calculations but copied our Jeremy’s principles as our houses were similar in size, (sucks finger and holds it up 🤣).

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In simple turns your heat pump is sized to heat the house and also have time to sort the hot water out.


So if your -2/3 day requires 4 kW, and your cylinder heating takes an hour, it's (4 x 24) / 23, so you need a heat pump that can give 4.2kW at -3.


Saying the above, my heat demand is just over 3kW and I have a 6kW heat pump, but have designed my system so the heat pump doesn't short cycle, when heat demand is lowest.



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Just got home from my weekly 600 mile drive, so hopefully this will make sense.


The number of bathrooms/showers and basins is irrelevant, it is the usage.  Think about that and then use the government issue of 150 lt of water a day, half of which is hot.


Heating sizing in not just to do with the maximum load you will ever see, it is also to do with how often you can expect to need that maximum load.

So down here, my minimum temperature is -2°C, but that is only for a few hours, and generally not the same few hours i.e. non contiguous.


Luckily the solution to both the spasmodic bathing and inconsistent heating is the same, supplementary heating.

The hot water can have a 6 kW heating element in the cylinder, and maybe an inline heater or two.

A wet heating system can also have a secondary supplementary heater i.e. not one that is built into the heat pump.

There is nothing, once the design is sorted, to stop both the heat pump and the supplementary heaters running at the same time, or sequentially i.e. ASHP to raise the temperature by 30°C then an electric heater to raise it another 15°C in the case of DHW.  May work out more expensive than the ASHP filling the cylinder to 45°C in the spring, summer and autumn, and most of the winter, but for those few hours when it is silly cold, and it is generally hours (144 of them in a week) not days, it is overall cheaper.

It also give you a back up system if something goes wrong with the ASHP.

Just has to be designed now and not as an after thought.


Hope that makes sense, my eyes are tired and my ears are ringing.

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3 minutes ago, Big Jimbo said:

That is tragic. I have had at least 14 cups today.

Been drinking Lidl version of redbull, called Kong, should keep me up for a while.

I always open a tin at Swindon, keeps me alert till past Bristol. Would hate to have an accident there and wake up to @Pocster bringing me grapes. Or gropes in his case.

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