Done Heat loss calc but what does it mean for ASHP sizing

[Chanmenie]

So have filled in Jeremy's Heat loss spreadsheet 

does this mean my heat loss is 3.4 KW per day in December 2 -10deg  ?   if so what size heat pump should I be looking at 

 

[ProDave]

It means when it's -10 outside and +20 inside the house will need a continuous heat input of 3.4kW or averaged over 24 hours 81.6kWh

 

Sizing the heat pump, you have to allow for it also heating hot water (when it won't be heating the house.

 

So a 5kW ASHP for instance, assume it will be heating hot water for 2 hours per day, and lets guess that it might only manage 4kW output at the low temperature, it could deliver 88kWh so should be adequate. But would need to run almost 24/7 to manage when it is that cold.  So if you wanted the heating off over night for instance you might consider something a little larger.

 

I don't imagine it gets that cold for very long in Norwich so you might not need that much heat very often.

[JohnMo]

If you read MCS manual it states

"Heat pumps should be selected as closely as possible to the design heat demands. MCS Standard MIS 3005, requires the unit to achieve 100% of the duty at an external temperature 
condition exceeded for 99.6% of the year, if reasonably practicable. "

 

So the delta T of 25 should really be used - 2.8kW.  not 30 degC which possibly only occurs for a day or so per year.

 

So you need to look at the data sheets and make sure at the output can match that.

 

You need to make sure you can turn down enough at warmer temperatures (much more normal than the lowest extreme) so that you don't short cycle

 

[Nick Laslett]

I got the impression that DHW reheat time was also something to consider when sizing an ASHP?

 

Of course you can just switch on the immersion if you need to replenish hot water in a hurry.