Sizing ASHP according to heat loss calcs

[flanagaj]

1 hour ago, MikeSharp01 said:

Ah yes but will you also be showering and cooking at the same time? Anyway as you say just dump the electric showers and all will be well.

 

Back to sizing the ASHP - you have all the data you need so looks to me like you need a 5kW unit. The one we are looking at is 6kW the modulates down to 1.8kW - according to the blurb, we have a heating load of of 1.2kW at -1^oC (We are in Kent so not the -10^oC of @JohnMo and @ProDave )

So a 5kW unit which can also operate in cooling mode will then suffice for both heating and cooling.  Regarding the modulation.  What about when it's 10C outside and your heat loss is < 1kW.  If the unit does not modulate that low, does it simply mean it will be cycling?

[ProDave]

6 minutes ago, flanagaj said:

So a 5kW unit which can also operate in cooling mode will then suffice for both heating and cooling.  Regarding the modulation.  What about when it's 10C outside and your heat loss is < 1kW.  If the unit does not modulate that low, does it simply mean it will be cycling?

You have a room thermostat somewhere.  The heating will run until the rooms are warm then shut off.  With a well insulated house it will be a while before it turns back on again.

[JohnMo]

6 minutes ago, flanagaj said:

So a 5kW unit which can also operate in cooling mode will then suffice for both heating and cooling.  Regarding the modulation.  What about when it's 10C outside and your heat loss is < 1kW.  If the unit does not modulate that low, does it simply mean it will be cycling?

Here is mine, put out 22kWh in 24 hrs, so less than 1kW average.  In cooling mode, note the cop when running and average CoP over day.

 .

 

Zoomed in to show cycling, which all nice and controlled. That's how they do capacity control below minimum modulation.

 

[JohnMo]

8 minutes ago, ProDave said:

You have a room thermostat somewhere.  The heating will run until the rooms are warm then shut off. 

You don't even need that. As you see my heat pump was off (except spike doing DHW at 0500) from 0100 to 1300. That's just letting the ASHP run when it's controller says to - nothing in house at all controlling that, just sensing return temperature and outside temperature influences.

[sharpener]

2 hours ago, flanagaj said:

So a 5kW unit which can also operate in cooling mode will then suffice for both heating and cooling.  Regarding the modulation.  What about when it's 10C outside and your heat loss is < 1kW.  If the unit does not modulate that low, does it simply mean it will be cycling?

 

Yes. E.g. the Vaillant 5kW will go down to about 1/3 output or about 1.7kW

 

Their 3.5 kW unit is simply the same with a s/w limit to 85rps compressor speed, so does not modulate any lower than the 5kW does. Below that they will cycle but if there is a reasonable system volume that is not a problem, the (adjustable) Energy Integral algorithm limits it to about 2 cycles/hr, anything less than 3 is generally thought to be OK.

 

I believe you have already discovered this supply sizing tool but it may be useful to others 

https://www.ssen.co.uk/our-services/tools-and-maps/demand-calculator/.

 

I still think you are worrying unnecessarily, you won't need to be cooking with both ovens and charging the car while using the machinery in your workshop. 100A will be plenty, no need for 2 or 3 phases which may cost £1000s

 

[JohnMo]

10 minutes ago, sharpener said:

Vaillant 5kW

One on eBay at moment for about £1k nearly new, removed as under sized for heat load. Billy bargain if you are down south.

[Dillsue]

6 hours ago, flanagaj said:

So a 5kW unit which can also operate in cooling mode will then suffice for both heating and cooling.  Regarding the modulation.  What about when it's 10C outside and your heat loss is < 1kW.  If the unit does not modulate that low, does it simply mean it will be cycling?

Remember that the 5kw is the heat output to the house. What youre interested in is the energy input as that is the electrical load. If you're heating the DHW to a high temperature on a cold winters night you might get a terrible COP of 2 so the electrial input will be 5kw/2 = 2.5kw. That's the worst case load your 5kw heat pump is going to draw, not the 5kw output.

 

For your workshop all your equipment will only draw the rated full load power when it's working at it's hardest....under full load. Unless you're an Octopus you'll only be drawing full load on one machine or the other. The machine left running will be drawing a fraction of the full load.

 

The only significant continuous load will be your EV charger but you can schedule that to run over night when the only other load may be your ASHP and a few hundred watts background load for the house. 7kw +2.5kw + 0.3 = 9.8kw overnight load when the ASHP is working hard.

 

Like you we have double ovens and an induction hob but once heated all of those switch intermittently so they will only ever draw the full rated load if they are all switched on at once. Diversity says that won't happen but if it did its unlikely to cause a problem as you're still well away from the 25kw supply rating.

 

Occaisional realistic daytime load might be 1 oven,  0.5 hob, 1 workshop machine working hard, 1 workshop machine running idle, ASHP, house background = 3kw + 2kw + 2.2kw + 1kw + 2.5kw + 0.3kw = 11kw. That's less that 50% of a 25kw supply