Going back to the heat loss calculation process. As we’ve discussed re the grant and trying to match fuzzy number 1 the heat loss and fuzzy number 2 the heat output capacity of the unit. I think thinking of the first as a particularly fuzzy number is a useful way of looking at this.
Here’s the process I try to follow:
-
Calculate a more likely to be reflective of real world design temperature heat loss first. Just use an air change rate of 0.5-0.6 ACH for this for all rooms in the dwelling (assuming basic double glazing has been fitted sometime in the last 20 years). If terraced or semi-detached, assume neighbours are heated to 18C if you do have neighbours occupying the properties next door. This comes out at 3.3 kW for our solid stone mid terrace. This is the heat loss that is most likely to correspond the maximum heat demand on the coldest day in reality, in kWh this is 3.3 x 24 = 80 kWh of heat.
-
Use any information that you have on previous heating, ideally daily gas smart meter readings if you have them to cross check this figure. Alternatively with annual gas consumption e.g 11,000 kWh of gas consumed (we didn’t have gas previously so this is more an example) divide by 2.9 (re michael podesta conversion factor) = 3.8 kW. Reasonable agreement, we’re just checking here for multi kW differences.
-
Applying a 30-40% margin on top of 3.3 kW gives 4.3 - 4.6 kW for the heat pump unit size. In reality that will be a 5 kW unit but the real world max output capacity during defrosts will likely be in the 4.3-5.0 kW range depending on the model and conditions.
-
Work out what the margin between 3.3 kW and say 4.3 kW gives you in terms of the capacity to deal with more unusual events such as a combination of a strong wind storm with sub zero conditions. In my case my heat loss would rise from 3.3 kW to 4.5 kW if the neighbors were unheated and the air change rate increased to 0.8 ACH. I would push beyond the capacity of the heat pump if the air change rate was 1.24 ACH and both neighbors were unheated = 5.1 kW (perhaps consider here what other sources of heat you have available e.g a 3kW fan heater removes any concern about a missing few 100W of heat pump capacity, and electric may be cheap and green during strong wind storms, assuming of course that there isn’t a power cut!)
-
Finally: Use the most likely heat loss (e.g 3.3 kW in my case) to work out the design temperature of the system and starting point for the weather compensation curve. If you are designing a new system use this heat loss to spec a radiator system that can run at 35-40C flow temperatures at design temp if you can. You can always bump up the curve if needed.