I think you’re on the money with your analysis Tristan.
Alongside the loss in efficiency that obvious comes from a higher flow temperature due to an overstated heat loss there are two other effects:
- The system cycles on the room thermostat. This isn’t really a problem from an efficiency perspective but its a lost opportunity. Leaving aside any efficiency benefits, running your heating where the radiators produce a constant heat is just nicer to live with than the traditional blast of heat for an hour or two and then waiting for the room temperature to drop before doing it again. When talking to other people about my heat pump one of the things I try and get across is that it is simply better than a gas boiler.
- By overstating the heat loss you may force a home owner to go down a more disruptive and expensive retrofit than what could have been done. Reducing the heat requirement by 30-50% might mean that existing pipework and radiators are suitable, vastly simplifying a switch from boiler to heat pump. Yes, replacing everything will allow the lowest possible flow temp and most efficient install but we also need to be pragmatic about what is realistic for mass adoption of heat pumps into existing UK housing stock.
I think one of the key issues in heat loss surveys producing overstated results is that there is a cumulation of errors all in one direction. The U values for things like windows & insulation err on the side of caution, the surveyor errs on the side of caution when assessing the house, the MCS methodology errs on the side of caution by excluding solar gain and heat produced by living in the house, etc. It then errs on the side of caution with their selected desirable room temperatures, and again with air change rates. Then finally once you have the heat loss (which is already overstated) you then select the next available heat pump size up.
I’m not sure what the answer is. Does the heat loss methodology get improved to produce a more accurate result, or do we just learn to apply a scaling factor to it?
I did my own heat loss calculation using the MCS spreadsheet when designing our system. The end result was very similar numbers to what the installer produced. They probably overstate the heat loss by 30-40% from what it seems to be at MCS specified room temperatures, and ends up being more like double what our actual heat loss is for our chosen room temperatures. Having said that the theoretical calculation correctly predicted the relative room temperatures. The rooms we wanted to warm up easily do, and the rooms which were a bit marginal on the heat loss spreadsheet are the ones we struggle to keep warm at very low flow temperatures.
The crux of Tristan’s question though is how do we make the best of an installed system, if the system is a bit oversized and a weather curve that is a bit steep. I don’t think more accurate calculations is the way forward, there’s too much reasonable variation in people’s desires. I think the answer is moving towards a system which works out the best way to drive the heat pump for the house it is in. Some heat pumps have auto adaptions or WDC modulation based on what the inside temperature is doing which go some way to achieving this. There are other people on this forum, and companies like Homely, working on similar systems where there is a specific control system which then interfaces to the heat pump.