Is Your Heat Pump the Right Size Based on Your Heat Loss Calculation? 

I think one of the main reasons heat loss calculations overestimate the size of the heat pump required is that the calculation gives the gross heat loss, and the heat pump is sized to supply this amount of heat, but this does not take into account other sources of heat in the house! Heat is supplied to the house from all the electrical appliances (including lighting, fridges/freezers, washing machines and tumble dryers, audio visual systems, etc. etc..), cooking, hot water use, people, and the biggie, passive solar gain through windows. A house with large south facing windows, and/or, as I have, a large glass roof lantern over a kitchen dining extension, will gain a lot of heat. Think about doing a heat loss calculation for a greenhouse, you'll come up with a very big number for the heat loss through the single pane glass, but completely missed the solar gain that the greenhouse is specifically designed to provide! You need to add a heat GAIN calculation from all the non-heatpump sources of heat to get a net heat requirement for the heat pump to supply.

The heat loss survey we had suggested a 16kW heat pump, as we have data from our gas boiler for the last 12 months we have a fair idea of the power needed by gas boiler, about 9kW. Heat loss engineers at a heat pump company dismissed the real data. If we had followed the suggested 16kW heat pump recommendation we would have had a heat pump cycling and reducing its COP. Net result we are sticking with our gas boiler for now.

Interesting comments about air exchanges. When I did a heatpunk survey the defaults seemed so high. My 1988 house just isn't draughty. I guesstimated it was more likely 1 or lower. A proper test is the only way to really know though.

I provide heat loss surveys for heat pumps and always try to benchmark my room by room heat losses against gas smart meter data where possible (80% of homes), which I then adjust downwards for gas boiler inefficiency and upwards or downwards depending on whether the homeowner keeps rooms cold or hot compared with MCS room temperatures. The MCS air change defaults are far too high, averaging 1.5 ACH (air changes per hour) whereas the average home at average wind speeds is typically 0.5 ACH, so the MCS by default overstates air permeability by a factor of 3. The MCS also has default U values which are far too high for many elements e.g. 2.1 for triple glazing whereas modern good quality triple glazing Uw's are typically 0.8 to 1.0. I spend a lot of time during the survey verifying fabric, trying to observe cavity wall depth and insulation, asking home owners when the insulation was installed, building reg certificates, invoices for new replacement windows etc. Generally using this more careful methodology my room by room heat losses are within 10% of the smart meter data. The MCS also doesn't account for internal heat gains: metabolic, appliances, hot water, solar etc., which is typically 0.4 to 1.0 kW which the heat pump won't need to produce. Where I have been asked to do heat loss calculations to check other installers calculations I see them typically 50% to 200% too high, worst case has been 400%. Often the root cause as suggested by the homeowner on the video is completely missing cavity wall insulation. The typical side effect of this is the heat pump short-cycles inefficiently, and that the weather compensation curves are typically set inefficiently too high. Generally oversizing the heat pump slightly is ok, some short-cycling appears not to reduce performance too much but getting weather compensation curves too high does reduce performance, but can be fixed post installation. Government research suggests that the average UK heat pump is 280% efficient, compared with Heat Geek installations which have more careful design which typically achieve 400%.

Wall insulation question - can you not take the temp of the internal wall and the same spot on the external and have a calculation of the potential loss of heat?

Is there a workaround within the MCS system of oversizing? The default desired temp feel so high (22 degrees). Is this really a requirement in the UK. If not can it be reduced at the design stage? Impacting the heat loss as a result.

Would like to see an episode discussing door blower tests and how to go about getting one and MVHR and how it works together with a heat pump

Guys, you said divide gas usage by 2000 while this guys says 2900: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-hCeghKa3liM.html (see 8 mins 7 secs for the final calculation and 6 mins 45 secs for the explanation). That's quite a difference and I was wondering why?