@cathoderay thank you, that is a very helpful reply.

shark infested water...

1. manufacturers can mislead with model sizes
2. installers serve themselves first and not your needs
3. MCS nudges heat loss estimates too high with overstated ACH guidelines

no wonder I'm finding this forum and associated YouTube so rewarding!

@matwin - a good summary, though in fairness I should add there are also a few friendly dolphins out there as well. The problem, as many have discovered, is finding them, because it is not unknown for sharks to dress up as dolphins.

This forum is a great resource. It has both accounts of individual heat pump 'journeys' with lessons learnt, and a lot of more generalised practical expertise. It is also remarkably good natured, largely due to the tone set by our most esteemed leader, Chairman Mars. We also follow the 'only dumb question is the unasked question' maxim.  

Posted by: @matwin

↑

Heating Degree Days

Heat degree days are not always that straight foreward. There are websites that will automatically calculate. But you have to be careful the weather station data is actually representative of your house, plus you need to set the base temperature, this is the temperature you actually stop needing to heat the house. 

Things that affect the above

Local micro climate for house or weather station in a micro climate

House airtightness and insulation levels of house. Well insulated and airtight, this drive down the outside temperature at which heating is needed. Very poor insulation the opposite. Either end of the scale gives very different HDD output numbers - how do you know what is the correct number

House inside temperature also changes the outcome

Posted by: @matwin

↑

MWh

To be representative, the heating system operation needs to be similar to the heat pump, heat pump design (to get grant money) needs house to be at given temperatures, if you operate the house at 16 to 18 then gas consumption will not be representative.

So there is a possibility of a calculation that is very miss leading, house heated differently, miscalculated HDD, equals rubbish in, rubbish out.

A heat loss calculation, using the best known data isn't generally that far out, add in a bunch of assumptions, again rubbish in rubbish out.

A simple whole house heat loss calculation is fine for sizing heat pump. Room by room is required for emitter sizing. Emitter sizing is required for a balanced heating system.

Posted by: @cathoderay

↑

heat pump puts out less than it should according to its badge rating, but at the same time the spreadsheet based heat loss was an over-estimate of the actual heat loss

Not saying you are wrong, but what temperature should they pick, different countries have different minimum design temperatures. Even in this country it varies, your design temperature is most likely -3, while mine is -9.  So do they sell me a different badge, to one they sell to you? The designers job is to to match a heat pump to the location and house. They have to read the performance chart for each and every install if they are designing correctly. So really doesn't make a jot if difference if they add a number or just call them small, medium, large and extra large, or well insulated, average or leaks like a bucket house.

My view who cares what number they feel they should add to a heat pump, you still need to go back to base data and pick a heat pump suited to the application.

Posted by: @johnmo

↑

what temperature should they pick, different countries have different minimum design temperatures. Even in this country it varies,

My installer used the Freedom heat pump spreadsheet, which until recently was widely available and widely used. It is highly MCS focused (for grant applications) with design outside air temp (OAT) and room temps in effect set by MCS, who if I remember correctly get them from somewhere else, possibly a crystal ball. This particular spreadsheet also has output data for a number of heat pumps, and allows you to match heat loss to the heat pump output. What it didn't say on the tin (it is instead on hidden sheets which you have to unhide) is that the Midea heat pump outputs were guesstimates.

Another source of spreadsheet error not to be overlooked is typos. For a while my largest room with three external walls (the kitchen) had an external wall length of 2 metres. It is actually 12 metres.

I remain convinced that if you can do an empirical heat loss, it always trumps a spreadsheet. Spreadsheets are in my view, as many will know, a form of whatiffery (what if the ACHs are 3, what if the OAT is -2 etc etc), and as you say, the output is totally dependent on the input. Get something in the input wrong, be it a typo or a wrong what if assumption, and the output will be wrong. An empirical (ie measured) heat loss on the other hand if done properly involves few or even no assumptions. Done properly here means steady state room temps plus accurate enough outside air temp and energy delivered to the house data. Given steady state room temp, the energy delivered to the house is the heat loss at each outside air temperature. Plot energy delivered against OAT and then read off the heat loss at your design OAT.         

Posted by: @cathoderay

↑

design outside a

ir temp (OAT) and room temps in effect set by MCS, who if I remember correctly get them from somewhere else, possibly a crystal ball

Room temps are set temperature for different rooms apply to all buildings. OAT is a simple equation to cover 99.7% of likely minimum outside temperature. So ignore the lowest temperature for 2 days.

Posted by: @cathoderay

↑

An empirical (ie measured) heat loss on the other hand if done properly involves few or even no assumptions. Done properly here means steady state room temps plus accurate enough outside air temp and energy delivered to the house data

Perfectly acceptable way to do it, but to do it properly, you and your family get to go into a hotel for several days, while data collection is completed. Several thousands of pounds later, you get a nice spreadsheet of data showing heat loss over a range of temperature. And an even more expensive install when you factor in this added cost. So you need to be realistic.

A spreadsheet model is perfect acceptable, as long as the person completing know what they are doing. And have suitable data to input.

Posted by: @cathoderay

↑

Midea heat pump outputs were guesstimates.

Midea like every company that manufacturer, publish full data for their heat pumps. Midea chart every flow temp and outside temp on a table for each model.