1) No you don't need a new EPC - you just need a recent one (less than 2 years) for the RHI. So you will be fine, he doesn't know what he's talking about.

2) No, you don't need to change the rads if you don't have to. We kept literally everything the same in our house bar the ASHP and water tank.

Source: I applied for the RHI in August 2021 with the Green Homes Grant. My ASHP was installed in July

@batalto is right and I would be a bit wary of your second installer.  You shouldn't have to 'suck it and see' because an accredited installer should be doing a full room by room heat loss calculation that will tell you whether your rads are big enough to heat your house. That's part of the MCS process.  To calculate your RHI payment, the SCOP of your system is used.  If your radiators are too small and you need a constant, say, 55 degrees flow to heat your house your SCOP will be lower and you'll get less RHI.  Plus, your ASHP will cost a fortune to run. It sounds like they either don't know what they're talking about or think they can fiddle the application for you (which they probably could). 

I'd stick with the original installer.   

Thanks for the replies, pretty much confirms what I thought. Seeing him pull up in a new Merc  put me off too... 🙄 

@batalto hi. re not changing rads - what did your heat loss survey suggest? Ours is suggesting some of our current rads are between 80-95% under the ideal wattage output. Were yours all roughly 100%?

@ianjrw, that's a good question. Our survey indicated that two rooms needed rad changes, which we didn't do. As it turned out they were undersized – new K3s fixed the issues. A properly conducted heat-loss calculation should be accurate.

Kev and Batalto are spot on with their responses. 

An interesting comment was posted on our My Home Farm site 1 or 2 days ago about something similar-ish. Rad sizing and RHI payments. Granted, they have a bivalent system, but still very interesting. Scroll down to the comment from Di: https://myhomefarm.co.uk/going-bivalent-ashp-hvo

Posted by: @ianjrw

@batalto hi. re not changing rads - what did your heat loss survey suggest? Ours is suggesting some of our current rads are between 80-95% under the ideal wattage output. Were yours all roughly 100%?

Best efficiency from a heat pump should be achieved when the heat emitters provide the maximum heating capacity, so that the heat pump can operate with the lowest water flow temperature.

Obviously, in the real world, the heat loss of different rooms varies, as does the size and type of heat emitters, so for best overall performance it is important to try to match the heat emitters to the heat loss, such that a similar ratio is maintained from room to room.

A larger room with a heat loss of 1000W at an outside air temperature of -2C, may have a radiator with a maximum heat output capacity of 2000W, whereas a smaller room with a heat loss of only 500W at -2C, would only require a radiator with a maximum heat output capacity of 1000W. If a 2000W radiator was installed in the smaller room, then its output would need to be scaled back, otherwise the room would become too warm.

By calculating the heat loss of each individual room, and then obtaining the specification for all heat emitters installed, it should be a simple matter to calculating the ratio. Then see if it is possible to change the heat emitters in any rooms with a low ratio, so that they more closely match the rooms with a higher ratio. The closer the match from room to room, should provide a system that operates in a more balanced and efficient manner.

@derek-m gotcha ta. I guess my heatloss survey has surprised me a bit. We're a 3 person, 3+1 bed, smallish 110m2, fairly well insulated home and its calculated  4366kWh for water and 15829kWh for heating. This (especially the water) seems high to me and i need to trust it to trust that the rads its specing are right? Have members posted their HLs anywhere so i can check whether mine's seeming reasonable?

@editor thanks. Blog helpful! Yeah, I guess I'm questioning the HL survey...

Posted by: @ianjrw

@derek-m gotcha ta. I guess my heatloss survey has surprised me a bit. We're a 3 person, 3+1 bed, smallish 110m2, fairly well insulated home and its calculated  4366kWh for water and 15829kWh for heating. This (especially the water) seems high to me and i need to trust it to trust that the rads its specing are right? Have members posted their HLs anywhere so i can check whether mine's seeming reasonable?

Hi Ian,

I think that there may be some confusion between heat loss calculations, and what you have stated in your post, which I believe to be an annual energy demand assessment.

The annual energy demand for hot water is based upon set criteria, which should be stated in the assessment. So in your case, it has been assessed that on average, it will require approximately 12kWh of energy each day to supply your hot water requirements.

Annual energy demand for heating should be based upon the detailed heat loss calculations. In your case, the assessment has stated that throughout the year, it will require 15829kWh of heat energy to keep the rooms within your home at the specified temperatures. Obviously the bulk of this energy will be required in the Winter months, with much less in the Summer.

The detailed heat loss calculation is a room by room assessment of the amount of energy that would be lost from each room, at a specified indoor temperature (normally 21C) and a specified outdoor temperature (in the region of -2C or -3C). Totaling the heat loss of each room gives the overall heat loss for your home. Obviously the heat loss will vary with changes in indoor and outdoor temperature.

If the overall heat loss from your home is say 6000W at -3C, then at +3C it will be in the region of 4500W. To maintain a constant indoor temperature over a 1 hour period, would require a heat energy input of 6kWh or 4.5kWh respectively.

If the overall heat loss of your home is 6000W at -3C, and the total capacity of the heat emitters is 12000W, then a heat pump would require an electrical energy input of approximately 3kW, to produce a water flow temperature of approximately 50C to meet the heat demand.

If the total capacity of the heat emitters is 18000W, then the electrical energy requirement would be 2.45kW and the WFT would be 42.5C.

If the total capacity of the heat emitters is 24000W, then the electrical energy requirement would be 2.2kW and the WFT would be 38.2C.

It can therefore be clearly seen that the sizing and overall balance of the heat emitters is vitally important in achieving the most efficient operation of a heat pump based system.

I hope this makes things clear, but please ask about anything that may need clarification.

@derek-m really helpful Derek, ta! You may yet hear from me again 😉