£6200+ for oil boiler change - is quote reasonable?

lohr500

Netexporter said:

Size of the house, changing nearly all the radiators, the need for two units in parallel and upgrading the house electricity supply to three phase all played a part.

Most people won't need a 3-phase heat pump. New, higher temperature, ASHPs may not require radiator enlargement.

Don't think they were an option when we were looking in early 2021. I guess things are continually evolving.

Netexporter

 I guess things are continually evolving.

They are. Propane (R290) has always been known as a very good refrigerant but it is also inflammable, so can be used indoors. As monobloc heat-pumps are entirely outside, any leakage wouldn't be a problem, so the regulations were changed a year or two ago. R290 is also not a powerful greenhouse gas, if it does get out, so you don't need an R-engineer to gas-up the heat pump, saving more money. You just have to take precautions to ensure any leakage won't pool in drains, as it is heavier than air, but that's not too onerous.

I assume the new Octopus Cosy 6 heat pump uses propane and it can achieve flow temperatures as high as a gas or oil boiler. Obviously, the lower the flow temperature, the greater the efficiency, but it does open up more options, like being able to use existing radiators, in some cases.

matelodave

The Cozy 6 is a 6kw heatpump, so you'd need to do the proper sums to see if its enough for a bigger property.

Netexporter

matelodave said:

The Cozy 6 is a 6kw heatpump, so you'd need to do the proper sums to see if its enough for a bigger property.

I was just giving it as an example of a high(er) temperature heat pump. Presumably there will be larger capacity models coming along, but 6kW will probably satisfy a high proportion of situations.

Reed_Richards

Netexporter said:

ASHPs that are run at high temperatures will be less efficient

Not necessarily. R290 can give similar SCOPs at higher flow temperatures.

Or even better SCOPs at lower flow temperatures, presumably.  Whilst there is some scope to make improvements in how close a heat pump comes to the maximum possible efficiency, the maximum possible efficiency goes down as the difference between the outside temperature and the temperature you are heating your water to goes up.  Ye cannae change the laws of physics and the laws of physics say that high temperatures mean lower efficiency.   

Netexporter

It's relative, though. If the R290 pump is a SCOP point higher at any given temperature, then it will more efficient than an HCFC machine.

CO2 is even better for efficient high temperature delivery but, sadly, only works at district heating scales of machine.

Reed_Richards

I'm not disputing that.  What I am saying is that R290 or HCFC, you will get a better efficiency and lower running costs if you operate the heat pump at as low an output water temperature as possible.

Apparently, the maximum possible efficiency (COP) of a heat pump is Th/(Th-TC) where Th is the temperature of the hot side, Tc is the temperature of the cold side and temperatures are measured on an absolute scale, such as Kelvin.  So, heating water from 50 C with an ASHP when the outside air temperature is 0, Th - Tc = 50 C/K and 50 C is 323.15 K so the maximum possible COP is 6.663.  That's a lot more than any domestic heat pump will achieve but I have no idea how much practical room for improvement there is.  We just know that you'll never do better than 6.663.

Heating water to 60 C when the outside air temperature is 0 C, the maximum COP is 333.15/60 = 5.5525

Heating water to 70 C when the outside air temperature is 0 C, the maximum COP is 343.15/70 =  4.9021

Or if you have Underfloor heating that you can run at 35 C (0 C outside) you can aspire to a maximum COP of 308.15/35 = 8.8043  

Netexporter

And I'm not disputing that lower flow temperatures are better. Just that R290 gives more options to be able to re-use existing heating systems at an acceptable COP. The capital saving offsetting the slight loss of COP.

Here's a good example:

https://www.youtube.com/watch?v=kxKEx1z00-4

Reed_Richards

I saw the first 1/3 of that video (then it got too late and I went to bed).  The man bought a fair few new radiators and redeployed some of his old ones so I believe the majority were replaced by different ones, although he already owned some of those.  In my personal experience the cost of replacing radiators was a small fraction of the total cost (I also replaced the majority of the pipework) and I think that getting a high temperature heat pump and keeping your existing radiators is more likely than not to be a false economy.     

smallblueplanet

Netexporter said:

And I'm not disputing that lower flow temperatures are better. Just that R290 gives more options to be able to re-use existing heating systems at an acceptable COP. The capital saving offsetting the slight loss of COP.

Here's a good example:

https://www.youtube.com/watch?v=kxKEx1z00-4

This is too complicated for me but I am interested in finding out more about re-using our existing CH system with an ASHP. We won't be eligible for the govt. grant because they would require our semi to have extra insulation including cavity wall, which I do not want.

So as we will very soon need a new oil boiler and tank as our current install is 20 years old, what sort of ASHPs should I look into instead do you think @Netexporter ?