Replacing oil boiler with a mix electric/heat pump

[Ben100]

Hi,

 

I'm looking at options to replace my old oil boiler/radiator system with an electric boiler/under floor heating, with the option to add an air source heat pump later on and have the electric boiler as backup/top-up.

 

I've done a bit of research, but this is not an area I'm an expert in. I think the best option would be to go for an electric boiler with large hot-water cylinder and under floor heating. Eventually I'd like to have an air source heat pump and use the electric boiler as a backup/top-up, but as I'm in the middle of building my house I just don't think my budget can stretch to one initially. So my question is "is this possible and is this a good idea?"

 

cheers!

[ProDave]

I am not sure what you are asking?

 

You talk about building your house so you have the perfect opportunity to get the insulation right and design it to work from under floor heating to which an ASHP is perfectly suited.

 

But I am confused because you mention replacing an oil boiler, so is this a new house and old house or 2 separate houses?

[Carrerahill]

I would say an electric boiler is not a good idea unless you have some solar/wind on site to generate electric for you. If you are going electric then I wouldn't bother with the boiler, just fit electric heating but then I don't much like the idea of electric UFH from a fault and longevity point of view.

 

Any figures I look at for electric boilers just seem insane. You are about, say, 4p for a kWh of gas and about 13.5p for kWh of electricity. Boilers are in the 10's of kW's range. Our gas boiler has a max output of 32kW - even if we disregard all the conversions and just take some simple numbers if it was running about 50% capacity on electric that is 16kW - that is going to cost £2.16 an hour! 

 

Then think about all the losses in getting the water to the heating area etc.

 

 

Edited February 3, 2020 by Carrerahill

[Ben100]

2 minutes ago, ProDave said:

I am not sure what you are asking?

 

You talk about building your house so you have the perfect opportunity to get the insulation right and design it to work from under floor heating to which an ASHP is perfectly suited.

 

But I am confused because you mention replacing an oil boiler, so is this a new house and old house or 2 separate houses?

 

So I'm refurbishing an old property and adding a large extension to it. Almost like building a house next to an old house and knocking through. The new part will be built to a good standard of insulation, and the old property will be refurbished with insulation on the walls, between joists, etc.

 

I've also got plenty of room inside and out for boilers, cylinders, etc.

[Temp]

Could you keep the oil boiler until you can afford ASHP? Electricity is 2-3 times the cost of oil at the moment. 15-20p vs about 6-8p per kWH I think.

 

 

 

 

[Ben100]

3 minutes ago, Carrerahill said:

I would say an electric boiler is not a good idea unless you have some solar/wind on site to generate electric for you. If you are going electric then I wouldn't bother with the boiler, just fit electric heating but then I don't much like the idea of electric UFH from a fault and longevity point of view.

 

Any figures I look at for electric boilers just seem insane. You are about, say, 4p for a kWh of gas and about 13.5p for kWh of electricity. Boilers are in the 10's of kW's range. Our gas boiler has a max output of 32kW - even if we disregard all the conversions and just take some simple numbers if it was running about 50% capacity on electric that is 16kW - that is going to cost £2.16 an hour! 

 

Then think about all the losses in getting the water to the heating area etc.

 

 

 

The UFH will be a wet system, but just heated using electricity. I'm not on the mains gas grid so it's oil or electricity for me. The reason I'm considering high running cost electricity is I'm planning to add an air source heat pump later down the line. I also really hate the big oil tank in the garden.

[Ben100]

4 minutes ago, Temp said:

Could you keep the oil boiler until you can afford ASHP? Electricity is 2-3 times the cost of oil at the moment. 15-20p vs about 6-8p per kWH I think.

 

 

 

 

 

Yes, this might be a sensible option. I guess I could upgrade the hot water cylinder (as it's too small for us) but keep the oil boiler? I'm not sure the boiler would work with the new UFH though... Also, am I correct in thinking that the boiler and heat pump would both connect to the cylinder (heat pump as primary and boiler as backup/top-up)?

[Carrerahill]

18 minutes ago, Ben100 said:

 

The UFH will be a wet system, but just heated using electricity. I'm not on the mains gas grid so it's oil or electricity for me. The reason I'm considering high running cost electricity is I'm planning to add an air source heat pump later down the line. I also really hate the big oil tank in the garden.

Yes, I fully understood that.

 

But my point was, "If you are going electric then I wouldn't bother with the boiler, just fit electric heating". Electricity has the benefit that it can just be wired to where required and used, whereas oil, gas & solid fuels need to be burnt centrally and conveyed via water to the UFH/Rad's for obvious reasons but I just would not use grid electricity to heat water centrally as said above the costs would be high - probably painfully so.

 

However, I understand what you are saying, if you think you may eventually heat the water with an alternative source of heat then can you not just keep your oil boiler going for a bit longer?

 

How long are we talking? Bearing in mind it will be summer before we know it, could you do something about it in September? Or do you foresee another winter or two on the oil?

 

If it was me, and I had the intention of going ASHP fairly soon then I would fit a wood burner with back boiler for now, which would then be a backup and supplementary source going forward. Other benefit is they can be run if there is a power failure with a pump running on an inverter. 

 

 

 

 

Edited February 3, 2020 by Carrerahill

[Ferdinand]

YWorth a note that oil prices are on a steep downward trend while China is partially closed down, which may help in the short term as it is affecting heating oil too.

 

If you are planning to stock up...

 

Were there not news reports at the W/e of the Saudis having kittens?

 

 

Edited February 3, 2020 by Ferdinand

[ProDave]

Indeed oil prices are falling https://www.boilerjuice.com/heating-oil-prices/

 

Can't say I have noticed much fall in petrol prices at the pumps?

[Temp]

Our system is an oil boiler feeding a thermal store which in turn feeds UFH and DHW. That works for us but I don't think it's an optimal set up when it comes to ASHP. In a well insulated house I think it's better to connect ASHP direct to UFH. That's because DHW needs higher temperatures than UFH and higher temperatures reduce ASHP performance. Eg you don't want you ASHP producing high temperatures all the time, only when it's heating DHW. 

 

 

 

[joe90]

BUT a few of us heat water with an ASHP to 48’ fir both UFH (blended down to required temp) and DHW and I don’t believe my unit has ever defrosted or lost COP because of this, we may have slightly bigger DHW tanks as we don’t blend with cold water so much.

[ProDave]

1 hour ago, Temp said:

Our system is an oil boiler feeding a thermal store which in turn feeds UFH and DHW. That works for us but I don't think it's an optimal set up when it comes to ASHP. In a well insulated house I think it's better to connect ASHP direct to UFH. That's because DHW needs higher temperatures than UFH and higher temperatures reduce ASHP performance. Eg you don't want you ASHP producing high temperatures all the time, only when it's heating DHW. 

 

 

 

Most (though not all it would seem) heat pumps operate at a lower set point when doing heating, than when doing DHW.  So they run efficiently.

 

However, a thermal store is a poor choice for an ASHP. you need the water in the thermal store a lot hotter than your target DHW temperature, otherwise your hot water will start to cool down quickly.  An UVC on the other hand you can heat the water in it to 48 degrees and then draw off hot water and the water you get will remain at or very close to 48 degrees constantly until the cylinder runs out of water and it will turn cold very quickly.