Greetings from off the gas grid in West Norfolk! We replaced the 1990s oil boiler in our 1980s house at a very fortunate time, by the looks of oil prices now.

Burning HVO isn't as green, is it? Seems more like a not-quite-as-awful hail-mary option for people who can't yet get a suitable heat pump.

@editor Yes, the rads jump from 100mm thick to 160mm for a triple-panel, but that’s not that bad in the scheme of things.

I can’t really do much until I actually own the property, beyond think about the subject. I did manage to take some measurements and look at the existing rad sizes in the main large living/dining room though, so I can do rough what-if estimates and think about general approaches. I’d need to upsize 2 out of the 3 rads in the main space and convert all to triples to get the output I think I’d need for a low-temp heat pump. 

The current oil boiler is non-condensing and floor-mounted inside the utility room, so if I was staying with oil, I’d want to change to an outdoor modern condensing unit and adjust the flow/return temps to get it as efficient as possible as a minimum system change.

It looks like the HVO approach may be a dead duck anyway unless there are exceptions when/if the 2026 new oil boiler ban comes in. I’d rather not lock into a dead-end if there is a sensible alternative.

The problem with a lot of the current policy things is that they are so uncertain and meanwhile life needs to go on - I’ll be doing whatever I need to do to the heating system next year after I have lived with the current system for a winter, not waiting to see what happens in 2026.

Posted by: @cycleneil

Hi All, my name is Neil.

I am in the process of buying a small detached house in a rural village in North Norfolk off the gas grid. Currently the house has an oil boiler, the original non-condensing model fitted dating back to the new build in 2001/2.

First step will be to replace the windows and doors with energy efficient modern products, add extra loft insulation and then carry out thermal loss calculations and measurements to see what can be practically done to improve the heating system. I'm open to any green alternative to heating oil - i.e. HVO biofuel or ASHP could both work.

So far it is very difficult to get an engineer's view (retired) of the subject due to the amount of disinformation and BS around the topic. I'm hoping to learn a lot from the postings on here.

Hi Neil,

As a fellow Engineer I feel certain that you will appreciate that heating a home in a reliable and efficient manner is not quite as simple as many may think. The problem is the wide variation of conditions under which any system is required to operate.

I am pleased to see that your starting point is improving energy efficiency, which is the first thing that I always suggest. Carry out the detailed heat loss calculation on a room by room basis, which will allow an estimation to be made with regard to the size of the heat emitters in each room. Then if necessary increase the size of any of the heat emitters to balance the heat output to the heat demand for that particular room, so that it is not necessary to throttle the output of any heat emitters, and hence reduce overall efficiency.

Whether you decide upon an oil boiler or heat pump, the above estimations should allow you to assess the type and size of system required to meet your needs.

@derek-m Thanks, that was very much my plan - make the place as energy efficient as practical and then work out what heat input the house really needs.

The way the house is laid out maximises space for the footprint by removing the hall completely, so the living area is essentially a large L- shaped room with a non-enclosed staircase up to a landing with bedrooms and bathroom off it. This main room has a single door to a large dining kitchen and I bet we will probably leave this door open most of the time creating one large space which would be all at the same temperature. My initial thought is that this layout should lend itself well to a heat pump solution as the other zones are relatively small in comparison (bedrooms etc.) and can run cooler with trvs to control their local temperature without this massively changing the circulating water volume as they turn on and off.

One thought: I’ll be doing a bathroom refurb before I move in that involves moving a radiator, so the heating system will be completely drained down. Plus all the carpets are being replaced, so the house will be in an ideal condition for any messy/disruptive work.

While I am at it, would it be worth having a heat loss analysis and all the radiator sizes checked and replacing those necessary to run the heating at delta t30 rather than the delta t50 it is currently? I bet that will end up being most of them, but it’s an ideal time to do it while everything is in bits and I’m not living in the property.

That does assume that if I went ASHP later it would have to be a model that could work at a 50deg flow temperature. Is that a safe assumption for a rad-only system with the way heat pump products are going? 

(it also assumes the current oil boiler doesn’t cycle itself to death in the meantime with the increased rad efficiency. That would need some further research later. I don’t know the make and model and what output it is set to yet, so can’t even guess.)

Hi @cycleneil

Initially I would suggest that you look at how you can improve the energy efficiency, and then carry out a heat loss calculation. It may then be possible to select radiators that match, as closely as possible, the heat demand in each room, such that they all achieve the indoor temperatures with approximately the same water flow temperature. It is rather pointless fitting a larger radiator in a room, and then having to turn it down because the room is too hot, when compared to other rooms within the property.

@derek-m, I agree with you. The one thing I will say, however, is that if you have the luxury of space in rooms, I’d still consider putting in an oversized K3. From our experience, the K3s we put in the coldest rooms that the ASHP struggled to heat, came up to temperature comfortably with the larger rads. 

This winter, with the HVO, that’s running a bit hotter at 50C flow, the rooms come to temperature faster, and the smart TRVs turn the rads off when the temperature has been hit. I guess this only works in zoned heating systems where you still have TRVs. 

Posted by: @editor

@derek-m, I agree with you. The one thing I will say, however, is that if you have the luxury of space in rooms, I’d still consider putting in an oversized K3. From our experience, the K3s we put in the coldest rooms that the ASHP struggled to heat, came up to temperature comfortably with the larger rads. 

This winter, with the HVO, that’s running a bit hotter at 50C flow, the rooms come to temperature faster, and the smart TRVs turn the rads off when the temperature has been hit. I guess this only works in zoned heating systems where you still have TRVs. 

The point that I was making is that it is pointless having the disturbance and expense of replacing radiators if they then have to be turned down using TRV's, which may reduce the system capacity and water flow rate, and could cause heat pump cycling.

Replacing the radiators in your colder room was indeed the correct thing to do, since you now have a more balanced system.

All change - that house purchase fell through when the vendors changed their minds about selling. Today we complete on a 1977-built property in a different rural village in North Norfolk.

This house was (unusually for the time) built with insulated cavity walls and has a recently installed oil boiler, so the pressure to change things isn’t as urgent. Physically, it needs extra insulation in various places and I plan to re-jig some of the heating pipework at the same time to make it easier to convert to a heat pump later.

The garden is much bigger and there is a perfect spot for an ASHP where the oil tank currently sits with a great pipe route into the HW tank cupboard on the 1st floor where the boiler is located. Physical location and pipe runs were going to be a problem with the previous property.

So, I’ll be doing the work to insulate and live in the house for a bit first before doing anything else. Hopefully the heat pump market will mature more in the meantime and we’ll get some sensible electricity tariffs rewarding the use of more efficient heating systems.