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Hi I live in a Victorian semi and I am looking for some advice please. The house has a modern two storey extension at the back. There are two boilers in the house. A water tank boiler for the old part and a combi for the new part. It was like this when we moved in. The previous owners just said if was easier to add a new combi rather than change pipes?!? and buy a bigger boiler as boiler in old part wasn’t that old. We replaced the combi, but it’s not coming to the time when we will likely need to replace the water tank boiler. Do you think we should replace for one boiler for the whole house? What would we need to do in terms of pipes please? Or if two should we replace water tank with another combi? Thanks for any advice all appreciated.

Hi hillslack,
Stick with two boilers and keep one as a water tank one.
If you go to one boiler, if this breaks down you will loose all heating capability. If you fit two combis you will probably have to increase the size of your mains supply pipe.
Regards S

My house three stories, bottom is a granny flat 4 rooms, upper two are main house with a further 4 bedrooms so three bathrooms, two kitchens, five bedrooms, two living rooms, dinning room, three normal outside doors, and three double patio doors, all heated with a single 20 kW oil boiler, which also does domestic hot water.

OK I use smart TRV's so can select what rooms and when they are heated, but it is enough to heat whole home.

Larger boilers would heat the home faster, but to maintain the temperature does not need that much energy.

I have looked at a larger boiler, but the existing one still cycles on the coldest days, so clearly still has more it could give out.

But what we are really looking at is cost. To combine will cost, I have looked at a better control system, likely cost £1000, and have decided not worth it.

In the main gas boilers modulate, so not as easy to see how close to the wind you are sailing, with oil control is by turning boiler on/off not up/down so easy to hear when running so one knows how close to the limit.

I do worry about the boiler failing, so it does have battery back up, one reason I would not entertain a heat pump. But in your case would heat in half the house help?

But it would have been easier setting up two boilers than one with two pumps and motorised valves so flat and house can be independent, but oil boilers are not cheap.