BISF House Back Boiler Replacement

Hi all, we’re thinking of replacing our old Baxi Bermuda SL3 Back Boiler and I was wondering what other BISF-dwellers have done – what type of boiler, placement etc.

Originally we were planning on keeping the back boiler as the engineer who serviced it said they are very reliable and easy to repair. However the air vent it needs lets in a lot of cold air which makes the whole downstairs draughty in winter.

back boiler
Original BISF gas fire, with integrated rear back boiler

At the moment the Baxi Bermuda SL3 back boiler heats hot water in a vented cylinder in the airing cupboard (the cupboard next to the flue in the main bedroom) and runs an 8 mm central heating system. I’ve replaced the oldest radiators with modern ones (they were probably more than 30 years old) and fitted TRVs to all radiators except the one next to the thermostat (there were none originally).

I’m thinking it will be best to stick with the present central heating system as it works, the radiators are all in sensible positions (on internal rather than external walls to minimise heat loss – nothing worse than a radiator right under a window!) and 8 mm pipe is the most that will fit between the ceiling and steel I-beams.

So the question really is stick with the hot water cylinder and get a heat-only boiler, or replace the cylinder too and get a combi boiler. And where to put the new boiler? The local housing association seem to have put them in the airing cupboard, based on the flue position on the roof, but as we still have an asbestos roof making a hole for a new flue doesn’t seem a good idea.

I was thinking of removing the cast iron flue upstairs and in the loft and fitting the new boiler where the boxed-in flue is now in a new cupboard that is the same sort of size. Then the new flue could use the old chimney.



  1. Hi Marc,

    the back boiler is indeed horribly inefficient – upgrading it to a condensing boiler should save a third of the gas that the Baxi Bermuda uses, and that’s not even taking into account all the heat that’s lost at the moment having a massive hole in the wall that I’d be able to seal up!

    In the last 12 months we’ve used 9100 kWh of gas so at current prices we’d save at least 180 per year, but the main issue I think is the discomfort of cold draughts from the vent. You may be interested to know our gas usage is down about 20% since I started upgrading the loft and wall insulation and fitted TRVs to the radiators, so with two rooms wall insulated and the rest still to go there should be big savings.

    I understand that a condensing heat-only boiler is suitable to use with a traditional vented cylinder system like the one we have got, basically it’s like a back boiler but without the fire front it seems, so there’s more flexibility in where it can go – hence thinking of putting it upstairs in the place where the boxed-in flue is now. Remeha have a good reputation, but the other boiler makes do them too: .

    Probably wouldn’t want to go down the route of switching to 15mm pipework due to cost and disruption and also I can’t see a straightforward way of getting 15mm pipes to the bathroom and small bedroom as there is only just enough room for the 8mm pipes between the steel floor beams and ceiling.


  2. Hi Ed, I do agree that the old Baxi boilers are pretty easy to maintain but they are very inefficient by today’s standards but I’m sure you’re aware of this already.
    I would always recommend a new condensing boiler as they are very efficient, my only concern is if they would be able to adapt it to feed the microbore pipes you have. In every install that I have replaced they have always replaced the pipework to 15mm. In that last image I showed you of the landing, the visible pipework was the remnants of the old system. I would think that your current radiators may still be suitable, they would probably just need different connectors to accommodate larger pipes which would save some money.

    What I don’t know, is to what degree Baxi have upgraded their back boilers as they may be far more efficient than the old ones. The fires used to be interchangeable, ie you could keep the boiler and change the front fires but this may have also changed.

    I have seen many boilers fitted into the airing cupboards and the old hot water cylinder removed, it does free up some space but as always it’s down to cost considerations.
    This may be the perfect opportunity to take a look at the Green deal or even ECO funding options but for ECO you do need to be on certain benefits. There are several free boiler replacement schemes available but the criteria does vary and as your boiler is over 10 years old you should have no problem.
    For me it’s all about the savings on running costs. Even if you did go down the Green deal route, it’s not a very large sum that is added to your bills and it should pay for itself over a short period. I think the Green Deal is a far better option for those not planning to move in the near future.