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Heresy? or pragmatic engineering? - a suggestion for the a segment of the 'failed boiler/distress purchase' market'

JamesPa · 2026-05-29T11:25:13Z

OK so Im going to put something controversial 'out there'. The trigger is a decision by the Hertfordshire Retrofit Strategy Steering group to include a proposition for the 'failed boiler' scenario as one of its targets. Its a market that must be tackled if heat pumps are ever to replace boilers, we install 1.6M boilers each year and build only 200K houses, implying 1.4M boiler retrofits of which I would venture to suggest a fair proportion are distress purchases. The problem with this is clearly timescale, one person on the team commented that their typical lead time was 6 weeks, to which I responded, only slightly tongue in cheek, that the local plumber will fit a replacement boiler next week, and its not the local plumber that needs to change! One approach is to encourage people to be prepared, but this will be a niche only. Using electric heaters temporarily is hardly attractive mid winter, both for comfort and cost reasons, so while that may work for some its another niche and we still aren't addressing the real issue, which is lead time. That said I do think that its valid to start off with a particular segment (much as Octopus did) and build out from there. So here are some observations which might point at how the lead time issue could be addressed: The Ofgem standard house consumes 11,500kWh gas per annum, this puts it at <6kW (actually closer to 4kW) loss at design temperature. This suggests that a fairly high proportion of houses are <6kW Most manufacturers minimum capacity machine* has a nominal output of 5kW or more You generally want 5kW+ for DHW reheat anyway so in a house that is definitely 5kW or less, in practice a 5kW (or a machine with a minimum capacity the same as a 5kW machine) is going to be fitted, at which point whole house sizing becomes irrelevant If radiators are 'undersized' its always possible to compensate by increasing the flow temperature, and R290 pumps can run at 70C Radiator upgrades are an easy 'stage 2' retrofit, in fact there is a good argument for measuring what actually happens before deciding which radiators to upgrade, because its a real measurement not a gigo spreadsheet. Thats what Adia advocate and make the kit for! provided primaries in a <=6kW house are 22mm and there is no microbore, the plumbing is going to work (or, worst case, there are no further checks that would be done as part of a survey that will detect a reason why it wont work) Real world SCOP is dependent more on installation quality/operating mode than on FT - see graph below from openenergymonitor (I suspect that this is what is behind the zero disrupt heat geek offering and Octopus's philosophy!) So, taking all the above together I propose that the following is a technically practical way of dealing with the 'failed boiler' scenario, which crucially does not compromise quality so far as I can see. Do some very basic checks to be certain that the house loss is <=5kW, no microbore, 22mm primaries fit a 5/6kW heat pump and UVC plus th a 50l volumiser to ensure defrost works Fit Hhomely or Passiv, or if you want the full monty Adia, leave it to self adjust Householder now warm, panic over! Over next few months monitor flow temperature etc and then return in summer or a few weeks later to upgrade radiators if necessary. This requires some derogation from MIS3005-D, but this is justifiable from an engineering standpoint. Obviously, in addition to the technical viability, we have to consider if its commercially viable (or make it commercially viable). I suspect that this might not be attractive for the 'top installers' who employ engineers that are expert in everything, but maybe its a very useful 'entry point'/training ground. Do a dozen of these before you do anything else. From a learning point of view I would think this would actually work better - instead of swallowing the whole elephant in on go you first get some understanding of the basics nd of commissioning in a relatively 'safe' context, and only once this is 'under your belt' do you need to learn the more detailed design stuff. Maybe it should be a separate 'MCS light' qualification which can be upgraded to the full month later, targeted at your local plumber who normally fits boilers? I would comment further that doing this this way poses no threat to most existing installers, this is a new market that many cant actually access at present. Constructive comments invited - we have to solve the 'failed boiler' problem if heat pumps are ever to succeed in displacing fossil fuel burners, is this the basis of a start? *with the exception of firmware limited versions of higher capacity machines, the minimum output of which generally be the same as its larger cousin

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Air Source Heat Pumps (ASHPs)

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AndrewJ

(@andrewj)

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Joined: 1 year ago

Posts: 216

31/05/2026 12:13 pm

@temperature_gradient my point was to minimise the emergency install stress by giving people access to the grant beforehand to make their house “renewables ready”. That way the decision is easier, delay significantly reduced if not totally, and less disruptive in what is already a difficult situation. If a cylinder doesn’t need changing then it doesn’t - makes more of the grant available for other things - that list was just meant to be ideas. I was glad to get rid of a noisy pump and a tank in the loft and would have done so regardless even though the existing tank was only 8 years old. Incidentally many of these tanks aren’t insulated underneath for reasons that make no real sense - can you imagine that volume of water freezing in this country! I couldn’t work out if building regs still applied to that but you don’t really want that amount of loft space uninsulated.

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