A while ago I asked about the possible perils of buying a listed house (17thC timber-framed) - the survey results were so bad that I ran away bravely. It now seems likely that I'll be buying an unlisted 1930s "arts and crafts" style house that seems to be pretty solid (but the survey is next week - :eeek!). This house has a fairly old (20+ years?) Mistral oil-fired boiler, which is certain to be non-condensing. I need to check but I think the output is 160k BTU. My previous boiler experience has all been with gas boilers. What do I need to know that I may not know? What real world efficiency savings am I likely to get by changing it for a modern, condensing, boiler?

Do they make condensing oil-fired boilers now? AIUI they used not to, as the benefits in terms of improve efficiency were marginal (oil, having a composition with less hydrogen and more carbon than gas, per molecule or per unit weight, doesn't generate as much water vapour when burnt, so the energy to be recovered by condensing that vapour is less). But there may be other benefits in a new boiler, especially if you have mains gas available and install a condensing gas boiler. If no mains gas, you could always install an LPG boiler but that would mean a gas tank in the garden and an expensive gas supply relative to mains gas.

Worth replacing an old oil boiler?

Mar 18, 2017 65 Replies

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John Rumm 9 years ago

Conventional wisdom was that the smaller fully loaded boiler will be more efficient. However that assumed non modulating boilers. It also assumed no pump run on. So each time the boiler cycled, the (considerable) residual heat in the large lump iron HE would be left to waft out of the flue.

With a modern boiler it will load match much based on the actual demand (if it has a decent modulation range - not so true for oil boilers), and the pump run on should at least dump the residual heat into the heating system (and newer boilers typically having lighter HEs there is less of it anyway)

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tabbypurr 9 years ago

Stupid, adj.: one who asserts a point of view without bothering to even get the facts.

It was about 2m higher up than I was. It might well experience a little more wind, but not much more.

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tabbypurr 9 years ago

Strange. Last time I looked there were lots for around £100. Maybe it' s the time of year.

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tabbypurr 9 years ago

It depends on the boilers. Modern wins over historic, greatly so if the old boy is cast iron.

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John Rumm 9 years ago

Or worse, other bill payers...

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Fredxxx 9 years ago

The efficiencies of boiler on full output, reduced/modulated output and cycling is less obvious that it used to be.

All boilers are now fan flued so there is minimal passage of air within a boiler when not lit.

That suggests a cycling boiler with a fanned flue is not as bad as an old boiler where natural convection within a boiler would carry heat away.

I might suggest a boiler running in modulating mode might be most efficient from a slower flow of hot gases over the heat exchanger leading to better heat transfer.

Its then not so obvious which is the more efficient, one running at max or one that is cycling.

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