Old boiler advice - replace or not

coffeehound

Jeepers_Creepers said:

I think Coffee's efficiency ratings are ambitious; I can't see any new boiler - unless installed in a careful tailor-made system - realistically achieving anything close to 98% eff when installed to an existing system. For max eff, I guess you'd be talking a completely underfloor heating system running at below 30oC or similar.

Yes, of course, it will be more efficient than the old boiler it replaces in an existing system, perhaps 15% or so as a guess? When- if - you do come to replacing it, make sure that any rads that also need replacing are sized for max efficiency, which basically means 'oversizing' them. By fitting 'larger' rads - either width or extra panels/fins - you will get the same heat out of them as before, but running on cooler water. A cool return temp is what provides the efficiency for modern boilers. 

Any sources to back those numbers up, Jeepers?  As long as a modern boiler is running in condensing mode, why should it not be running at near peak efficiency?  I acknowledged above that it would need to exceed condensing mode for the final degrees of heating of the DHW tank, but it should always be below that for a balanced CH circuit.

In any case, ratechaser's existing boiler is probably not running near the 73% figure, so the £400 pa saving is a reasonable, and probably conservative, guide figure for 3p gas.  There was one example on here where it was calculated that an old gas boiler was using 8 kWh to put 1½ kWh into a DHW tank!

Jeepers_Creepers

No I don't have specific figures, Coffee, but a quick glance at realistically attainable figures suggests I'm not far off.

I can't see a condensing boiler achieving anything close to 98% eff when installed to an existing system. Why? Because the return water temp will be too high if the 'old' rads are going to be hot enough to provide the levels of comfort the owner was used to. The original non-cond boilers will often be firing out hot water at 80oC+. The floor-mounted Mexico that came with our house was kept 'kettling' by the elderly sellers. No surprise that the GlowWorm I replaced it with struggles to get some rooms up to temp in Winter.

I've no idea what efficiency that old Mexico was running at, but what I do recall is that the enamel flue coming out t'top would have taken the skin off your hands if you touched it.

Modern condensing boiler efficiency figures:

Return temp of 40oC = ~94% eff. (Generally only achievable with underfloor heating as I said)

RT 50%  = ~90% eff. Flow too cold for most systems in Winter.

RT 60% = ~87% eff - that's about doable.

grumbler

I think higher efficiency can be achieved when heating mains water.

Jeepers_Creepers

grumbler said:

I think higher efficiency can be achieved when heating mains water.

As in a combi, Grumb?

grumbler

Yes, combi, as if the "return temp" was always very low. I guess in this case the efficiency can get very close to 100%.

Jeepers_Creepers

grumbler said:

Yes, combi, as if the "return temp" was always very low. I guess in this case the efficiency can get very close to 100%.

I certainly get a huge plume when drawing hot water!

I wonder, tho' - it's not the cold mains water that's going through the main exchanger and which could theoretically draw that condensing heat out of the combustion process, but the constantly recirculating 'system' water which is driven between the main and P2P exchangers within the boiler. I'm guessing that water must be kept piping hot so that it can instantly heat the incoming cold. The main exchanger will be pretty darned hot at all times during this process, so perhaps not as efficient as we'd hope.

That's the way i understand it, but I don't know if it's accurate.

Grenage

Even at £400pa saving I suppose it's not an obvious choice (but quite an attractive one).

I'd do it if I divided the costs of the new installation by the savings and it paid back in 8 years or less, maybe.

The modern combi in my last place was forever going wrong and being looked at.  My current old boiler had been faultless and I can replace the external stuff myself.  Very anecdotal, but old and inefficient tends to out-live modern and efficient.

grumbler

Jeepers_Creepers said:

grumbler said:

Yes, combi, as if the "return temp" was always very low. I guess in this case the efficiency can get very close to 100%.

...

I wonder, tho' - it's not the cold mains water that's going through the main exchanger and which could theoretically draw that condensing heat out of the combustion process, but the constantly recirculating 'system' water which is driven between the main and P2P exchangers within the boiler.

My understanding is that mains water goes through the 'secondary' heat exchanger where it's heated by both the circulating hot water (going through the 'main' heat exchanger) and flue gases.

Jeepers_Creepers

Certainly not on my GlowWorm Gumb, as i've had most of it apart including - recently - the hydrobloc which handles the DHW.

That diagram you have also seems to suggest the cold water goes straight to the 'water-to-water' exchanger and out as DHW - no link to the combustion or flue other than via the main system's water.

I wonder why they don't have the cold inlet coiling around the lower flue? That would surely drag lots out of it? Effectively a stone-cold return flow!

FreeBear

coffeehound said: Any sources to back those numbers up, Jeepers?

https://www.homeheatingguide.co.uk/efficiency-tables?model=000284 - Gives an efficiency rating of 65% for the Ideal Mexico.

A random selection (Worcester Greenstar 30 system) - https://www.homeheatingguide.co.uk/efficiency-tables?model=016807 - 89% efficiency rating.

I would take the efficiency ratings with a pinch of salt as there are many factors that can influence the figures.