Do Condensing Boilers operate in condensing mode?

If the plume of condensate visible at the exhaust, then ours works in condensing mode most of the time.

Don't know anything about the theory, but I believe the boiler needs to be sized accurately to the house to achieve the condensing mode.

Dave

surely there shouldn't be a plume of condensate if it's condensing.

talked to a guy at grants once and he said once they are hot they stop condesing unless you have something like underfloor heating which lowers the return temp low enough. as I was querying standard boilers ratings and he said there wasn't much difference with standad rads just the ratings were artificially capped for non condensing boilers this was 20 years ago.

winter 2010/2011 killed that boiler

You are supposed to have a 20 degree differential between flow and return for condensing to occur. But you can usually achieve that on radiators if you put the effort in to balancing them correctly. Most of the time it’s not done correctly.

Note however that even when not condensing, modern condensing boilers are still a good bit more efficient than a decent standard efficiency boiler from 20 years ago. The secondary heat exchanger in the vortex for example has flue glass temps down to about 50 degrees vs 200+ on the older equivalent.

I am not sure what active condensing does on top of that, but I’d be surprised if it was any more than 1 or 2% more to be obtained.

AFAIK the return temperature has to be below the condensation dew point. Normally around 55 degrees.

So no, boilers installed and commissioned with a flow temp of say 70 degrees likely won't go condensing mode.

Don't know how and I don't think I need to. I run the flow temp at 55 degrees so it will return sub 50.

I'll try find a good YT video on it. On a crisp cold day if you can see a large white plume you are seeing condensing that had not happened inside the boiler.

If that works for you then happy days.

Just watch your hot water cylinder, ideally it should be a little hotter to deal with legionella. In some cases houses need higher flow temps than that as radiators wouldn’t have been sized for 50deg.

Thanks for the advice. Should have told you mine is a combi! Hot Water set to 62 degrees. Rads 55.

The house has two zones with heating controls. The root of the problem was it was not condensing because the flow was too high at 70 degrees. You can argue all you want but when this was changed and only this the boiler became super efficient. And after this I read in almost all literature that a condensing boiler should not work at more than 60 degrees and that is why you have separate piping for the hot water tank because it needs to be hotter. But I guess the plumber installing this unit was thinking like you.

Lowering the boiler flow temperature from 70 to 55 will reduce the output of the radiators and cylinder by about 30% (roughly), and you will have a fuel saving of approximately the same amount. That is where your saving is coming from.

It has little or nothing to do with the efficiency of a new boiler or condensing. The flue gas temperature is already so low leaving the boiler the remaining latent heat to be recovered by condensing is actually quite small.

My 20 year old SE (non condensing) Firebird Oil Fired boiler has a flue gas temperature of ~ 230C, if I was to install a HE (condensing) boiler I would expect the flue gas temperature to fall to at least 100C (without trying too hard) due to the extra heat exchanger and a efficiency gain of ~ 6%, boilers don't start condensing util the flue gas temperature falls to 50/55C and won't fully condense until the flue gas temperature falls to ~ 25C/30C, reducing the boiler flow temperature is the easiest way to get at least partial condensing as most rads are oversized to T50 standards, you will often see the T50 standard quoted at, flowtemp/returntemp/roomtemp, 75C/65C/20C which is a T50 rad, ((75+65)/2)-20, 50C. Quite often, depending on rad sizing, house insulation, a outside air temperatures of say 7C to 10C may only require rad outputs as low as 25% of their rated T50 output, simply adjusting the boiler flow temperature (which weather compensation does automaticall) can result in extra condensing and a nice gain in boiler efficiency. You can see, below, that once down to ~ 50% rad output that the rad/boiler return temps are down to ~ 47C, leading to a reasonable boiler efficiency but once down to 25% rad output then return temp is only 36C ( the very limit of OF boiler return temp) to give very high boiler efficiencies.