Boiler efficiency

I've been pondering the above topic in regards to my circa 20 y/o Camray oil burner. I assume boiler efficiency is calculated as calorific value of fuel burned versus how much of that actually ends up as hot water. Googling suggests my boiler is probably only at about the 70% mark.

But is that really such a big deal depending on where the lost heat goes? My boiler chucks out enough waste heat to keep the utility room warm (it has no radiator) and even the toilet and shower room leading off that doesn't get too cold to sit at the throne if I leave the door open although I open up the TRV in there for half an hour before having a shower in this weather.

So really the only truly wasted heat in winter time is that which goes out of the flue. I concede that in summer if I'm just heating the immersion tank then waste heat is more of an issue although a relatively minor one as not much oil is being burned.

So how much of the "wasre" heat of a low efficiency boiler like mine goes out of the flue and how much goes into the house anyway to contribute to general heating?

Reply to
Dave Baker
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Is that from the SEDBUK site:

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similar sort of age massive cast iron lump manages about 80% according to the engineer. IIRC he measures the flue gas temperature as it leaves the boiler. This more or less agrees with the SEDBUK database.

Our boiler room can be the warmest place in the house... but then the pipes to from the thermal store are unlagged and there is a 18" or so of single skin flue pipe as well. It's also not very big 6 x 10 ish, no windows, just two doors normally closed. There are a couple of free air air vents.

I suspect most of the waste heat goes up the flue. Ours is a nominal 38kW output, so wastes nearly 10kW. At a guess from how warm the boiler room gets I doubt it's getting more than 10% of that waste, ie 1kW.

Reply to
Dave Liquorice

Most of it goes out of the flue, as steam. The large cast iron Ideal Standard my parents used to have in the kitchen directly output about the same as one of the medium large radiators, which is much less that it would have lost up the flue.

I don't know if SEDBUK counts heat lost from the boiler casing as useful or waste. However, I observe that many new boilers lose much less from the casing. My Keston gets slightly warm on the front, but it's constructed so that most of the heat which leaks out from the internal parts into the case is used to preheat the outside air which is forced back into the burner, so it's recycled internally.

Reply to
Andrew Gabriel

The Mexico I just ditched, got quite hot to the touch, but due to location (in a corner under a worktop, and boxed in a bit on all sides) did not add much heat to the utility room - enough to keep it warmish, but not as good as the rad I put in there when the boiler was swapped.

Sounds similar to my ecotec - the lower bit of the front casing gets slightly warm to the touch (its close to the pump and main flow and return pipework), but most of the rest of the case is room temperature. The concentric flue pipe is cool (not surprising since the cold comes in around the outside). That seems to discharge directly into the case of the boiler. The fan then draws air through a pipe assembly that does a lap of the HE prior to sucking on air from in the case. (presumably ensuring the case is at a slight negative pressure)

Reply to
John Rumm

I have quite a large bathroom (16x12ft) with two external 9" solid brick walls and a large double glazed window. Roof is reasonably well insulated. The boiler and hot water cylinder are sited there. The old cast iron Potterton kept the room toastie warm at all times - the only other heating is a towel rail, fed from the central heating.

Replaced the boiler with a condensing one and it's freezing. ;-)

Reply to
Dave Plowman (News)

Newer boilers achieve greater efficiency by using lower flue gas temperatures.

I replaced my boiler a few years back and I definitely use less oil. The house temperature is the same. I would think that my reduced oil usage has paid for the boiler - not to difficult at the current prices.

Reply to
Michael Chare

If it saved me 25% of what I use in gas it would take about 5 years to pay for a new boiler if I fitted it myself. If I have it fitted it will never pay for itself before it is end of life.

Reply to
dennis

Partly depends on the location of the system.

The house I used to live in (small semi) had a gas-fired back boiler, which in over 30 years never broke down.

This was located in the lounge, and the central chimney breast adjoined the lounge, dining room, and kitchen. Upstairs it adjoined the two larger bedrooms and the airing cupboard, before disappearing into the loft before exiting on the ridge-height chimney. This is modern terms was inefficient.

However, on snowy days my roof had as much snow as anyone else's, including those with modern boilers, and it didn't melt any faster either. I concluded that the (hot) flue gases were being cooled by the chimney, and little if any was reaching the loft or external chimney as evidenced by the lack of snow melt. So the heat that wasn't going in to the water was heating something in the house, mostly keeping the masonry warm as a heat-sink/ supply depending on the individual room temperature.

I now live in a more modern house with dry-lined walls. These allow impressively-fast warm-up, matched only by impressively-fast cool-down. In the depths of winter the average minimum overnight indoor temperature in my old house was 16 degC, but in this present house it's 13 degC for the same outside temperature as there's very little heat available from the masonry due to the dry lining.

ISTM that 'boiler efficiency' is put forward these days as the key issue, but I maintain that it's 'system efficiency' that counts. In those terms my old system might have been just as 'efficient' as my current one, but somewhat harder to measure.

Reply to
Terry Fields

Have you noticed what's happened to gas prices over the last few years? And these rises are likely to continue.

Reply to
Dave Plowman (News)

So does the cost of boilers, the economics stay the same. If I had a really poor heating control system it might pay to change but not ATM.

Reply to
dennis

Can't say I have noticed boilers jump in price by anything like the amount of rise seen in gas...

I seem to recall paying about the same for the 35kW combi I installed in the last place in 2004, as I paid for a 24kW system boiler recently. That is without allowing for inflation, and also ignores the fact that the first was an Ideal, and the recent one a Vaillant.

Reply to
John Rumm

I maintain that it's 'system efficiency'

as my current one, but somewhat

Indeed. Its probably worth noting that the SEDBUK figures do actually look at the efficiency in seasonal terms, and in the context of a system, rather than just the best possible figures that the boiler itself can attain under ideal conditions.

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said that, how a system is installed and used will also make a potentially significant contribution.

Reply to
John Rumm

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