UK gas boiler (furnace) efficiency - or lack of it.

[confused2]

Lilly's Youtube about the price of energy woke my dragon.

My annual heating bill for gas (4,500kWh) at todays prices is about £575 or almost exactly 5.7% of the income of a single pensioner - honestly not very scary. Taking away the amount used for hot water leaves the heating with about 3,500 kWh of raw gas per year. The official average house in the UK uses between 8,000 and 12,000 kWh raw gas per year.

'Raw gas' because how much heat you get out of it depends on what you do with it. For example my '90% efficient' boiler designed in Germany with matching thermostat has soldiered valliantly on for the last ten years and really is 90% efficient. Unfortunately I also have a less than ideal boiler "Designed and manufactured in the UK" which generally means "Primitive and no quality control".

The UK designed thermostat works by turning the boiler on so the room fills with hot air from the top down. When the layer of hot air reaches the thermostat it starts to warm up the thermostat until it reaches a temperature high enough that it will turn the boiler off. After a while, maybe an hour, the thermostat will have cooled down enough to turn the boiler on again and the cycle repeats. Imagine the bimetallic strip (invented in 1759) and an electrical contact being replaced by a thermistor and the sort of software a child would write. Of course the German thermostat is rather more advanced - by about 250 years.

The combustion fan. About 70 years ago people started fitting fans in boilers to give the gas enough air to burn in without having to rely on a chimney to draw air through the boiler. Imagine the idiot child that designed the thermostat is telling the boiler it wants water at 65C and the water is already at 65C? We have to turn the gas off but do we need to turn the fan off? Much simpler to carry on taking cold air from outside, blowing it through the heat exchanger and send it back out into the garden. If you have a radiator with an output of 0.5kW and a boiler with a minimum output of 8kW you can spend quite a lot of your time watching your boiler heating the garden. Of course the Germans wouldn't ..

There's more but I'm kind of burnt out for now.

[Zinjanthropos]

What have you got there C2? Sounds like you’re describing a power vented hot water heater that also heats the air in the house? I’ve seen a few of these in Canada. The room thermostat in this case is just a switch that opens and closes a valve that provides hot water to lines located in a warm air plenum where the heat is exchanged to the air being circulated thru it. IOW your hot water heater (one appliance) also heats the air in your house. Probably turns on a water circulation pump as well.

90% efficiency ( hmmmm?) is good number for a hot water system but since you have ductwork, a hi/eff forced air (FA) gas furnace could do much better, up to 98% in some cases. In my house the water heater serves only the sinks and tubs. In your place does it seem like the hot water heater( boiler) hardly ever shuts off during cold weather? Personally I don’t like these but if you don’t have the space for a stand alone FA furnace then you’re stuck with it. I call it the poor man’s furnace, no offence. An FA furnace would also extend life expectancy of the boiler as you Brits call it. If it’s something different then please disregard.

[confused2]

What have you got there C2? Sounds like you’re describing a power vented hot water heater that also heats the air in the house? I’ve seen a few of these in Canada. The room thermostat in this case is just a switch that opens and closes a valve that provides hot water to lines located in a warm air plenum where the heat is exchanged to the air being circulated thru it. IOW your hot water heater (one appliance) also heats the air in your house. Probably turns on a water circulation pump as well.

90% efficiency ( hmmmm?) is good number for a hot water system but since you have ductwork, a hi/eff forced air (FA) gas furnace could do much better, up to 98% in some cases. In my house the water heater serves only the sinks and tubs. In your place does it seem like the hot water heater( boiler) hardly ever shuts off during cold weather? Personally I don’t like these but if you don’t have the space for a stand alone FA furnace then you’re stuck with it. I call it the poor man’s furnace, no offence. An FA furnace would also extend life expectancy of the boiler as you Brits call it. If it’s something different then please disregard.

What you're saying sounds very interesting ..I'll come back to it.
The 'traditional' English way is a thing that burns gas to heat water that is pumped out to radiators round the house. The only air involved should be the air to mix with gas to burn in the heat exchanger which comes in cold from outside and leaves as cool as possible to keep the thermal efficiency high. Recent designs 'condensing boilers' try for an output temperature below about 40C to reclaim the latent heat in the water vapour formed as the gas burns. Basically a water heater. A 'combi' or combination boiler will divert mains water through the heat exchanger for baths and showers. With small houses in the UK the heat required is generally low (less than 2kW) .. my seaside house averages well below 1kW except on the very coldest days so the efficiency at low output is more important than the claimed efficiency at some output chosen by the manufacturer. The UK attitude is still that only rich folks could afford central heating and (being rich) the efficiency didn't matter. The price of electricity has driven folks in small houses to buy gas boilers and we now think about global warming and the price of gas as being very good reasons to have efficient heating.

Back to your comments..

We don't have 'ductwork' - just a 'flue' about 4" in diameter with a central tube - cold air for the burny bit comes in on the outer bit and the exhaust goes out on the inner bit - the boiler its lf is sealed so no inside air mixes with outside air.. A crap (UK) design will use that air to cool the heat exchanger even when there is no combustion hence my rant heading in the direction of boilers with lower than 10% efficiency.

"The boiler hardly ever shuts off" is a favoured situation where the water (radiator) temperature is varied to match the likely heat required after measuring the outside temperature. In reality the boiler will burn gas until the set temperature is reached and (very likely) will then blow cold air through the heat exchanger until the heat exchanger falls (say) 5C below the set temperature. I can see there is a problem with people wondering why you would want to blow cold air through a hot heat exchanger..we have to go back to 1960 when relays were expensive .. the entire budget went on getting the gas to light and go off again and no relay to turn the combustion fan off was ever fitted. If it was good enough then it is good enough now despite the catastrophic effect on efficiency at any output other than that for which a (90%) compliance certificate is given.

"a hi/eff forced air (FA) gas furnace could do much better, up to 98% in some cases." Now really interested - if you want to know about heating ask someone who lives in a cold country. When Americans say 'furnace' I'm thinking 20kW or more and hot exhaust so maybe 80% at best - how do you get the 98%?

[Zinjanthropos]

Been awhile since I’ve been in the field I forgot the combi term, thanks for that.

Hi/eff FA furnaces have a secondary heat exchanger, we call it a tertiary using technical terms. The exhaust is run thru the additional heat exchanger to scrub damn near all of the remaining heat left in it before exiting. Of course as it cools the water vapor in your exhaust turns liquid and this water is usually removed from the exhaust via a waterline piped to a drain of some kind (floor or sink). Next to no heat exits as exhaust, practically all heat generated is used to warm house. The main heat exchanger plus tertiary are located in the warm air plenum downstream from circulating fan. If it’s hi/eff then you should see a water drain from the appliance…that’s a big clue…lol

My own furnace is 90% efficient and it’s not even a high eff furnace. Input 60000btu and output 54000. It does not draw air from outside for combustion. Furnaces that take C&V air from outside are primarily used in airtight construction, in Canada its called R2000 home, very well insulated . There’s no air communication between home and appliance. Trouble with that is you can’t have any other appliance in the house that uses air from inside and that includes a wood fireplace. Why? Because the house air will go negative and not being able to draw from outside means trouble with ignition. Believe it or not there are R2000 homes that have wood fireplaces. They won’t vent combustion products to the outside unless you open a window which kind of defeats the airtight purpose. There are also electric appliances that can make an airtight home negative such as clothes dryer or any exhaust fan. So if your home is not airtight, having C&V air from outside piped in doesn’t really mean much. People in airtight homes don’t like being told they should have an outside air opening in the house somewhere, preferably in the cellar and usually a 4” diameter opening is enough. Anything exhausting air will draw in the air and if it’s cold then so be it.

You say you use a bimetallic thermostat. Does it have a heat anticipator on it. It’s usually a little dial located under the cover. Mercury stats have them and I can’t remember if bimetallic stats do, I never saw very many. A heat anticipator shuts off the main burners before it actually comes up to the setting. This is because there was always residual heat left in the heat exchanger that still had to be distributed to the house air. It basically fools the t-stat into shutting off a bit early so the leftover heat can bring it up to the setting. Whatever you set it at controls the length of heating cycle, long or short. Programmable stats? Well they’re a bit smarter…lo9

If you want to know how efficient your gas oiler or furnace is then check the rating plate attached to every appliance. Output divided by input…simple as that.

[confused2]

If you want to know how efficient your gas oiler or furnace is then check the rating plate attached to every appliance. Output divided by input…simple as that.

Poor folks are turning off radiators because they can't afford to pay for the gas. Let's leave one radiator valve open - the radiator is maybe 500W at 65C so that is your heat output. Your boiler says minimum output 8kW so that is your input. Your room needs 250W to stay at a constant temperature so that's the output you actually want.

With a German thermostat you get a burn every 10 minutes .. because we have a minimum output of 8kW I'm going to change that 10 minutes to 8 minutes to make the arithmetic easier. The German thermostat knows how much heat it put in 8 minutes ago and knows the temperature change so it knows how much heat to put in this time. A 1 minute burn would be 1 kW (average) so the desired 250W would be a 15 second burn . The boiler knows a 15 second burn isn't efficient (it's German) so will miss out this burn altogether and do a 30 second burn (which is OK) after 16 minutes. The thermostat is watching the temperature so it may wait longer (or shorter) to fit in that 30 second burn After 16 minutes (or even just 8) the water coming back to the boiler is cool so the primary and secondary heat exchangers are at maximum efficiency so the exhaust gas maxes at about 30C with virtually all vapour (and latent heat) extracted. After a 30 second burn the radiator temperature will be about 45C which will give you (surprise!) exactly (ja!) the 250W average that is what you wanted. The 30 second burn is a bit short - losing maybe 6% but everything else is good - about 96% .. which gives you the claimed 90% efficiency even at 250W output.

In the British system .still wanting 250W ... the thermostat calls for heat..
The combustion fan turns on and gas is lit
The boiler burns at maximum output of about 25kW and brings the water up to 65C in about 15 seconds - short burn penalty of about 10%.
The boiler needs to cool by about 5C before turning the gas on again .. so cooled by the 500W radiator and cold air being blown through the heat exchanger (another 500W?) until..
We get another burn, with the heat exchanger now hot this will be at the minimum of 8kW.but still enough to raise the temperature by 5C in about 15 seconds.
The water returning from the radiator is now hot (say 50C) so the thermal efficiency goes down and there's little if any heat extracted from the vapour in the exhaust
Not only is the thermal efficiency around 70% but 50% of that heat is going out through the flue while the boiler waits for the heat echanger to cool.
The boiler carries on for maybe 15 minutes until the room warms enough for the thermostat to turn the boiler off.
Just under/over heating the room is a 10% loss.
Overall around 25% conversion of gas to useful heat.
I'd appreciate it if you could point out anything I've missed or exaggerated.

Not many bimetallic strips used nowadays but we've managed to copy the bad features (and none of the not-so-bad ones - you had 'accelerators'?) into our electronic thermostats.

"My own furnace is 90% efficient .. It does not draw air from outside for combustion." ...surely it must you'd run out of oxygen if it didn't.

The UK is very damp. There is a thing over here for airtight houses. I once went out with a girl who could fart for England - I'll pass on the airtight house thing.

[Zinjanthropos]

We’re losing something in the translation. Sounds like it’s not standardized worldwide. Haven’t the time this morning to elaborate much. Do you run on natural gas? Is it metered by the local distributor? Our meters here measure gas used in cubic meters. How is it done there? Perhaps you use propane?

Go to rating plate attached to boiler. It gives you the input for the appliance and also the output. Difference between the two is the heat lost during operation of the unit. My furnace is rated at 60000 btu/hr input, 54000 btu/hr output. 6000 btu’s lost for every hour the machine operates. Only 54000 btu used for heating. Makes it 90% efficient. Sometimes gas burned here measured in cfh…cubic feet per hr. 1 cu ft gas equal to roughly 1000 Btuh. So for every hour my furnace runs it burns 60 cu ft of gas. My meter measures gas use in cubic metres so 60 cu ft is roughly 1.7 cu meters for every hour my furnace runs. 1 cu meter = 35 cu ft approx. Yes you guys obviously use different UOM but the principles are the same. Input vs output. Have to go.

I think your problem is how well the residence holds the heat provided. The appliance efficiency can also change with usage but regular maintenance should keep it pretty close to the factory efficiency rating.

[confused2]

[Image: Condensing-boiler-example-benefits.jpg]

Basically a water heater. If you burn gas at 8kW rate the water inside will get hotter until a sensor switches the gas off. If your load is (say) 1kW the boiler will burn gas for (say) 10 seconds and be off for 70 seconds .. there is a claimed efficiency of 90% so a claimed 8 in and 8x0.9 out which is what I am suggesting is fictional unless you have a load of 8kW and the boiler runs continuously at a stable temperature with water coming back to the heat exchanger at 50C or less.

Best pic was from NZ so NZ dollars. UK boilers can 'modulate' .. for hot water and max heating you have around 32kW and a gas flow that can be reduced for heating to maybe 8kW.

[Zinjanthropos]

[Image: Condensing-boiler-example-benefits.jpg]

Basically a water heater. If you burn gas at 8kW rate the water inside will get hotter until a sensor switches the gas off. If your load is (say) 1kW the boiler will burn gas for (say) 10 seconds and be off for 70 seconds .. there is a claimed efficiency of 90% so a claimed 8 in and 8x0.9 out which is what I am suggesting is fictional unless you have a load of 8kW and the boiler runs continuously at a stable temperature with water coming back to the heat exchanger at 50C or less.

Best pic was  from NZ so NZ dollars. UK boilers can 'modulate' .. for hot water and max heating you have around 32kW and a gas flow that can be reduced for heating to maybe 8kW.

Guess I should have mentioned that over here a hi/eff forced air furnace is called a condensing furnace also, pretty much same principle as your diagram there. So what’s the problem? House not warming up fast enuf? Boiler itself highly eff but will cost you more the longer it runs? Won’t shut off? Not boiler’s fault. Seems either the residence is not well insulated or system (pipes, pumps, etc) losing some heat. Have you insulated water lines? Is thermostat poorly located? Survival Lily wears a lumberjack shirt when not naked.

Also if your house is not 100% airtight then you don’t really need air from outside. Lots of leakage in many houses. You just can’t isolate such a furnace in an airtight room without C&V air openings communicating with rest of house.

People blame their furnaces and boilers but they just provide what they’re rated for. It’s what you do with the heat they deliver.

[confused2]

@Z.
Thanks for your time - I have to admit it confirmed what I rather expected to happen.

So what’s the problem?

The problem is I have two boilers both with 90% efficient written on the side and when used at low output one is good and the other is more like 50% efficient. I can afford it but there are people going cold and/or hungry because their boiler is woefully inefficient to say nothing of the pure waste and extra greenhouse gases emitted as a result of that 90% efficiency label. I would have liked to make a case for requiring manufacturers to provide an efficiency curve as well as the current best efficiency figure. While I try to make clear how and why 30% is an inevitable consequence of poor boiler and control design you quote the label and suspect something is wrong with the room. I really don't know how to get past that - any non-expert will ask an expert and the expert will tell them the efficiency is already written on the side of the box. Possibly (OK probably) my presentation doesn't help but I'm not sure I can do much about that. I have designed a thermostat to replace the 'bad' one - if I can get the efficiency up to about 80% I'll be happy otherwise I'll probably scrap the boiler and replace it with another one from Vaillant which I already know work 'properly'.

[stryder]

Setting the timer(s) to output heat for 15 minutes every 3 hours, leads to 2 hours heating a day (It can be cheaper when using a timer than just a thermostat in that instance) however it's really more useful just for keeping the chill out of rooms rather than heating the place up.

In the case of my combo boiler, it's basically on it's last legs and has a nasty habit of not wanting to work at all. So I don't spend that much on gas even with the price hikes (in fact I think more gets spent on the connection surcharge than on the gas itself)

There are obvious things that people miss though, like bleeding radiators in centrally heated homes. Not bleeding them means you can end up with air in your radiators which means they both aren't as efficient at heating and can cause the boiler to trip out (cease functioning) more often.

The problem now however is that over the next 15 or so years it was suggested they (UK gov) would phase out gas boilers, however it still lacks a decent replacement and anything on the market isn't ready to be switch from say methane to hydrogen etc.