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

#41
Zinjanthropos Offline
If your getting 90% for a boiler than I think your doing alright. The guys I worked with always used to say there’s no such thing as a high efficiency water heater. Were they correct?

Best I leave you to it. Good luck.

Do they make heat reclaimers for power vent boilers? Never seen one but who knows if they do or what it’s called.
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#42
confused2 Offline
(Feb 24, 2023 07:24 PM)Zinjanthropos Wrote: If your getting 90% for a boiler than I think your doing alright. The guys I worked with always used to say there’s no such thing as a high efficiency water heater. Were they correct?

Best I leave you to it. Good luck.

Do they make heat reclaimers for power vent boilers? Never seen one but who knows if they do or what it’s called.

I'm left with my non-communicable concept - I can't actually go to anyone and say things need to be improved if I can't explain why - and I obviously can't. Thanks for being patient while I did my best.

flue gas heat recovery unit. It pre-heats the cold water feed but doesn't store much - more gimmick than useful.


flue-gas-heat-recovery-unit

[Image: Greenstar_Xtra_FGHRU_400px_x_230px_003-400x_.jpg]
[Image: Greenstar_Xtra_FGHRU_400px_x_230px_003-400x_.jpg]




https://www.worcester-bosch.co.uk/profes...essories/f
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#43
Zinjanthropos Offline
We’ve come a long way from a thermocouple/powerpile, millivolt gas valve, fan/limit control, 1/4 hp motor and a fan belt. That was all you needed to fix most furnaces in the old days…lol

You sound like a smart fellow C2. Hope you get it done.
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#44
confused2 Offline
At least I don't have to think about furnaces that heat 5 tons of of water up by 1F in an hour.
I was starting to think about 4 large dead moose indoors and one small one outside .. how quickly would the outside moose heat up?
Good news is you'll have more time to devote to the cleaner fish song.
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#45
confused2 Offline
Z. not accepting private messages .. the content would have been

me Wrote:I think you were right not to to accept my .. everything stays the same when the gas isn't burning .. a lot of the waste heat is caught up in the water vapour from the chemical reaction involved in burning. If someone else spots my error all well and good and we compare what we think .. else I don't think it matters except "You were right!" is always quite nice to hear.
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#46
Zinjanthropos Offline
(Feb 25, 2023 12:03 AM)confused2 Wrote: Z. not accepting private messages .. the content would have been

me Wrote:I think you were right not to to accept my .. everything stays the same when the gas isn't burning .. a lot of the waste heat is caught up in the water vapour from the chemical reaction involved in burning. If someone else spots my error all well and good and we compare what we think .. else I don't think it matters except "You were right!" is always quite nice to hear.

From experience I couldn’t give in. I joked about it earlier that the difference between repairman and designer is that I could ask what the hell were they thinking. I guess that will never change. How do some things get past the drawing board idk.

There’s a furnace out there made by Carrier. On every other furnace I’ve worked on the flame rod sensor, besides being the cheapest part, is readily accessible….one little screw and it’s in your hands. Except for this particular Carrier furnace where one has to remove the entire ventor motor assembly to access the screw. A 5 minute sensor cleaning job turns into a couple hours or more. Common to hear guys ask if designers ever work on their creations….lol
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#47
confused2 Offline
The problem I am attempting to address is that the efficiency of UK gas boilers falls dramatically when they are working far below their rated output.
+ trying to cover points that have previously caused confusion..

The UK 'boiler' has no function other than to heat water.
The water heated by the boiler goes into a loop where it is continuously pumped out hot to water filled 'radiators' and return slightly cooler to be warmed up before carrying on round the loop.

Piccy here:
https://www.scivillage.com/thread-13684-...l#pid56694


Air used by the boiler comes in cold from outside and is vented hot to the outside. It will be heated either by burning fuel gas or just by being passed through an efficient heat exchanger full of hot water.

Any air heated by the boiler is heat wasted. Fuel gas is what comes in from the gas main. Flue gas is what comes out of the flue when gas is being burned - there's mostly nitrogen (80% ish), some CO2 and a few other bits and bobs in it. As will hopefully become clear - a lot of what comes out of the flue is simply hot air.

A typical UK thermostat will call for heat while the room temperature is below the indication on the dial (set-point) and will continue to call for heat until the room temperature rises considerably above the set-point.

When the thermostat calls for heat a typical UK boiler will burn gas in a flow of air to raise the temperature of the water in a heat exchanger to (say) 65C - above 65C the gas turns off but the flow of air does not. The boiler will again burn gas when the temperature of the heat exchanger falls below (say) 62C. The cycle with continue until the room thermostat stops calling for heat.

By law UK boilers are required to be able to work in 'condensing mode' where the temperature of the outgoing air is below the dewpoint of water (55C). To comply with the law some manufacturers retained old designs but tacked on a second water/air heat exchanger so hot exhaust air preheats cool(ish) water returning from the radiators. Obviously the boiler only 'condenses' when the returning water is substantially below 55C which very often it isn't. It is possible the main heat exchanger in these modified boilers would corrode rapidly if condensation occurred in them so advisors are split 50-50 between "Run the boiler as cool as possible" and "Don't go below 65C".

To get a reasonable amount of heat from a radiator it has to be hot (!) but we also want cool water (below 55C) to allow the boiler to condense water out of the flue gas .. the trick is to reduce the flow of water so it comes in hot (say 65C) at the top and leaves cool (below 55C) at the bottom. This can work well with ten radiators but with one .. reducing the flow also reduces the flow through the heat exchanger in the boiler which with (say) a 10kW burner will raise the temperature of the small amount of water it contains by 3C in a matter of seconds - after which time the burner turns off. I don't know how much inefficiency these short burns (blips) add.

I can't make the point without numbers .. I'd foul up if I tried to use American units so its in metric - hopefully the gist of it will still be clear.

1 m^3 of UK gas gives roughly 10kWh when burned.
For combustion the ratio of air to fuel gas is 10 to 1

In what follows the 'air' is not used to heat the building - it has no function other than to allow fuel gas to burn and to transfer heat to the water which is what we actually use to heat the house.

A 10kW boiler theoretically uses 1m^3 of gas per hour and while it does it it heats 10m^3 of air per hour
Assume a rise of 50C .. to heat the air requires 1kJ/C/m^3
10x50 = 500 kJ or 0.14 kWh or just 1.4 % of the claimed output.

Now we attach a 500W radiator to the boiler .. the thermostat calls for heat for an hour so we still blow 10m^3 of air through the hot heat exchanger with the gas burner blipping on and off.
So we have a useful heat of 0.5 kWh and we've lost 0.14kWh in the hot air .. now 28% lost.

Now for the real crunch.. many UK boilers (mine) can run at high power (say 24kW) because they can also provide hot water for showers. They 'modulate' - lower the output when high output isn't needed. So the fan can blow enough air through to burn gas at 24kW .. more than twice what it needs at 10kW. Does the fan blow less air when it isn't needed - can't say I've seen it do it. Maybe with 10 radiators and given time to settle under nice test conditions it might .. but I haven't seen it do it. So maybe I'm burning my gas with 24 m^3 of air for every 1 m^3 of gas. My loss is 2.4 times greater than the 0.14kWh calculated above which gives 0.336 kWh lost .. 60% efficient .. with the gas blipping on and off and probably no condensing I think 50% efficiency would be a fair estimate.
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#48
confused2 Offline
Finally the home grown thermostat has been tested.
Two houses a few miles apart are in constant use
Normally the Test House uses 40 to 80% more gas for heating than Reference House (which has a 'good' thermostat).
Replacing the Test House thermostat with the own design thermostat .. after one month the Test House used just 17% more gas than the Reference House. Hardly a lab test but certainly the potential to cut fuel use by 20% with no loss of comfort looks very promising.
1st April to 30th April 2023 Devon (South) Uk

New thermostat:-
Reference House 584 kWhr
Test House 683 kWhr (+17%)

Old thermostat (typical)
Reference House 636 kWhr
Test House 982 kWhr (+37%)

The Mk2 version won't rely on the house WiFi network which turned out to be unreliable and will be further optimised to increase condensation from bullshit mode to something more efficient and also reduce the waste caused by the boiler using cold outside to cool itself after a heating cycle.

In UK English
Both houses average under 1 kW which is where the 400W loss due to air forced through the boiler either for combustion or 'cooling' is a substantial loss.

In American English
Both houses average under 3,400Btu/hr which is where the 1,400 But/hr loss due to air forced through the boiler either for combustion or 'cooling' is a substantial part of the fuel used.
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