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

[Zinjanthropos]

My furnace is a mid eff, no secondary heat exchanger, no water drain lines so there’s a need to remove water Vapor before it condenses inside unit. That is exhausted at a vent terminal on side of house. So 6000 btus out the exhaust vent. Power vented appliances have made the chimney obsolete in most cases.

When I say "We burn gas in a furnace (UK boiler) to make hot air and the heat from the air is transferred to the water which we use to heat the house" I get the impression there's nods all round.

Can’t recall ever seeing anything like that. A boiler that heats air to heat water to heat house but I’ve been out of the field for many years. Do you have hot water lines in floor for heat?

We have combi units. A storage water heater (may be you call it a boiler) heats water for bathing, washing dishes etc) and in this case is also piped separately to a heat exchanger located in a plenum downstream from a circ fan which on a call for heat from a house t-stat moves house air thru to be heated and distributed.

Europe has been using instantaneous water heaters instead of storage for years and they have arrived here and slowly becoming more popular. If you use the term water heater here it only refers to bathing, washing water. Here a boiler heats and sends hot water to radiators to heat house air.

[confused2]

Can’t recall ever seeing anything like that. A boiler that heats air to heat water to heat house but I’ve been out of the field for many years.

'Air' is 80% nitrogen .. nothing you can do about that. To ensure complete combustion gas is burned in an excess of air .. roughly ten cu ft of air to one of gas - or so I am told.  Unless you have access to pure oxygen whatever goes on in the furnace pretty much 'air' - you disagree? In fact it doesn't really matter what happens inside the furnace as far as efficiency is concerned .. all we need to know is what temperature the air went in at, what temperature it came out at and how quickly it flows through. We can simplify the process by reading the difference between energy in and energy out (I think you said this was written on the furnace somewhere) - the difference is what has been used to heat the air - yes?

[confused2]

Edit..

For natural gas-fired burners, the stoichiometric air required is 9.4-11 ft.3 / 1.0 ft.3 of natural gas or approximately an air-to-gas ratio of approximately 10:1. In this case, there is an excess oxygen level of 2%.

https://sagemetering.com/combustion-effi...fficiency/

[Zinjanthropos]

Can’t recall ever seeing anything like that. A boiler that heats air to heat water to heat house but I’ve been out of the field for many years.

'Air' is 80% nitrogen .. nothing you can do about that. To ensure complete combustion gas is burned in an excess of air .. roughly ten cu ft of air to one of gas - or so I am told.  Unless you have access to pure oxygen whatever goes on in the furnace pretty much 'air' - you disagree? In fact it doesn't really matter what happens inside the furnace as far as efficiency is concerned .. all we need to know is what temperature the air went in at, what temperature it came out at and how quickly it flows through. We can simplify the process by reading the difference between energy in and energy out (I think you said this was written on the furnace somewhere)  - the difference is what has been used to heat the air - yes?

If you put it that way then sure. Air is drawn into the burner, mixed with gas entering the Venturi and burned at the ports. Any other air in burner compartment gets heated of course and that heats the metal which heats the medium. These are sealed combustion chambers you’re talking about I think. Usually with a useless little round window you can’t see much thru. We call it direct venting…..no communication with house air.

I think I get what you’re saying. The air being heated in the combustion chamber is responsible for the lost heat or the difference between input and output. Something like that. Never thought of it that way. People were burning gas and making calculations long before I got here.

[confused2]

Can’t recall ever seeing anything like that. A boiler that heats air to heat water to heat house but I’ve been out of the field for many years.

'Air' is 80% nitrogen .. nothing you can do about that. To ensure complete combustion gas is burned in an excess of air .. roughly ten cu ft of air to one of gas - or so I am told.  Unless you have access to pure oxygen whatever goes on in the furnace pretty much 'air' - you disagree? In fact it doesn't really matter what happens inside the furnace as far as efficiency is concerned .. all we need to know is what temperature the air went in at, what temperature it came out at and how quickly it flows through. We can simplify the process by reading the difference between energy in and energy out (I think you said this was written on the furnace somewhere)  - the difference is what has been used to heat the air - yes?

If you put it that way then sure. Air is drawn into the burner, mixed with gas entering the Venturi and burned at the ports. Any other air in burner compartment gets heated of course and that heats the metal which heats the medium. These are sealed combustion chambers you’re talking about I think. Usually with a useless little round window you can’t see much thru.  We call it direct venting…..no communication with house air.

I think I get what you’re saying. The air being heated in the combustion chamber is responsible for the lost heat or the difference between input and output. Something like that. Never thought of it that way. People were burning gas and making calculations long before I got here.

Yes, sealed combustion chambers. Alas no window to check for pilot light and blue flame.

Many many thanks for your input and staying with this. I can (now) see that my assumptions about what happens when the furnace isn't burning gas would seem crazy without total clarification of what goes on when the furnace is burning gas. I'd greatly appreciate any suggestions for how we might have got there faster (I'll take me being smarter as read). If we have some agreement about what is going on when the furnace is lit .. what about blowing the same amount of air through the heat exchanger while it is hot and there is no gas burning..?

[Zinjanthropos]

what about blowing the same amount of air through the heat exchanger while it is hot and there is no gas burning..?

Already done. On all forced air furnaces the house air circulation fan continues to run for a period of time after burners shut down but for some reason I think you may be talking about something else. Remember the heat anticipator or accelerator as you call it? How much air would vary from furnace to furnace. Substitute water for air and pump for fan if boiler.

My ‘for some reason’ makes me think your version of air thru heat exchanger is different than mine. Are you suggesting somehow allowing the heated air in a sealed combustion chamber to enter the house after burner shuts down? If so can’t see it happening because if burner(s) is having combustion issues you could repeatedly release CO into the house.

Amusing Anecdote: I was in a real filthy house. They had no heat. Hi eff with sealed combustion chamber. An idiot light was telling me flame failure, IOW not sensing auto pilot flame. Important because main burner needs it sensed to turn on. I had a sight glass I could see through. I reset the ignition sequence and observed. I saw the pilot light sensed and main burner turn on. It was a massacre as several cockroaches had somehow penetrated the combustion chamber. They were running around like little lit torches. Found a rubber grommet missing and the little devils must have crawled in thru there. It wasn’t sensing pilot because of a draft thru the opening affecting the pilot and obviously someone had tried to fix it by taking it apart. Cue the cockroaches. I also opened the fan compartment door and found an army of cockroaches. Best part…homeowner owned the restaurant directly across the driveway next door. Unreal

[confused2]

Are you suggesting somehow allowing the heated air in a sealed combustion chamber to enter the house after burner shuts down?

No. You really think I'm crazy don't you.
Still with the same sealed off from the inside of the house furnace...

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

The fan blows air through to give the gas some oxygen to burn in and to force the hot air to exchange heat with the cold bits.
There's 5 openings altogether.
Hot water out (We call it flow)
Cool water back (we call it return)
Cold air in (we call it air in)
Hot air out (we call air out)
Let's imagine the furnace is doing exactly what it was specified to do - we read off the loss which is written on the boiler somewhere.
Cold air is taken in, heated by the burning gas and forced through the heat exchanger to heat the water .. ok so far?
Now we turn off the gas leaving everything else the same
Cold air is taken in, forced through the exchanger and it leaves hot ..still ok?
With a 90% efficient heat exchancher it leaves darn nearly as hot as when there was actually gas burning so what we read off the boilerplate as loss when the gas is burning is pretty darn close to the loss when the gas isn't burning.
Of course when the boiler cools by a few degrees (say to 215F) we burn gas for a bit until it warms up to say 220F.
As long as the temperatures stay the same within a few degrees the loss we read from the boilerplate is very nearly constant regardless of whether of not we are burning gas. .. You agree?

[Zinjanthropos]

Are you thinking the loss is unnecessary or can be lessened to some degree? You think it should be redesigned? Other than that not sure what….maybe I’m just stupid to not get your point.

Have you forgotten two basic operations that already occur for safety purposes?

Power vented or induced draft furnaces have a sequence of operations. The first is a prepurge to rid the combustion chamber of any unburned gases present and to create a pressure in the vent for exhaust and possibly prevent pre ignition, that sudden boom or vibration you might hear. It won’t light without proof of venting to outside.The last operation is a postpurge to rid chamber of any lingering products of combustion like water. It has to be hot to begin with. Both of these operations take around a minute or so. I have smelled gas at the exhaust terminal during these operations, likely main valve not closing properly or leaking.

Prepurge in case fuel gas has leaked into chamber and postpurge to get rid of the water and prevent corrosion, scrub out remaining heat and any gas not burned. Both operations occur with burners off.

Other than what I’ve mentioned over many posts I don’t know what else we can do to the air entering the chamber except mix it with the fuel and light it on fire. We may have to sacrifice some gas and heat for safety purposes.

[confused2]

Prepurge in case fuel gas has leaked into chamber and postpurge to get rid of the water and prevent corrosion, scrub out remaining heat and any gas not burned. Both operations occur with burners off.

OK there are goblins that live inside furnaces and yes they will jump out of the furnace and burn your house down if you provoke them.
I'm not going to hurt them or interfere with them in any way - I'm just trying to establish what they doing with all the gas I give them

What with the goblin warnings I'm not sure if I've hit the jackpot and you agree that the heat loss/waste will be roughly the same (except no gas) if all the flows and temperatures stay the same.

[confused2]

Jumping ahead..
What can you do about ..?
A goblin called Etsaw (!) has built a nest in the hot gas outlet. The heat that is good for Etsaw is bad for me. Etsaw lives in air that is at very nearly the same temperature as the water returning from our radiators. To heat the inside of the house we pump hot water into the radiators and shut the boiler down as soon as the temperature of the returning water starts to rise. Etsaw might get a breath of air slightly above room temperature but that's all. We choose the temperature of the water and how often we send it out to keep the house warm and Etsaw very unhappy.

Until we're on roughly the same page with boiler heat losses I don't think there's any point in looking at ways to reduce the efficiency to 50% or lower.