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

[Zinjanthropos]

The unit/appliance burns 30cfh . Gas pressure, orifice sizes, burner ports etc. That's all its going to accomplish once confgred. That's 30000 btuh. 3000 of that goes up chimney. No question. The eff rating does not include heat loss in rads or whatever else that isn't air in the heat distribution range that gets warmer. That's what I was intimating earlier, the system also loses heat but that doesn't affect the appliance efficiency. Manufacturers do their bit to reduce gas consumption but the owner of the appliance has to do their part I. E. insulate, t-stat location, air returns, maintenance etc

[confused2]

30,000 btu 3000 of that goes up chimney. No question.

Air goes in at 32F and comes out at 120F .. 3,000 btu
Turn the gas off and blow air through the hot furnace
Same amount of air goes in at 32F and still comes out at 120F (now heated by the hot water in the boiler) .. is that a 'free' 3,000 btu or something else?
The hot water gets cooled 3,000 btu by the air (and another 3,000 btu at the radiator) and when the water temperature falls below 115F the boiler burns gas until it is back up to 120F.
A 3,000 btu radiator is common in the UK .. the 3,000 btu up the chimney is also common .. as I see it that's 50% of gas used wasted in heat going up the chimney - you don't see it like that?
The things you suggest start after the basic efficiency is 50% (or less). The problem is the thermostat .. try explaining that and .. we know what happens.

[Zinjanthropos]

The rating plate is saying you have 90% of the unit's input to work with. That's it. Trick is for you to make the most of it. Many times.I've heard someone say their house is no warmer than it was before hi eff Furnace installed. And they're right but they will use less gas in the process. Years ago govts provided incentives to manufacturers to decrease nat gas usage and this is what we're seeing. Everything from eliminating constant pilots to modulation. Less gas used for the same results basically. Up to owner to improve their resident/system heat loss .

[confused2]

Many times.I've heard someone say their house is no warmer than it was before hi eff Furnace installed.

The old systems had what we call an accelerator in the thermostat (a little heater by the bimetallic strip) which encouraged the boiler to turn on and off instead of turning it on until the room has heated by 2F. An old furnace running at a real 80% efficiency could easily beat (by 30%) a modern one (rated 90%) but actually 50% efficient. Edit old boiler and thermostat using 30% less gas for the same amount of heat.

[Zinjanthropos]

Many times.I've heard someone say their house is no warmer than it was before hi eff Furnace installed.

The old systems had what we call an accelerator in the thermostat (a little heater by the bimetallic strip) which encouraged the boiler to turn on and off instead of turning it on until the room has heated by 2F. An old furnace running at a real 80% efficiency could easily beat (by 30%)  a modern one (rated 90%) but actually 50% efficient. Edit old boiler and thermostat using 30% less gas for the same amount of heat.

We call that a heat anticipator. I think the digitals have it built in but you can vary it just like the old stats that have a little dial you can set. You can adjust the length of one heating cycle with it also. I still use old mercury stat. The anticipator is a heater in itself, warming the mercury in my case so it moves the bulb that trips the furnace switch. All it does is allow you to use heat leftover after furnace off and not overheat or go higher than your setting. It doesn’t shut off your t-stat which is a switch by the way, but aids it and the furnace will use less gas to get to that point, indisputable.

You mixing gas used with actual heat and time for efficiency I think. Efficiency is about gas consumed. 80 cfh is 80000 btuh. 1000 btuh roughly equals 1 cu ft gas and that may be a source of confusion. 80000 btuh furnace will burn 80 cu ft of gas in one hour of operation whether it’s hi eff or not, simple fact.

Let’s say you have a 100000 btuh natural draft furnace and it’s 75% efficient. Its burning 100 cfh and using 75 cfh to heat the home. Rest is lost up chimney. You’re worried about gas bill too high. So you buy a hi eff furnace. Good thing is you don’t need to burn 100 btuh for same results. You can buy a 95% power vented hi efffurnace that is only 80000 btuh and retire the chimney in some cases. You still get your 75 cfh to heat home but burn 20 cfh less. It’s a no brainer. Survival Lilly would do it I’m sure.

What’s happening to the heat in the home is a bigger problem. Does the snow on your roof melt whereas your neighbours doesn’t. If so you have heat loss. Old houses have plenty of leaks bringing in cooler air. Window and door seals, all kinds of factors will increase the time req’d to heat house. Heat loss has absolutely nothing to do with furnace other than some parts absorb a little of the heat.

[confused2]

Heat loss has absolutely nothing to do with furnace other than some parts absorb a little of the heat.

Oh dear.

Maybe try a heater that uses hot water to heat air. It has a very efficient heat exchanger though that really isn't important for now. A fan blows air through the heat exchanger. Hot water flows into the device at 120F and comes out at 118F .. the heated air leaves the device at very nearly 120F. The plate on the back of the device says 3,000 btu/hr at 120F. The 'device' runs continuously (24/7) and is our furnace with no gas being used. The device (our furnace) takes the cold air in from outside and sends the hot air back outside - this hot air makes no contribution to heating the house.

Sharing the same hot water is a radiator also rated 3,000 btu which is inside the house and provides useful heat. When the water temperature falls to (say) 115F we burn gas which heats the water up to 125F. The inside radiator and the furnace then take heat from the water until the cycle repeats.

Of the gas used by the furnace only 50% provides any useful heat.

[Zinjanthropos]

No way you’re getting free gas …lol. You sure it’s not cycling on high temp limit switch instead of t-stat. Overheating will cause continuous run of circ pumps, take longer to come up to temperature, boiler appear to never shut off plus maybe other probs and higher gas bills. Something ain’t right there. That limit switch could be saving you from burning down the place. Cycling on the limit switch could make you think the t-stat is wonky. Limit switch will shut burners off but as soon as this switch cools it will make and relight burners. During that time the stat will continuously call for heat until it eventually comes up to temp. You will also hear vent motor continuously for long periods.

If you look at boiler and burners go off, unless you have a voltmeter, you don’t know if t-stat is satisfied. Use schematic to find limits location and put leads across contacts, it’s usually a normally closed switch.

If it is cycling on limit you need to find out why. Blockage of water, circ pump impellers, over firing from some poor adjusting of input pressure etc. There are other safeties too. If your outside air openings are partially or fully blocked then vent pressure switch may shut down burners intermittently but be still running.

Other problems could be reverse polarity or no ground wire. Some of these things sense pilot flame by pushing microamps thru the flame to ground. If there’s a flame rod immersed in your pilot or main flame make sure it’s clean and flame hitting it well. Could be intermittent and if so boiler will run on and off until desired temp reached at stat. Cheapest part on a gas furnace or boiler these days. Sandpaper works too.

[confused2]

No way you’re getting free gas …lol. You sure it’s not cycling on high temp limit switch instead of t-stat. Overheating will cause continuous run of circ pumps, take longer to come up to temperature, boiler appear to never shut off plus maybe other probs and higher gas bills. Something ain’t right there. That limit switch could be saving you from burning down the place. Cycling on the limit switch could make you think the t-stat is wonky. Limit switch will shut burners off but as soon as this switch cools it will make and relight burners. During that time the stat will continuously call for heat until it eventually comes up to temp. You will also hear vent motor continuously for long periods.

If you look at boiler and burners go off, unless you have a voltmeter, you don’t know if t-stat is satisfied. Use schematic to find limits  location and put leads across contacts, it’s usually a normally closed switch.

If it is cycling on limit you need to find out why. Blockage of water, circ pump impellers, over firing from some poor adjusting of input pressure etc. There are other safeties too. If your outside air openings are partially or fully blocked then vent pressure switch may shut down burners intermittently but be still running.

Other problems could be reverse polarity or no ground wire. Some of these things sense pilot flame by pushing microamps thru the flame to ground. If there’s a flame rod immersed in your pilot or main flame make sure it’s clean and flame hitting it well. Could be intermittent and if so boiler will run on and off until desired temp reached at stat.  Cheapest part on a gas furnace or boiler these days. Sandpaper works too.

Something ain’t right there.

Yep. I don't know whether it was the air heater example or just persistence that lead to that.

Before looking at at possible equipment faults could we look at design problems?

My installation is probably fairly typical of those which are causing people to choose between eating and staying warm.

Big boiler. Small radiator. Not much water in the system. Room with high thermal mass. Thermostat with wide switching band and slow response. A boiler which does the 'air heater' thing for as long as the thermostat calls for heat.

How we got here.. Boilers have to be 90% efficient at some output chosen by the manufacture and anything that can make and break contacts when temperature changes is good enough for us (scratch resistance is very important).

British boilers are designed (using the term in its loosest sense) around a flow temperature of 67C and a return temperature around 42C (a 15C drop which doesn't translate to F). With low flow and very little water around the boiler will be up to set-point and cycling rapidly before you can say "How crude is my boiler.". On a fairly warm day you have about 15 minutes of this cycling madness before the thermostat cuts off about 2C above where you started followed by 45 minutes of peace before the madness repeats. What with the constant loss up the chimney and the cycling madness to say nothing of the temperature overshoot - I think 50% efficiency would be about right. Are any of these things actually 'faulty' or were they just 'not fit for purpose' from the start?

[Zinjanthropos]

It’s probably my fault. We do things different over here so some terminology Brits use I’m trying to decipher. Maybe American readers here might benefit from what I’ve been saying, if only for their own appliances. You the designer, me the repair guy, big difference, Although I get to say ‘ who the hell designed this’.

I don’t think I’ve ever measured water temperature or flow rate in a boiler. For me if the water’s not flowing then the first place I might look is a flow switch if available. Flow switch paddles jam up at times. Before that I’d check the circ pumps & pressure gauges. Water temperature & relief valves blowing water a good indication water too hot. Easiest thing to check is input of gas if unit is firing. Enuf of that. I’ll tell you a story and you tell me if related to what your dealing with.

I changed a gas meter at one house during the winter. I don’t leave until I check the safe operation of all gas appliances downstream of meter. However in the winter, outside pool heaters(water boilers) are impossible without water in the pool so I just make a note of that to cover my ass. At this house I noticed the gas supply piping to the unit seemed awfully small so I checked the input for the heater. It was 250000 btuh, the gas supply piping was 1/2” and over 100’ long. Doesn’t take a genius to see that’s too small.

I talked with the owner about the pool heater’s performance during the summer. I asked him if it ever shut off. Answer was no , and that made sense. This boiler because of the gas supply piping being undersized, worked at about 50% rated input I figured. Maybe that’s what you mean by efficiency. In this case rated at 250000 but burning at 125000.

Side note: Same day by chance, I had coffee with the gas fitter licensing inspector for our area and told him about it. Saw him again couple months later and he told me he not only checked the installation I looked at but at recent jobs by the installer. For almost every installation the gas piping was too small. Company was given a cease & desist order and told to replace piping on all installations with the undersized pipe.

So undersized gas piping can affect performance of units. Water lines no different I’d imagine. The nat gas code book I carried with me had pipe sizing charts for specific inputs and lengths etc. Underfired equipment can run for long periods. Besides gas pipe size there are other things that can reduce rated input but for glaring issues, pipe size usually the culprit.

Could that be what you’re telling me?

[confused2]

OK some confusion about efficient and effective. I'll try not to muddle the two up from now on.

A while back you started with something and had to go.

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.

1 cu ft gas equal to roughly 1000 Btuh.

Later I got the impression you don't have a chimney (flue) so you breathe the air that has been through the furnace. So your furnace burns 60 cu ft gas which you say gives 60x1000 - 60,000 btu (when you burn it) yet when you burn it you only get 54,000 btu - where is the 6,000 btu going?

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.

When I say "When we pass air through a furnace (UK boiler) full of hot water the heat from the water is transferred to the air" .. nothing .. no nod .. nothing. Can we confirm that the concept of water heating air is non-communicable?