Explanation of thermostat vs control temp

[Johntheo5]

I have seen numbers recently on some gas boiler manual where I think it showed that at full firing, the flue gas temperature was ~ 45/55C higher (its actually only 27C higher) than the return temp and if this is say 35/45C then as some of the gases will be in contact with the combustion chamber then some condensing will take place IMO. On minimum output the gases I think were within 10C (actually 7C) of the return water temperature. It would be well worth for someone with a gas boiler whose interested to actually measure the condensate flow at various boiler outputs and return temperatures.

from page 5 of attachment.

 

[vulcancontinental]

I have seen numbers recently on some gas boiler manual where I think it showed that at full firing, the flue gas temperature was ~ 45/55C higher (its actually only 27C higher) than the return temp and if this is say 35/45C then as some of the gases will be in contact with the combustion chamber then some condensing will take place IMO. On minimum output the gases I think were within 10C (actually 7C) of the return water temperature. It would be well worth for someone with a gas boiler whose interested to actually measure the condensate flow at various boiler outputs and return temperatures.

from page 5 of attachment.

View attachment 282795

Tried that a long while ago on one of ours and metered the gas consumptionn but really only useful on boilers with a separate drain from flue collector and heat exchanger. When combined the condensate will be included from the flue into the hex.

 

[Koolpc]

So, if one has a condenser boiler, set the flow (rad symbol on boiler) to 55 and that will make it more efficient?

 

[Johntheo5]

Yes.

 

[Johntheo5]

Tried that a long while ago on one of ours and metered the gas consumptionn but really only useful on boilers with a separate drain from flue collector and heat exchanger. When combined the condensate will be included from the flue into the hex.

Yes, but probably not a lot from the flue if fully hot as its not a heat exchanger.
My basic calcs assume the following, that 10% of the fuel input (gas) is the maximum recoverable if fully condensing, the latent heat of steam at atmospheric pressure is 2256kj/kg, call it 2256kj/litre or 0.627kw/litre, so max condensing per kw of gas burned is 1X10%/0.627, 0.16litres/kw. So a boiler running at a output of 30kw, say 31kw fuel input fully condensing should produce 0.16X31, 4.96 litres of condensate/hour??, it will obviously less in practise. If condensate flow is ~ 2.7 LPH then gaining 5% due to condensing gain.

 

[JonathanM]

the latent heat of steam at atmospheric pressure is 2256kj/kg, call it 2256kj/litre or 0.627kw/litre, so max condensing per kw of gas burned is 1X10%/0.627, 0.16litres/kw.

Thanks, I was trying to work out how to do that calculation yesterday. I had the latent heat figure, just couldn't work out how to turn that into the answer!

 

[Johntheo5]

Latent heat is fascinating nnd makes steam such a powerful heating medium for heating as it gives up its heat isothermally, ie, no change in temperature. The industry norm (or used to be) 8bar which is a temperature of 175.4C, if heating say a HW cylinder, all its (latent) heat, (0.563kw/kg) is given up at 175C as it passes through a coil/Hx, its just getting wetter and wetter until it gives up all its latent heat.

 

[vulcancontinental]

Yes, but probably not a lot from the flue if fully hot as its not a heat exchanger.
My basic calcs assume the following, that 10% of the fuel input (gas) is the maximum recoverable if fully condensing, the latent heat of steam at atmospheric pressure is 2256kj/kg, call it 2256kj/litre or 0.627kw/litre, so max condensing per kw of gas burned is 1X10%/0.627, 0.16litres/kw. So a boiler running at a output of 30kw, say 31kw fuel input fully condensing should produce 0.16X31, 4.96 litres of condensate/hour??, it will obviously less in practise. If condensate flow is ~ 2.7 LPH then gaining 5% due to condensing gain.

I think more from the flue actually as the heat exchanger has hot surfaces and the flue tube is surrounded by cold combustion air (concentric flue) all of which is far below the flue products dew point. I'm told one manufacturer include this in the calculation of the boiler efficiency as it warms the combustion air.

Look at a plume from a boiler running at <55°C or use a hygrometer to see the moisture content in the flue gases minus the moisture content of the combustion air.

Until latent heat ceases to be latent heat and becomes sensible heat it cannot improve the efficiency of the boiler.

 

[vulcancontinental]

Thanks, I was trying to work out how to do that calculation yesterday. I had the latent heat figure, just couldn't work out how to turn that into the answer!

Discharge condensate into a container separating flue from heat exchanger and run a m³ of NG through the appliance. It was easy for us as we used to always separate flue condensate from exchanger condensate keeping them separate.

 

[JonathanM]

Yes, but probably not a lot from the flue if fully hot as its not a heat exchanger.
My basic calcs assume the following, that 10% of the fuel input (gas) is the maximum recoverable if fully condensing, the latent heat of steam at atmospheric pressure is 2256kj/kg, call it 2256kj/litre or 0.627kw/litre, so max condensing per kw of gas burned is 1X10%/0.627, 0.16litres/kw. So a boiler running at a output of 30kw, say 31kw fuel input fully condensing should produce 0.16X31, 4.96 litres of condensate/hour??, it will obviously less in practise. If condensate flow is ~ 2.7 LPH then gaining 5% due to condensing gain.

So when you look at efficiency data, and the 30% part load test figure is usually about 10% higher than the full-load test figure, is that all due to condensing. If so, that would mean almost 100% of the theoretical latent heat has been recovered, which seems unlikely. But, if not from condensing, where does some of that extra efficiency come from?

EDIT: it's usually higher by 8% to 9%+, but not quite 10%

EDIT2: just checked the condensing graph, and it does look like at 30C return, an extra 9% efficiency is possible.

 

[JonathanM]

I think more from the flue actually as the heat exchanger has hot surfaces and the flue tube is surrounded by cold combustion air (concentric flue) all of which is far below the flue products dew point. I'm told one manufacturer include this in the calculation of the boiler efficiency as it warms the combustion air.

Look at a plume from a boiler running at <55°C or use a hygrometer to see the moisture content in the flue gases minus the moisture content of the combustion air.

Until latent heat ceases to be latent heat and becomes sensible heat it cannot improve the efficiency of the boiler.

I thought there would be a standard test for efficiency, rather than each manufacturer doing it differently.

Following on from the post immediately above, when I was googling boiler efficiency a few weeks ago, I came across a test result, and coincidentally it was for one of your boilers, an Easi-Heat Plus 29Ci. The bits I understood were really interesting, but it mentioned things well beyond me, like a direct and indirect method of measuring part-load efficiency.

 

[Johntheo5]

I think more from the flue actually as the heat exchanger has hot surfaces and the flue tube is surrounded by cold combustion air (concentric flue) all of which is far below the flue products dew point. I'm told one manufacturer include this in the calculation of the boiler efficiency as it warms the combustion air.

Look at a plume from a boiler running at <55°C or use a hygrometer to see the moisture content in the flue gases minus the moisture content of the combustion air.

Until latent heat ceases to be latent heat and becomes sensible heat it cannot improve the efficiency of the boiler.

Your calcs above show a max latent heat recovery of 0.93kw/M3 or 0.93kw/10.73kw. this is only 8.7%, looking at typical nat gas analysis (UK), LHV & HHV thought it worked out almost up to 11% wet gas loss.

Re heat gain to combustion air from concentric flue gas/air trunking, some of the marine/industrial boilers that I was very familiar with had a rotary air wheel which was full of corrugated metal strips which was turning very slowly (about 1.5RPM) one half was always in the flue gas flow and the other half in the incoming combustion air flow so incoming air was heated, this temperature was measured to give the correct calculations for air/fuel ratio.
Other boilers had a economiser where the gases exiting the boiler convection bank made a few passes through the economise and this heat raised the incoming feed water temperature, the aim was the same, to get the flue gas temperature as low as possible.

 

[Johntheo5]

I thought there would be a standard test for efficiency.

Following on from the post immediately above, when I was googling boiler efficiency a few weeks ago, I came across a test result, and coincidentally it was for one of your boilers, an Easi-Heat Plus 29Ci. The bits I understood were really interesting, but it mentioned things well beyond me, like a direct and indirect method of measuring part-load efficiency.

A fairly simple method (assuming no condensing) is to use a loss of 0.45% per 10deg flue gas temperature so a boiler with a flue gas temperature of say 60C would have a sensible heat loss, assuming combustion air at 20C, of 0.45X(60-20)/10 or 1.8%, add the 11% wet gas (condensing) loss so a total loss of approx 12.8% giving a boiler efficiency of 87.2%?.

The indirect method we used om our boilers was simply output/input, we measured the energy input in the fuel (HHV) flow into the boiler, and calculated the net energy in the steam output from the boiler, effic was output/input X100%

 

[JonathanM]

A fairly simple method (assuming no condensing) is to use a loss of 0.45% per 10deg flue gas temperature so a boiler with a flue gas temperature of say 60C would have a sensible heat loss, assuming combustion air at 20C, of 0.45X(60-20)/10 or 1.8%, add the 11% wet gas (condensing) loss so a total loss of approx 12.8% giving a boiler efficiency of 87.2%?.

So, with my 32 year old non-condensing boiler, I've read that flue gas temperature is typically about 130C for these, so if true, that would be a 4.95% flue loss, +11%, so 84.05% efficient.

But it's supposed to be a lot less efficient than that. What are the other losses? Is it the heat given off by the boiler into the room? Mine's in a cold garage, so this is lost heat. But if it was in the house, it would be warming the house.

EDIT: the manual gives very basic input KW and output KW and that ratio is 78%. Is that the efficiency?

 

[Johntheo5]

I expect so yes, input/output, 130C is pretty good, I have a 16 year old Firebird SE oil fired boiler with a flue gas temp of 230/245C., been like this since day 1.Wet gas loss of oil is ~ 6.0%.