Gas rate confusion

[TECHY 44]

Hi all,

I am new to this forum and I must say straight away that I am neither a plumber or a gas engineer, just a confused citizen seeking your help.
I have a gas central heating boiler that is about 35 years old and I would like to work out how efficient it is.
I have used the formulas given on this and other forums but I am still confused by the results I am getting.
I am setting my boiler to run full pelt then timing how long it takes for the dial on the gas meter to do one revolution. I have included a picture of my meter.
The dial is taking 132 seconds per revolution ( checked several times ).
From all the formulas that I have used it returns an answer of about 8KW.
My boiler is an Ideal Concord WRS255. Heat I/P, min 17.9KW, max 21.4KW.
Heat O/P, min 13.2KW, max 16.1KW. These figures are from off the boiler door itself.
Here is my confusion how is it that I am getting an answer of 8KW when the boiler states Heat I/P max as 21.4KW.
Hope some kind sole can enlighten me.
Thanks.

 

[fuzzy]

answer 8.3kw. make sure the heating is cold before you gas rate again, then post your results back on here

 

[kirkgas]

i would expect it to burn 1ft3 in just over 50 secs, so first Q needs to be is the house hot enough? if it is burning less than 1/3 of the gas it should then i would say not, if you are a gas engineer the next thing would be to check standing and working pressures, then come back to us, your calculation is correct based on 132 secs, so perhaps do it again and make sure the reading on the left goes up by 1ft3 as well

 

[Kemble]

I think that the first question needs to be... "What are you expecting the answer to look like ?" If you are looking for an efficiency percentage such as 89%, implying that 89% of the energy that goes in, comes out as usuable heat energy in the home... then you already know that... At maximum output we have 16.1/21.4 = 75%.... However this is based on when it was new... so I'd say less now...

If you want to prove this calculation, not only will you need to know how much energy the boiler is consuming, you will also need to know the usuable heat output... Whilst the input is relatively easy (you have already calculated it)... the output figure will be far harder to calculate as you'll need to know who much heat it has imparted into the water... i.e. temp rise and volume... without any heat loss in the pipes and radiators...

On balance... unless you are retired and in need of mental stimulation, I'd settle for a figure a bit less than 75%...

And before the Scaremongers chirp up... you do not need to be GSR to calcululate this....

Hope this helps...

 

[fuzzy]

I think that the first question needs to be... "What are you expecting the answer to look like ?" If you are looking for an efficiency percentage such as 89%, implying that 89% of the energy that goes in, comes out as usuable heat energy in the home... then you already know that... At maximum output we have 16.1/21.4 = 75%.... However this is based on when it was new... so I'd say less now...

If you want to prove this calculation, not only will you need to know how much energy the boiler is consuming, you will also need to know the usuable heat output... Whilst the input is relatively easy (you have already calculated it)... the output figure will be far harder to calculate as you'll need to know who much heat it has imparted into the water... i.e. temp rise and volume... without any heat loss in the pipes and radiators...

On balance... unless you are retired and in need of mental stimulation, I'd settle for a figure a bit less than 75%...

And before the Scaremongers chirp up... you do not need to be GSR to calcululate this....

Hope this helps...

think your going a bit far, he doesnt need to know this, just wants to know why the boiler appears to be running at 50% its capacity and why?

 

[gasmarc]

sounds to me its under gassed a job for a GSR engineer me thinks if everything aint getting hot enough , it might be a one bed flat which would be ok

 

[GrahamM]

Firstly, it's age is an issue. You will not get near what it's heat input was new. 35 years old is very old.
Secondly, as gasmarc mentioned, it is probably under-gassed. Would the gas supply be 1/2" with a multitude of elbows by any chance.
Even if you could get a decent gas supply to it there would be a question of would a boiler of this age take the abuse of been run to a higher input after all these years. And if it would how long would it last.
Maybe it would be best to try bring it out of retirement.

 

[gasmarc]

i dont suppose you were running a gas fire at the same time , I agree with GrahamM but may well be worth getting an engineer to check HX clear and check the burner pressure etc

 

[fuzzy]

need a gsr to check working and burner pressures

 

[TECHY 44]

Thanks for the advise fuzzy.
I did as you sugested and made sure the heating was cold then did the test again, but I got the same result.
My house heating and hot water are okay.
Just as a matter of interest I did a similar test with my gas fire, which is rated; max I/P 6KW, max O/P 3KW.
With no other gas appliances turned on and the fire turned to full on, it took 225 seconds for one revolution of the gas meter dial.
I make that to be about 4.85KW which is a lot closer to the stated I/P than is that of the boiler.
Thanks again.

 

[TECHY 44]

Thanks for the advise kirkgas.
Yes the house is hot enough it will easily get to 20deg-25deg and the hot water is very hot dependant on the hot water tank thermostat setting.
According to my energy providers bill each unit on the gas meter = 100ft3, so if you mean the first figure on the left of the dial ( red figures ), I believe this represents 1/10th of this ( 10ft3 ), this is born out by the fact that it is taking 22 minutes ( 1320 seconds ) for these figures to change whilst the dial is going round at one revoluton every 132 seconds.
So I believe this is correct 10 revolutons =1/10th of a gas meter unit = 10ft3.
Thanks again.

 

[obseen16]

when did it last have a service? sounds like a long time ago.

 

[metaldust9]

132 sec is near as dam it to 8kw net probs 35 yr old boiler past its sell by date

 

[Reg Man]

sounds to me its under gassed a job for a GSR engineer me thinks if everything aint getting hot enough , it might be a one bed flat which would be ok

Sorry GASMARC, I don't think you meant the last bit. If it is a one bed flat it would not be OK. The boiler is manufacturered as a range rate appliance, so is designed to operate within its design margin (17.9kW to 21.4kW) Totaly agree with the first part though; This boiler is under gassed and requires attention of a GSR engineer. It could be something as simple as an adjustment and service, or gas pressure supply problem.

 

[kirkgas]

132 sec is near as dam it to 8kw net probs 35 yr old boiler past its sell by date

hoy not everything over 35 is past it (ALTHOUGH I BET MY MISSUS WOULD SAY DIFFERENT) he needs a gas engineer to have a look, potentially this could be repaired as it is likely to be the injector, burner or gas supply being inadequate
you are right but prob best option is to renew the boiler, but this would almost certainly require the gas supply to be upgraded anyway,

 

[kirkgas]

Thanks for the advise fuzzy.
I did as you sugested and made sure the heating was cold then did the test again, but I got the same result.
My house heating and hot water are okay.
Just as a matter of interest I did a similar test with my gas fire, which is rated; max I/P 6KW, max O/P 3KW.
With no other gas appliances turned on and the fire turned to full on, it took 225 seconds for one revolution of the gas meter dial.
I make that to be about 4.85KW which is a lot closer to the stated I/P than is that of the boiler.
Thanks again.

this would def lead us down the line of undersized/blocked/too long pipework as it will supply a 6kw appliance but not an 20+kw

 

[metaldust9]

yep kirkgas i would go with undersized pipework just was thinking along the lines of amount of gas a 35 yr old boiler would use (ie old technolagy) also safety devices ,getting parts etc

 

[kirkgas]

yep kirkgas i would go with undersized pipework just was thinking along the lines of amount of gas a 35 yr old boiler would use (ie old technolagy) also safety devices ,getting parts etc

yes i agree that the best option would be to renew, as a dinosaur im not 100% sure that the savings are all they are cracked up to be, by the time 6+ engineers visit under warranty and over the 5yr life cycle of the new boiler the cost to customer and ozone layer by all the transit vans