Gas pressure drop along pipework

iep

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I've recently been trying to work out why our heating provides so little heat output (details in popcorn boiler post).

The end result was that I measured the gas flow and found that it was using 43 cubic feet per hour compared to its rated minimum of 57 cubic feet per hour.

At this point I threw in the DIY towel and called British gas to see if they'd send someone out (after phoning three local plumbers none of whom returned my call). The woman on the phone asked what the issue was and I said the burner was running low in the boiler and that there might be an issue with gas pressure. Apparently this phrase sets alarm bells ringing at British Gas because 45 mins later a transco engineer was at my front door to look for gas leaks.

Needless to say there is no gas leak but he was good enough to test the gas pressure at the meter (21.5mB) and then quickly tested it at my boiler too (really nice guy). At the burner, the pressure measured only 9mB which is well below the minimum rating of 11.8mB for the boiler (glowworm energysaver 60).

The guys parting comment was that I should get my internal gas main looked at before replacing the boiler as it could be restricting the flow.

I measured the length of gas pipework from the boiler to the meter last night and it does take a pretty strangled route through the house comprising:

18m of 22mm pipe with five 90 degree angles.
2m of 15mm with three 90 degree angles.

Is there any way to calculate if this is too long a run?

iep
 
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Needless to say there is no gas leak but he was good enough to test the gas pressure at the meter (21.5mB) and then quickly tested it at my boiler too (really nice guy). At the burner, the pressure measured only 9mB which is well below the minimum rating of 11.8mB for the boiler (glowworm energysaver 60).
You should not have more than 1mB drop from meter to boiler!

I measured the length of gas pipework from the boiler to the meter last night and it does take a pretty strangled route through the house comprising:

18m of 22mm pipe with five 90 degree angles.
2m of 15mm with three 90 degree angles.

Is there any way to calculate if this is too long a run?
Yes. Domestic Gas Pipe Work tells you how to do it. But you will need info about all gas powered items (fires, cookers etc) fed from the meter.
 
The 9mB was measured after the multifunction control unit so that should be reduced to a min of 11.8mB (see table 2http://www.glow-worm.co.uk/stepone/data/downloads_sd/58/00/00/energysaver60.pdf). He didn't give me a measurement at the input to the boiler.

It's possible of course that the burner adjustment is set too low but it seems crazy that the original installer would have set it below the minimum level for the boiler.

Regards the link, that is a brilliantly useful document, thanks.

I'm not sure of the rating of our cooker (the only other thing on the gas main) but I guess I can start by working out the drop without the cooker since it has not been switched on during any of the measurement I have made.

Cheers,

iep
 
He may have been measuring the burner pressure ( thats measured after the gas valve ! )

He should have measured the working pressure at the input to the gas valve.

You are well out of your depth with gas problems and its not yet clear to me what your problem really is. All you have determined is that the boiler output power is about 80% of the maximum. Its quite common to reduce the power input to meet the heat output required.

Have you calculated the heat loss of your house?

Tony
 
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Okay. Used the document and came to the following conclusions:

18m of 22mm pipe with 6 (I miscounted earlier) 90 degree angles = 20.8m
2m of 15mm pipe with 3, 90 degree angles = 2.9m

At full tilt, the boiler should use 20kW of gas = 1.87cubic meters per hour flow rate. For simplicity, I'll ignore the gas used by the cooker right now (since all testing has been done with it switched off).

At this flow rate and, according to the tables, I should be seeing 0.67mb of pressure drop over the 22mm pipe and a further 0.48mb over the 15mm pipe (man, the 15mm stuff kills you). So, a total of 1.15mb drop.

The gas pressure at the meter is apparently 21.5mb (thanks transco) so I should still have over 20mb at the boiler when it is running at full power (which it is not).

So, either the pipe is somehow partially blocked (seems unlikely to me?) or the gas control module in the boiler is goosed.

Either way, I realise that I'll need to change at least the last 2m of pipework from 15mm to 22mm if I want to get a larger boiler and possibly a bit of the earlier 22mm to 28mm to allow for the cooker.

Cheers,

iep
 
Hi Tony. Thanks for the advice. Don't worry, I know I'm out of my depth, with gas, not planning to adjust anything myself. Just looking to understand. And yes, he measured the burner pressure because it is the easiest to find on the boiler (see diagram 8.1 on p20). He was doing me a favour so I couldn't really complain.

http://www.glow-worm.co.uk/stepone/data/downloads_sd/58/00/00/energysaver60.pdf)

However, the boiler is running at a lot lower than 80% of its maximum power since the relationship between pressure and output power is not linear. The max setting for the boiler (at 20kW input) would require a flow of 68cubic ft per hour. I'm only measuring 43. That puts the boiler at 63% of its max value.

In its minimum mode the boiler should still use at least 57 cubic ft per hour. In this case it is still only managing 75% of rated output.

Having said that, in it's Low flame mode, it uses 33.6 cubic feet per hour which is very close to the rated value of 34. So, I guess that might suggest an issue with the high rate burner valve?

Yes, I did calculate the heat loss of my house using an online calculators (tried three in all and they came back with very similar figures). Final figure for heat loss was around 20kW so I'd have thought the boiler would have been set up to run at full power.

As ever, thanks for inputs.

iep
 
In non technical terms you are barking up the wrong tree!

You need an RGI to properly check the boiler settings, assess the heat requirements and set the boiler output appropriately.

No one in their right mind replaces gas pipework when:-

a. calculations show its correctly sized.

b. when the working pressure at the boiler has not been checmed.

c. when they are not gas registered.

If it really needs 20 kW then it must be quite a large house and ytou gas bill would be about £3000pa if you heat it properly.

Far better to keep to one topic so that anyone replying can see all the situation. I dont have time to go searching for your other threads.

Tony
 
Hi Tony,
I'm not planning to replace the gas pipe until I have has an RGI come out and inspect the boiler and inlet gas pressure (and only then, in the event that he/she finds something wrong with it).

I'm certainly not planning to do it myself. The purpose of this post was just to find out how to calculate gas pipe requirements.

It's not a very large house but we live in a location where the temp often drops to -10 in winter. It's also 270 years old with single glazed windows and solid walls. Obviously, I plan to improve insulation as I go along but the construction of the property does not lend itself well to insulating so we do need to have the facility to throw a bit of power at it when it gets very cold (and yes, the gas bills are terrifying).

Thanks again,

iep.
 
Yes, I did calculate the heat loss of my house using an online calculators (tried three in all and they came back with very similar figures). Final figure for heat loss was around 20kW so I'd have thought the boiler would have been set up to run at full power.
and
It's not a very large house but we live in a location where the temp often drops to -10 in winter. It's also 270 years old with single glazed windows and solid walls.
Did you take the -10C temperature into account when sizing the boiler?
Most calculators assume a minimum winter temperature of -1C. If the on-line calculator gives 20kW and assuming a required inside temperature of 20C, the revised boiler size for a -10C outside temperature will be [(20-2) x (20-(-10))/(20-(-1))] + 2kW

=[18 x 30/21] + 2kw = 25.71 + 2 = 27.71 kW = 28 kW.

The 2kW deducted from the boiler size and added at the end is the hot water allowance, already included by many on-line calculators.
 
The calculators include a 'Scotland' check box which allows for colder temperatures. I'm not sure whether they account for the full -10 that we saw back in Feb/March but they do go some way.

I realise they are not an exact science though and will definitely take the advice of an RGI when we come to put a new boiler in.

My own thinking though was to install something like a WB 24ri (24kW) and let it modulate down to the required load (which I understand they can do). That way we should have enough grunt to get at least 15 degrees on the really bad days and can supplement that with the wood burning stove.

As ever thanks, for the analysis. Useful to see how it is done.

iep
 
My own thinking though was to install something like a WB 24ri (24kW) and let it modulate down to the required load (which I understand they can do). That way we should have enough grunt to get at least 15 degrees on the really bad days and can supplement that with the wood burning stove.
You are correct about the WB boiler being able modulate down, in fact all modern boilers do this. This can be taken a stage further by using Weather Compensation controls. These measure the outside temperature and adjust the boiler output accordingly (high when it is cold, low when it is warm)

WB have a weather compensation control, but it is not compatible with the 24Ri.

Have a look a the Remeha range of boilers. They can all be used with weather compensation controls.
 
If the on-line calculator gives 20kW and assuming a required inside temperature of 20C, the revised boiler size for a -10C outside temperature will be [(20-2) x (20-(-10))/(20-(-1))] + 2kW

=[18 x 30/21] + 2kw = 25.71 + 2 = 27.71 kW = 28 kW.

I wish you'd never asked :eek: :LOL: :cry:
 
bodmas Dave...bodmas



at least that's what they used a very very long time ago when I were a lad.
 
Hope this doesn't turn into one of those "I do this - I do that" posts
ps I'm a bodmas man myself ;)
 

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