Gas supply intermittent problem?

[Lee Nowell]

Hi All,

I have a bit of a strange problem at the moment and was wondering if anyone can shed any light on it...

The setup... During our property renovations, we had our whole house replumbed including new boilers (2) and gas pipework from the meter. The pipework from the meter is 32mm (I think - essentially the largest a regular plumber can fit) which goes about 3m to the boilers where they are tee'd off and around another 5m to the gas hob. I can't recall the pipe size from the main run to the boilers (approx 0.5m each) but assume it is whatever the valve size is on the boiler.

The problem... Intermittently (approx twice a week) we have had one or both boilers locking out and the error code is something along the lines of "gas supply problem/ can't light etc.". In addition, on occaision (twice a month), when using the gas hob, the pressure goes low then usually returns after 30 second or so. We have switched one of the boilers off for just over a week now and no issue yet.

The diagnonsis... The plumber has checked the gas pressure a couple of times and it all seems fine. He seems reluctant to call out Transco until he can see the issue/ measure a pressure issue so is coming back in a few days to check the boilers.

Given the issue happens to both boilers and we have seen the hob issue, it seems to me that the likelihood of both boilers developing the fault is low and it must be something to do with the gas supply. Is this possible? Can the regulator intermittently work and fail?

Any other ideas?

All help appreciated.

Thanks

Lee.

[Paul D Smith]

What is the supply TO the meter like? We recently had the gas pipes in our street "replaced" which actually meant they shoved yellow pipe INSIDE the old iron (?) piping. For most householders this didn't cause problems but one of my neighbours had similar problems to you (but with a single boiler).

Turned out that there had been some sort of constriction in his pipes so the contractors had shoved smaller pipe onto his premises and it simply wasn't good enough. Required Transco to come and dig up the road and his entire drive to replace the pipe properly - but his supply is probably the best in the street now and he's very happy.

Paul DS.

[Andrew Gabriel]

Also, what's your max demand? I think they only expect to support about 55kW max on a domestic supply, and I'm guessing you might be going over that with two boilers and a hob. It might be that the pressure drop on your supply up to the regulator, plus other loads in the street, are causing the pressure at the regulator input to drop too low.

[Roger Mills]

But has he checked the pressure at the meter regulator with everything running full chat? It could be that the regulator simply can't handle the flow when everything is at full power.

[Roger Mills]

Lining the pipes is not normally a problem - other than in exceptional cases like that quoted above - because the supply-side pressure is

*much* higher than the regulated pressure - so the same mass flow can come through much smaller pipes than those between meter and appliances.

[Lee Nowell]

Thanks all....

In terms of demand, from memory, I believe the boilers are 24 kW each so easily within the 55 kW limit. The "lock out" seems to mainly be late in the evening/ over night as we wake up to a cold house so the hob is almost certainly not on at the time :(

The suggestions so far seem that the problem would exist all the time?

Any ideas why the plumber is reluctant to simply call Transco out to check it all over - do the black list him if there is no issue? He has furthe work to do on the house anyway so could do that whilst he is waiting.

thanks

Lee.

[Brian Gaff]

Well, the last time this happened to a friend it turned out, in the end to be water in the external poip, which meant the pipe had a small leak. The pressure was not always enough to shove the water out of the way. Brian

[David WE Roberts]

I assume that you have made sure that both boilers are running flat out at the same time and then turned on all rings of the gas hob at full blast - just to confirm that under normal conditions with everything on there is still enough gas? This would then point to an intermittent low pressure in the gas supply. If you see low pressure under these conditions then the failure may occur when the first boiler is lit and the second then tries to light but doesn't have enough pressure. Depending which boiler lit first, either could fail.

The timing is interesting - I assume you are having overnight failures with the boilers, but also have seeen problems with the hob during "normal" hours.

You don't say if the boilers failed when you had the low pressure at the hob.

I am mildly intrigued as to why you have two boilers - did you follow the advice of one well known contributor to this NG?

As you say, it is unlikely to be a boiler fault from the symptoms you describe.

I suppose what you would need to diagnose this fully is a continuously recording gas pressure gauge attached to the supply for a week. However a quick Goggle suggests that this is not a DIY tool.

[Roger Mills]

Is 55kW a universal limit for domestic supplies? My meter says 212 cu.ft/hr which, if my calculations are correct, would equate to about 66 kW. Could yours be lower? what does your meter say?

It could *potentially* exist all the time, but only actually manifest itself at peak demand. How are your boilers controlled? Are they timed to come on at the same time? Do they modulate, so as not to run at full power all the time? Could there be just particular sets of circumstances which cause them both to run at full power at the same time?

Presumably he's worried that it could be caused by something he's down, and he doesn't want to be 'exposed' by Transco. In that case, what is

*he* doing to try to fix it?

[Lee Nowell]

Thanks all for your good thoughts and advice... to answer some of the questions...

1. We have 2 boilers because the total heat required for the house exceeded the largest "normal" domestic boiler. We could have gone to a semi-industrial thing but the price of this was essentially the same as 2 normal domestic ones. So, to give some resiliency and the ability to switch one off in the summer (may be of little benefit since I guess they all modulate anyway) thought for no extra cost and some limited benefit might as well go down that path! Having said all this, I suspect we would be OK with 1 in normal operation with a bit of
2. The boilers are wired in parallel so if any device calls for heat they both activate. Seems to work fine and gives quick heat up times.
3. In terms of timing, yes... hob issues seen during the day, boiler seen at night but could be during the last heat cycle of the day or the first heat cycle (if you see what I mean)
4. I'll try the full on test this evening and let you know. I wasn't there when the plumber did the test but assume it was a static pressure (i.e. without anything on) but could be wrong.
5. The plumber is definately not worried about Transco identifying something he did wrong. We have designed/ troubleshooted other problems together so have a very good relationship. He wouldn't mind if the issue was his - in fact he upgraded the pipe from the meter to the boilers to 32mm in case that would fix the problem.
6. Will also check the meter and report back

Thanks once again.... this is an interesting puzzle :)

Lee.

[Lee Nowell]

[Lee Nowell]

Ok. I have run the tests.

- running everything full blast. I fired up the hob first with all burners on. Switched the heating on and the boilers fired with about

15 seconds between them. When the first one fired the hob pressure lowered for a few seconds then regained. Same happened when the second one fired.

- checked the meter and it says 212ft3 / m with max pressure 50 m bar

- checked boilers and they are both 30.9 kW max output.

If the max is 55 kW then this may be the issue. It doesn't explain why this happens when only the first one has fired unless it takes the gas before the light indicates it has fired.

Everything fired and all ok could be because the boilers modulated down for some reason. Thanks Lee.

[John Rumm]

Static pressure tells you very little. Its normally around 26 mBar over atmospheric, however it should fall to around 21 mBar when gas is flowing. The basic intention is that the appliances are designed to run at 20 mBar, and that leave 1 mBar available for dynamic pressure drop in the pipes. In reality things are a little more relaxed since most governors won't actually be able to maintain that output pressure a full volume, so the boilers will be designed to tolerate a couple of mBar less at the input.

So the boilers alone could kick in and demand best part of 62 kW...

So 62 x 3.6 = 223MJ / hour

If you take a cubic metre of gas as 37MJ, then that's 6m^3/h

The full capacity of the meter without the hob getting a look in.

Chances are your supply is simply running out of puff at peak demand times of day. You could probably help matters greatly by arranging for the call for heat to one boiler to be delayed by a few seconds - give the supply a chance to recover from the large step change in demand as the first boiler kicks in.

(mains timers suitable for the purpose are available off the shelf)

[Lee Nowell]

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Thanks John.

The bit I don't quite understand is why there is a sudden drop then it stabilises? Is this the boiller modulating? Also, when I run the test, there was a time delay already between the boilers which was seen with the "I'm fired" lights going one after the other as well as

2 dips in the hob pressure. I guess the former could just mean that both boilers turn their gas on at the same time just 1 took a little longer to ignite. Doesn't emplain the latter though I guess.

What I should have done is switched one of the boilers off and repeated the test.... Maybe one for tonight

Do you have a link to a suitable tmie delayer by any chance? I guess I could imagine something which is wired into the wiring centre to switch one, wait for a few seconds then switch the other but how would it work when something is calling for heat , water has reached temp so boilers are off, then the water reduces in temp and both boilers fire again?

Not sure what a regulator looks like but can they stick or something. It almost seems as though you get a constant static pressure then when there is demand (or extra demand) it takes a few seconds for the regulator to react and send out the right pressure/ flow. Whilst it is reacting, the overall output is the same/ simlar to before but now shared with more devices?

Any thoughts gratefully appreciated.

Thanks again

Lee.

[Roger Mills]

The regulator is the bulbous thing on the input side of the gas meter. It has a diaphragm and spring inside, and operates a restrictor valve in order to reduce the high incoming pressure down to a controlled low level.

If there is a sudden demand for gas, the regulator needs to open its restrictor valve a bit more, to compensate. This may not be instant, resulting in a temporary pressure reduction.

Your problem is almost certainly down to regulator response. Whether yours is faulty, or whether you're asking it to do something which no standard domestic regulator can actually do, I don't know. I tend to suspect the latter, because suddenly demanding 60kW worth of gas without any appreciable reduction in pressure is a bit of a tall order.

If there *is* a 55kW limit on domestic supplies, it may well be the regulator which imposes this - because the meter sounds like it's the same as mine (I presume you mean 212 ft^3 per *hour* rather than

*minute*) - which should be able to handle about 66kW.

Maybe you need a bigger regulator which can handle more flow? [That would probably mean a bigger meter, too, because the meter must presumably be able to measure everything which the regulator can pass.]

[Dave Liquorice]

Looks like it...

They'll only modulate down once the return water is getting too hot. If it's cold or cool they'll be running flat out.

A regulator that can more than meet the demand of about 7m3/hr should solve the problem always assuming the service pipe can supply the "high" pressure gas suffciently fast enough. Only Transco can answer that one and what tarrif or customer type you'll end up on might be interesting.

I'd be looking at ways of preventing both boiliers being at full chat at the same time. Some form of control system that leads with one or other boiler and then brought the other in if the one wasn't enough for the demanded heat. I'm not quite sure how one would do that. Must be possible as multiboiler setups are fairly common in the commercial sector

Another possibilty is a plumbing side one. Have one boiler heating one half of the property with its zone(s) and stat(s) and the other boiler the other half again with its own zone(s) and stat(s).

[John Rumm]

No... the boiler will only usually modulate as the system warms up and the return temperature starts to rise.

When the boiler initially fires, it may do it at full power, or it may do it at a slightly reduced power and then ramp up to full power.

The governor is in the end a mechanical device which will take a finite time to react to a sudden change in demand. Hence why you see the dip in pressure at the hob as the boiler fires.

Are you sure that they are just wired directly to the same demand - without any delay mechanism between them?

Yup, that certainly ought to be within the capability of the supply.

Do you know what power the hob is?

Well at the moment, it sounds like you have a call for heat signal that is fed to both boilers. This will be derived in the control wiring in the normal way from the outputs of the stats, programmers, valve switch positions etc.

A simple delay timer like:

be introduced prior to this signal being fed to one of the boilers. So when the call for heat goes live, the directly attached boiler fires then, and the delayed one will not see the live signal until a preset number of seconds later. When the call is satisfied, both boilers will turn off.

Typically a round silver thing about 4" in diameter that sits between the incoming pipe and your meter.

Yup - exactly. Some are better with high demands than others, however all will require a small amount of time to do their job when there are large step changes in demand.

[Lee Nowell]

Thanks Dave.

I guess.... even if I split the zones by boiler, I would still have to make sure that they are not on together. If our theory is right, it does seem odd that they are happy going full pelt together most of the time!

When you say it will only modulate when it gets hot, is this when it reaches (or nearly reaches) max temp? I was wondering whether we could fire the second boiler on demand + temp of the inlet pipe/ water?

thanks

Lee.

[Andrew Gabriel]

I would not assume the meter max rate is the limiting factor. I can't recall where I got the 55kW from now - it was over

10 years ago, and I asked because I was fitting a 25kW boiler and a 33kW instant water heater. I think the response I got was it should be good for at least 55kW, so it will probably work, and it does in my case, plus 7kW grill on the cooker, without any problem. My boiler always ignites on a low modulation setting, before cranking up to max over about 5 seconds if it's going to. The instant water heater always starts on max though, before modulating down if it's going to.

Regulators can also misbehave, and this tends to come to light (boom boom) when a higher powered boiler is installed. So it could be that.

What boilers are they? Can they be configured to set the max rate lower? (Some can)