I have a bit of a strange problem at the moment and was wondering if
anyone can shed any light on it...
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.
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 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.
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.
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.
[email address is not usable -- followup in the newsgroup]
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.
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 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?
Thanks all for your good thoughts and advice... to answer some of the
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
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 :)
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
- 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.
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)
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
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.
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.]
On Wed, 1 Feb 2012 05:32:49 -0800 (PST), Lee Nowell wrote:
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
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
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).
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
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?
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
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)
[email address is not usable -- followup in the newsgroup]
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
A simple delay timer like:
could 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.
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 Gaff - email@example.com
Note:- In order to reduce spam, any email without 'Brian Gaff'
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"
You don't say if the boilers failed when you had the low pressure at the
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
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.
No plan survives contact with the enemy.
[Not even bunny]
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