Combination Boilers

[Brian]

By Combination boiler I mean one which heats water for taps etc ‘instantly’
and for central heating radiators - in this case a sealed system.

The actual boiler is a Green Star, a 25 from memory. ( Not ours, it is our
eldest’s). The question is fairly general so the model shouldn’t matter.



Key question: If you have drained down the CH system, should you be able to
still use the boiler to heat hot water?

Second question: If not: How, in normal use, does the system avoid heating
the radiators when not needed when hot water only is demanded?

I am assuming there are two heat exchangers - one for hot water and one for
CH, each with their own burner.



Reason for questions: during some diy, son - in - law damaged CH pipe late
yesterday. The usual call home to get instructions etc. Typically, it is
at (almost) the lowest point in the system but in a room stripped for
refurbishment so no real harm done.

The complication is, the boiler was playing games before this, ‘cutting
out’ heating water etc so we aren’t sure if the lack of water now is just
the next step in this problem or something else.

A qualified plumber etc was booked to fix the boiler before the pipe
business so I’m more curious than planning to delve into the boiler.
Likewise, the damaged pipe is one which was going to be moved to fit a
replacement / different style radiator by a plumber.

[Alan]

On Sun, 15 May 2022 07:48:09 +0000, Brian wrote:

> By Combination boiler...

> Key question: If you have drained down the CH system, should you be able
> to still use the boiler to heat hot water?

No. It needs to have water , pressurised, in the primary circuit - the CH
side.

> Second question: If not: How, in normal use, does the system avoid
> heating the radiators when not needed when hot water only is demanded?
>
> I am assuming there are two heat exchangers - one for hot water and one
> for CH, each with their own burner.

No. There are 2 heat exchangers, but only a gas flame in one. The CH HE
is the larger of the 2, with the gas burner in it.
For HW, the CH HE is fired up, then the diverter valve opens, to allow
the heated water to enter the DHW HE where it exchanges its heat to the
water flowing on the other side of the exchanger, whoch then exits as DHW.

[Brian]

Ok, thank you.

[John Rumm]

On 15/05/2022 08:48, Brian wrote:
>
> By Combination boiler I mean one which heats water for taps etc ‘instantly’
> and for central heating radiators - in this case a sealed system.
>
> The actual boiler is a Green Star, a 25 from memory. ( Not ours, it is our
> eldest’s). The question is fairly general so the model shouldn’t matter.
>
>
>
> Key question: If you have drained down the CH system, should you be able to
> still use the boiler to heat hot water?

No not usually.

The way they usually work is by having the main Heat Exchanger (Hx) heat
a primary circulation loop. This heats the rads. When there is a demand
for hot water, a diversion valve redirects this primary flow to s
secondary water to water crossflow Plate Heat Exchanger (PHE), and that
transfers heat to the incoming mains cold water.

> Second question: If not: How, in normal use, does the system avoid heating
> the radiators when not needed when hot water only is demanded?

With the diversion valve.

> I am assuming there are two heat exchangers - one for hot water and one for
> CH, each with their own burner.

Two Hx yes, but normally only the primary is heated from the burner.

> Reason for questions: during some diy, son - in - law damaged CH pipe late
> yesterday. The usual call home to get instructions etc. Typically, it is
> at (almost) the lowest point in the system but in a room stripped for
> refurbishment so no real harm done.
>
> The complication is, the boiler was playing games before this, ‘cutting
> out’ heating water etc so we aren’t sure if the lack of water now is just
> the next step in this problem or something else.
>
> A qualified plumber etc was booked to fix the boiler before the pipe
> business so I’m more curious than planning to delve into the boiler.
> Likewise, the damaged pipe is one which was going to be moved to fit a
> replacement / different style radiator by a plumber.

--
Cheers,

John.

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[Brian]

Thank you.

A follow up to my original post.

As I mentioned, the system was drained following a ‘mishap’ causing a leak.


The system has a history of needing repressurising every month or so,
although there is no evidence of leaks.

Thinking I could perhaps drive up and cap the damaged pipe to restore hot
water, eldest FaceTimed the pipe to me so I could check what I needed to
take.

I’d assumed from the original phone call last night, the pipe had been
split or damaged. It hasn’t.

There is a junction at (concrete) floor level, I assume a 22mm to 15 mm T,
and the solder joint has failed. I suspect a half decent pull would free
the pipe.

I’m wondering if this has been the reason for the pressure loss.

They are having the radiator replaced so the plan is to leave the job to
the plumber - it will mean hacking up part of the floor etc. Resoldering
with concrete ‘sucking’ the heat away and probably water still in the pipe
( the drain is higher than this point) is unlikely to work.

If it fixes the loss of pressure, the mishap may be a blessing in disguise.

[John Rumm]

There are a few possibilities there. There may be a leak in a place it
is not obvious like under the ground floor, or could even be inside the
main heat exchanger of the boiler (the leak would then leak to the
condensate drain and never show up in the fabric of the house).

A common one is a failed expansion vessel (or one that needs
repressurising). That can cause the system pressure to rise too much as
the water is heated, resulting in some water being discharged from the
pressure relief valve (which typically vents outside).

Slow leaks, can also evaporate fast enough to never leave a "wet" tell
tale.

> Thinking I could perhaps drive up and cap the damaged pipe to restore hot
> water, eldest FaceTimed the pipe to me so I could check what I needed to
> take.
>
> I’d assumed from the original phone call last night, the pipe had been
> split or damaged. It hasn’t.
>
> There is a junction at (concrete) floor level, I assume a 22mm to 15 mm T,
> and the solder joint has failed. I suspect a half decent pull would free
> the pipe.
>
> I’m wondering if this has been the reason for the pressure loss.

Could be...

> They are having the radiator replaced so the plan is to leave the job to
> the plumber - it will mean hacking up part of the floor etc. Resoldering
> with concrete ‘sucking’ the heat away and probably water still in the pipe
> ( the drain is higher than this point) is unlikely to work.

You would normally need to break away some concrete around the pipe to
fix it. A wet'n'dry vac might be able to clear enough water to get it
dry enough to solder. Failing that, make a repair with a push fit or
compression fitting that will not be affected by water in the pipe.

> If it fixes the loss of pressure, the mishap may be a blessing in disguise.

[RJH]

On 15 May 2022 at 17:03:16 BST, "John Rumm" <see.my.s...@nowhere.null>
wrote:

>> The system has a history of needing repressurising every month or so,
>> although there is no evidence of leaks.
>
> There are a few possibilities there. There may be a leak in a place it
> is not obvious like under the ground floor, or could even be inside the
> main heat exchanger of the boiler (the leak would then leak to the
> condensate drain and never show up in the fabric of the house).
>

FWIW, that happened to mine (9 year old Ideal Logic) last winter.

> A common one is a failed expansion vessel (or one that needs
> repressurising). That can cause the system pressure to rise too much as
> the water is heated, resulting in some water being discharged from the
> pressure relief valve (which typically vents outside).
>

Not that - yet.

> Slow leaks, can also evaporate fast enough to never leave a "wet" tell
> tale.

Yep, had that here too :-)

--
Cheers, Rob

[Andrew]

On 15/05/2022 17:03, John Rumm wrote:

> You would normally need to break away some concrete around the pipe to
> fix it. A wet'n'dry vac might be able to clear enough water to get it
> dry enough to solder. Failing that, make a repair with a push fit or
> compression fitting that will not be affected by water in the pipe.

No-one has asked if this copper pipework is protected with
'horsehair' or whatever, insulation ?.

If is in direct contact with 'concrete' (I assume screed) then
I would expect some insidious hidden corrosion if there has been
a long-standing leak.

[Brian]

There is some wrapping visible around the joint that has failed.

It looks like that hessian like stuff used for insulation in the past but
not easy to see via FaceTime.

[Andrew]

That's what my house had around the pipes buried in the ground floor
screed. Fairly typical of the 1970's.

If it hasn't disintegrated to dust then the leak hasn't been that
serious (yet).

resoldering really means draining the system which might be a problem.
my house only had a drain cock combined with a radiator tail valve
so the the pipework down in the floor could never be drained unless
it was sucked out with a wet'n'dry vac.