If my boiler is running, and servicing the central heating, then after
some time the Hot Water is demanded, won't a flush of cold water end up
going through a very hot boiler? (From the HW Flow via common return).

Are boilers designed to cope with this or is there a possibility of
cracking the heat exchanger?

Are boilers designed to cope with this or is there a possibility of
cracking the heat exchanger?

They are designed to cope with this.

Christian.

In an earlier contribution to this discussion,
kmillar wrote:

If my boiler is running, and servicing the central heating, then after
some time the Hot Water is demanded, won't a flush of cold water end
up going through a very hot boiler? (From the HW Flow via common
return).

Are boilers designed to cope with this or is there a possibility of
cracking the heat exchanger?


I think there'd be even more unhappy combi owners than there are already if
they couldn't cope with that!
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Set Square
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Please reply to newsgroup. Reply address is invalid.

Cheers everyone, thanks for the replies.

kmillar wrote:
If my boiler is running, and servicing the central heating, then after
some time the Hot Water is demanded, won't a flush of cold water end up
going through a very hot boiler? (From the HW Flow via common return).

Are boilers designed to cope with this or is there a possibility of
cracking the heat exchanger?

Many combis have a separate heat exchanger for the hot water side
anyway. What happens if the burner heats the main exchanger as usual, a
diverter valve switches the the flow of heated water from round the
radiator circuit to a small circuit that just includes the secondary
heat exchanger, this then transfers heat to the mains cold water.

--
Cheers,

John.

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"John Rumm" wrote in message
...
kmillar wrote:
If my boiler is running, and servicing the central heating, then after
some time the Hot Water is demanded, won't a flush of cold water end up
going through a very hot boiler? (From the HW Flow via common return).

Are boilers designed to cope with this or is there a possibility of
cracking the heat exchanger?

Many combis have a separate heat exchanger for the hot water side
anyway.

I few don't and the main gas heat exchanger has cold fresh water running
through it. It is designed to cop with excessive low temperatures.


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kmillar wrote:

Are boilers designed to cope with this or is there a possibility of
cracking the heat exchanger?

It happens with commercial boilers, probably cast iron ones. It's
called thermal shock.

I've never heard it mentioned with regard to domestic boilers, if it
was a problem, there'd be big warranty claims. I think the answer is no
in that context.

"Aidan" wrote in message
oups.com...

kmillar wrote:

Are boilers designed to cope with this or is there a possibility of
cracking the heat exchanger?

It happens with commercial boilers, probably cast iron ones. It's
called thermal shock.

That is why they have back end protection, amongst others.






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"Aidan" wrote in message
oups.com...

kmillar wrote:

Are boilers designed to cope with this or is there a possibility of
cracking the heat exchanger?

It happens with commercial boilers, probably cast iron ones. It's
called thermal shock.

That is why they have back end protection, amongst others.








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IMM wrote:

That is why they have back end protection, amongst others.


Nope.
Back-end protection is intended to protect against back-end corrosion,
i.e., prolonged cold start return condensing temperatures of less than
60 degC. It wouldn't necessarily protect from excess dT/ thermal shock
caused by a slug of cold water from switching on a dormant zone,
although it may help. Protection against the possibility thermal shock
should be incorporated in other design details.
Finish.

"Aidan" wrote in message
ups.com...

IMM wrote:

That is why they have back end protection, amongst others.


Nope.

Yep.

Back-end protection is intended to protect against back-end corrosion,
i.e., prolonged cold start return condensing temperatures of less than
60 degC. It wouldn't necessarily protect from excess dT/ thermal shock
caused by a slug of cold water from switching on a dormant zone,
although it may help. Protection against the possibility thermal shock
should be incorporated in other design details.
Finish.

Having the back end protection sensor on the return would keep the water
temp into the boilers return higher than say 60C (condensation does not
occur above this). When a remote wing of the system is switched on. A slug
of water at say 10 C may be pumped into the boilers return, the sensor
immediately sees this as the return water temp drops below 60C and
immediately starts to close the valve so water from the flow mixes with the
cold slug to keep the temp above 60C.




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IMM wrote:


to close the valve so water from the flow mixes with the
cold slug to keep the temp above 60C.

Close the valve?

If it is a valve, it's generally a 3-port diverting valve, which
doesn't close, it diverts. Also, could be a shunt pump. Both are
intended to stop back-end corrosion, not thermal shock. Both stop
working when Tr60 degC, which might not prevent thermal shock.
Back end protection is nothing to do with thermal shock.

"Aidan" wrote in message
oups.com...

IMM wrote:


to close the valve so water from the flow mixes with the
cold slug to keep the temp above 60C.

Close the valve?

If it is a valve, it's generally a 3-port
diverting valve, which doesn't close, it diverts.

Of course it is'nt, it is a 3-way "mixing" valve. It is clear you only have
half a clue how it works.

Also, could be a shunt pump. Both are
intended to stop back-end corrosion,
not thermal shock.

But the 3-way "mixing" valve keeps the temp above 60C into the boilers
return.

Both stop working when Tr60 degC,
which might not prevent thermal shock.
Back end protection is nothing to do with thermal shock.

You can do "both" with a good controller, quick action sensor on the boilers
return, and a 3-way "mixing" valve. It used to design them.



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IMM wrote:

Many combis have a separate heat exchanger for the hot water side
anyway.


I few don't and the main gas heat exchanger has cold fresh water running
through it. It is designed to cop with excessive low temperatures.

Tis why I say "many" and not "all"....

--
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John.

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IMM wrote:

Of course it is'nt, it is a 3-way "mixing" valve. It is clear you
only have
half a clue how it works.

Could be either, a 3-port mixing valve or a 3-port rotary-shoe
diverting valve. Either way, it's still a 3-port valve and it doesn't
"close", as you had stated. You've obviously been Googling to try to
find out something about this. You're just digging yourself in deeper.


quick action sensor on the boilers return,

You've misunderstood that; whichever website you've got that from
probably mentioned quick action re the actuator.

"Aidan" wrote in message
oups.com...

IMM wrote:

Of course it is'nt, it is a 3-way
"mixing" valve. It is clear you
only have half a clue how it works.

Could be either, a 3-port mixing valve or a 3-port rotary-shoe
diverting valve. Either way, it's still a 3-port valve and it doesn't
"close", as you had stated. You've obviously been Googling to try to
find out something about this. You're just digging yourself in deeper.

You are a trolling idiot!! It is quite clear I know exactly what I am on
about. It is clear you only have half a clue, if that. Read what I wrote.
It is good for you, as you now know something new.

















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IMM wrote:

You are a trolling idiot!!

Hello pot meet kettle....

It is quite clear I know exactly what I am on about.

ROTFLMAO... go on tell us another, I could use another good laugh.


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John.

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"John Rumm" wrote in message
...
IMM wrote:

You are a trolling idiot!!

Hello pot meet kettle....

What a stupid thing to say.

It is quite clear I know exactly what I am on about.

ROTFLMAO... go on tell us another, I could use another good laugh.

Wow is this good old Yorkshire humour? By gum. Fraid it goes over all
humanities heads, that one.

Do you talk to dolls as well?




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IMM wrote:

a stupid thing

par for the course then...

Do you talk to dolls as well?

Other than you? No.


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John.

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"John Rumm" wrote in message
...
IMM wrote:

a stupid thing

par for the course then...

Do you talk to dolls as well?

Other than you? No.

You are that way inclined? Inverted? Boy!


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