UK diy (uk.d-i-y) For the discussion of all topics related to diy (do-it-yourself) in the UK. All levels of experience and proficency are welcome to join in to ask questions or offer solutions.

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
  #1   Report Post  
kmillar
 
Posts: n/a
Default Hot and Cold water in boiler

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?

  #2   Report Post  
Christian McArdle
 
Posts: n/a
Default

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.


  #3   Report Post  
Set Square
 
Posts: n/a
Default

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!
--
Cheers,
Set Square
______
Please reply to newsgroup. Reply address is invalid.


  #4   Report Post  
kmillar
 
Posts: n/a
Default

Cheers everyone, thanks for the replies.

  #5   Report Post  
John Rumm
 
Posts: n/a
Default

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.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/


  #6   Report Post  
IMM
 
Posts: n/a
Default


"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.


_________________________________________
Usenet Zone Free Binaries Usenet Server
More than 120,000 groups
Unlimited download
http://www.usenetzone.com to open account
  #7   Report Post  
John Rumm
 
Posts: n/a
Default

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"....

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/
  #8   Report Post  
Aidan
 
Posts: n/a
Default


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.

  #9   Report Post  
IMM
 
Posts: n/a
Default


"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.






_________________________________________
Usenet Zone Free Binaries Usenet Server
More than 120,000 groups
Unlimited download
http://www.usenetzone.com to open account
  #10   Report Post  
IMM
 
Posts: n/a
Default


"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.








_________________________________________
Usenet Zone Free Binaries Usenet Server
More than 120,000 groups
Unlimited download
http://www.usenetzone.com to open account


  #11   Report Post  
Aidan
 
Posts: n/a
Default


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.

  #12   Report Post  
IMM
 
Posts: n/a
Default


"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.




_________________________________________
Usenet Zone Free Binaries Usenet Server
More than 120,000 groups
Unlimited download
http://www.usenetzone.com to open account
  #13   Report Post  
Aidan
 
Posts: n/a
Default


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.

Reply
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules

Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On



All times are GMT +1. The time now is 05:29 AM.

Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 DIYbanter.
The comments are property of their posters.
 

About Us

"It's about DIY & home improvement"