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Secondary Circulation loop on a combi



 
 
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  #1  
Old September 28th 05, 01:43 PM
Mr Fizzion
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Default Secondary Circulation loop on a combi

Can you have such a thing or is this one of Doctor Evil's fantasies?

If you can...how would you do it?

TIA

Mr F.

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  #2  
Old September 28th 05, 02:49 PM
Christian McArdle
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Can you have such a thing or is this one of Doctor Evil's fantasies?

If you can...how would you do it?


You'd do it the same as any secondary circulation loop. It doesn't really
matter what the heating appliance is, provided there are no pressure
reducing devices such as TRVs or air breaks (i.e. cold water cisterns)
actually in the loop. These may be before the loop, however.

However, not all combis would be suited to a secondary circulation loop.
They work better on a storage based system as an instantaneous combi will
cycle with much greater rapidity than a system boiler off a storage
cylinder.

Christian.


  #3  
Old September 28th 05, 04:19 PM
Aidan
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Mr Fizzion wrote:
Can you have such a thing or is this one of Doctor Evil's fantasies?


Assuming youre referring to a secondary circulation loop on the hot
water supply system, then;

The combi fires on a draw off from the hot water outlets. The only way
I could imagine installing a circulation loop would be to install a
heat exchanger (tube-in-shell, preferably) on the secondary return and
pipe the primary side of the HX as a zone on heating. You'd need the
heating to operate 365 days. You could use an electric heater.

There are also electric trace heating system for hws, which are
installed under thermal insulation to minimize the heat loss from the
pipes. These don't need a circulation loop.

If you're doing that you'd be better off installing an unvented hot
water storage cylinder ( which has got a heat exchanger & has got a
large volume of water, so it won't discharge scalding water if the
controls overshoot the set-point slightly). If you're doing that, you
might as well scrap the combi and put in a conventional hot water
storage system.

You should realise that Drivel is a moron and his rantings should be
treated accordingly.

  #4  
Old September 28th 05, 05:59 PM
John Rumm
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Mr Fizzion wrote:

Can you have such a thing or is this one of Doctor Evil's fantasies?


It ought to work on "simple" combis so long as the manufacturer states
it is ok for the "cold" inlet temp to much hotter than it usually would.
You also need a combi that can operate on very low mains pressure (so
that it will work correctly when the flow is being driven by the pump
and not the mains).

However you could potentially run into all sorts of difficulties with a
modern combi that uses a more sophisticated control loop, or one that
includes a small store of tempered water so as to give "instant" hot water.

If you can...how would you do it?


Timer, pipe stat, check valves, secondary circulation pump, loads of
good insulation on the pipe run etc. You would then need to juggle stat
temperatures carefully to get the operation you require without ending
up in a deadlock state with the pipe stat demanding heat and the combi
HW stat satisfied and hence inhibiting the burner (and the result that
the boiler sits in HW mode for extended periods while the house gets cold).

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/
  #5  
Old September 29th 05, 12:21 AM
Ed Sirett
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On Wed, 28 Sep 2005 13:43:02 +0100, Mr Fizzion wrote:

Can you have such a thing or is this one of Doctor Evil's fantasies?

If you can...how would you do it?


For me this is just plain nonsense.

1) I doubt very much whether any manufacture would permit the boiler to be
used in this way.
2) The boiler is not designed to take luke warm let alone hot water as
it's input.
3) All the while the 2ndry pump is going the boiler is in DHW mode. When
does the house get heated?
4) The whole system would rely on the boiler shutting down because the
primary is at or over its maximum temperature for DHW mode. This will only
happen when the DHW temperature is very hot. Ergo the heat losses on the
2ndry loop will be huge.
5) Whilst the burner is shutdown the fan will still be going so that the
primary will be losing heat at a rate comparable with the burner rating
i.e. tens of kW. This is going straight out the flue!
6) The water bylaws
forbid the permanent connection of the mains to a closed circuit.

If a house needs a secondary loop it also needs a grown up heating system
that will include a stored HW of some type (vented, unvented or thermal
store).

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html


  #6  
Old September 29th 05, 02:56 AM
John Rumm
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Ed Sirett wrote:

For me this is just plain nonsense.


I would agree that the concept has serious flaws in many cases.

1) I doubt very much whether any manufacture would permit the boiler to be
used in this way.
2) The boiler is not designed to take luke warm let alone hot water as
it's input.


Those two are likely to be show stoppers anyway...

3) All the while the 2ndry pump is going the boiler is in DHW mode. When
does the house get heated?


That is where the pipe stat and the timer comes in I suppose. Once the
loop is up to temperature it kills the pump and allows it to return to
heating mode. The big problem will be the heat capacity (or more the
lack of it) of the pipework - it will get cold quickly.

4) The whole system would rely on the boiler shutting down because the
primary is at or over its maximum temperature for DHW mode. This will only
happen when the DHW temperature is very hot. Ergo the heat losses on the
2ndry loop will be huge.


Where do most boilers sample the DHW output temperature. in the DHW flow
on on the return of the primary for example?

5) Whilst the burner is shutdown the fan will still be going so that the
primary will be losing heat at a rate comparable with the burner rating
i.e. tens of kW. This is going straight out the flue!
6) The water bylaws
forbid the permanent connection of the mains to a closed circuit.


Circumventable with a doublecheck valve?

If a house needs a secondary loop it also needs a grown up heating system
that will include a stored HW of some type (vented, unvented or thermal
store).


Indeed ;-)

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/
  #7  
Old September 29th 05, 09:21 AM
Doctor Drivel
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Default


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

Mr Fizzion wrote:
Can you have such a thing or is this one of Doctor Evil's fantasies?


Assuming youre referring to a secondary circulation loop on the hot
water supply system, then;

The combi fires on a draw off from the hot water outlets. The only way
I could imagine installing a circulation loop would be to install a
heat exchanger (tube-in-shell, preferably) on the secondary return


Boy are you confused.

  #8  
Old September 29th 05, 09:27 AM
Doctor Drivel
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Posts: n/a
Default


"John Rumm" wrote in message
...
Mr Fizzion wrote:

Can you have such a thing or is this one of Doctor Evil's fantasies?


It ought to work on "simple" combis so long as the manufacturer states
it is ok for the "cold" inlet temp to much hotter than it usually would.
You also need a combi that can operate on very low mains pressure (so
that it will work correctly when the flow is being driven by the pump
and not the mains).

However you could potentially run into all sorts of difficulties with a
modern combi that uses a more sophisticated control loop, or one that
includes a small store of tempered water so as to give "instant" hot

water.

What difficulties might these be? Or did you make that up?

If you can...how would you do it?


Timer, pipe stat, check valves, secondary circulation pump,


Can be booster pump to achieve the pressure to operate the combi.
http://www.heatweb.com/products/pumps/homebooster.html
Plumb Center also sell them. You also need a time clock to switch off
during the night.

loads of good insulation on the pipe run etc. You would then need to

juggle stat
temperatures carefully to get the operation you require without ending
up in a deadlock state with the pipe stat demanding heat and the combi
HW stat satisfied and hence inhibiting the burner (and the result that
the boiler sits in HW mode for extended periods while the house gets

cold).

Set the pipe stat to 40C


  #9  
Old September 29th 05, 09:39 AM
Christian McArdle
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Default

2) The boiler is not designed to take luke warm let alone hot water as
it's input.


But I suspect most boilers would cope for the minute or two operation
period.

That is where the pipe stat and the timer comes in I suppose. Once the
loop is up to temperature it kills the pump and allows it to return to
heating mode. The big problem will be the heat capacity (or more the
lack of it) of the pipework - it will get cold quickly.


It only needs to be on for about one minute to heat the loop. Then the loop
will stay hot for many minutes with a high hysterysis thermostat satisfied.
Obviously any secondary circulation system would have to be very well
insulated. This applies to all types.

It will still cycle the boiler much more than a storage based system,
though. However, I don't see this as a show stopper, just a disadvantage.

6) The water bylaws
forbid the permanent connection of the mains to a closed circuit.


I can't see how this applies to secondary circulation loops. They aren't
really closed loops.

If a house needs a secondary loop it also needs a grown up heating system
that will include a stored HW of some type (vented, unvented or thermal
store).


Indeed ;-)


Whilst I'm no fan of permanently on secondary circulation loops in most
cases due to energy efficiency reasons, I can see the benefit of a manually
operated (i.e. 2 minute timed) system in a domestic property, as it would be
very effective at water conservation. A manually operated system would also
actually save energy, as the reheated water will start at a higher
temperature than fresh mains.

Christian.



  #10  
Old September 29th 05, 09:49 AM
Doctor Drivel
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Posts: n/a
Default


"Ed Sirett" wrote in message
news
On Wed, 28 Sep 2005 13:43:02 +0100, Mr Fizzion wrote:

Can you have such a thing or is this one of Doctor Evil's fantasies?

If you can...how would you do it?


For me this is just plain nonsense.


On what does he base this wild claim on....we shall see....

1) I doubt very much whether any manufacture would permit the boiler to be
used in this way.


The boiler is not being monkied about with.

2) The boiler is not designed to take luke warm let alone hot water as
it's input.


Some are and most are really. See makers. Many accept very hot water from
solar preheated water. Eco-Hometec for e.g. The cold inlet to most combi's
is just a pipe to a plate heat exchanger and a flow switch. Nothing there
to say 40C water will screw the system up.

3) All the while the 2ndry pump is going the boiler is in DHW mode. When
does the house get heated?


You really can't figure this out can you. A highly insulated loop from the
outlet to the inlet with pipe stat set to 40C means that the loop will be
warm within a minute or so and then be switched off, amd stay off for a long
time. There are keep warm boilers around. The same thing but extended to
outside the boiler casing.

4) The whole system would rely on the boiler shutting down because the
primary is at or over its maximum temperature for DHW mode.


What are you on about?

This will only
happen when the DHW temperature is very hot. Ergo the heat losses on the
2ndry loop will be huge.


Again. What are you on about?

5) Whilst the burner is shutdown the fan will still be going so that the
primary will be losing heat at a rate comparable with the burner rating
i.e. tens of kW. This is going straight out the flue!


More confusion.

6) The water bylaws
forbid the permanent connection of the mains to a closed circuit.


Not so. Thermal store are done this way. Here are The DPS Heat bank makers
giving advice on uk.d-i-y on how to connect up a heat bank using "exactly"
this method.
http://tinyurl.com/cfet8

If a house needs a secondary loop it also needs a grown up heating system
that will include a stored HW of some type (vented, unvented or thermal
store).


What tripe. The basin and washroom may be way off from the combi, so
looping the DHW makes sense. In fact a secondary circulation loop saves a
hell of a lot of water and gives instant DHW at the taps, which is a great
convenience.

Have a look at the one box AVC Heatmaster for a system in your view is not
grown up.

...and you are a professional? Struth.

 




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