Linking two Combi Boilers

SimonD, i hope youre not a heating engineer!

Judging by your questions here, I must come to the same conclusions about you. I own a pie shop. :LOL:

If this is a commercial application you could consider putting one boiler and running a DHW heat exchanger module off it. If there are no tanks and you need to run 6 basins simultaneously, you are going to need a good CW supply.

Even at 5l/m per washbasin you'll need 30 l/m. Check this is available!
 
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sturharv said:
What size header would you suggest for linking the 22 mil f & r's into.... 42mm?

I would split the heating system. Divide and rule. Or if they need two on the one system, for power and back up, then a low loss header. 42mm should be fine, even the next size down. There are various methods of doing this. One is a vertical pipe with the CH flow taken off the top using a pump - return at the bottom. At the top and sides of the pipe a tee each side for the flows of the boilers, same at the bottom. Make the pipe as long as possible. The CH circuit pump must "generally" be more powerful than the two CH pumps, so the heat is immediately taken out of the header and does not immediately return to the boilers lowering condensing efficiency.

The boilers must never be on with the CH circuit pump not running, as they will just short circuit.

Others may have another take on headers.

Re: the basins. The likelihood all are on at the same time is remote. Even a cylinder based system will have problems providing full flow on all at full flow.

If the basins are a way from the combis then you will have the dead-leg lag. Insulate the DHW outlet pipe to all the taps to keep the water warm.
 
get a combi that can take inlet water at 60'c then daisy chain the boilers.
eg cold inlet from one boiler from the hot of the other.

remember to remove the flow restrictor, i believe alpha boiler take 60'c inlet for there solar thing
 
SimodD, funny I own a pie shop too!

Even heating engineers ask questins on plumbing & heating too!? Continuous improvement!
 
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mehran said:
get a combi that can take inlet water at 60'c then daisy chain the boilers.
eg cold inlet from one boiler from the hot of the other.

remember to remove the flow restrictor, i believe alpha boiler take 60'c inlet for there solar thing

Two 12 litres/min combis can deliver 24 l/min Having them in series I doubt will.
 
Doctor Drivel said:
sturharv said:
What size header would you suggest for linking the 22 mil f & r's into.... 42mm?

I would split the heating system. Divide and rule. Or if they need two on the one system, for power and back up, then a low loss header. 42mm should be fine, even the next size down. There are various methods of doing this. One is a vertical pipe with the CH flow taken off the top using a pump - return at the bottom. At the top and sides of the pipe a tee each side for the flows of the boilers, same at the bottom. Make the pipe as long as possible. The CH circuit pump must "generally" be more powerful than the two CH pumps, so the heat is immediately taken out of the header and does not immediately return to the boilers lowering condensing efficiency.

The boilers must never be on with the CH circuit pump not running, as they will just short circuit.

Others may have another take on headers.

Re: the basins. The likelihood all are on at the same time is remote. Even a cylinder based system will have problems providing full flow on all at full flow.

If the basins are a way from the combis then you will have the dead-leg lag. Insulate the DHW outlet pipe to all the taps to keep the water warm.

I have seen two 24 kW system boilers (what combis are) coupled up by having a 28mm pipe for the flow and both went into this. The system dictated a 28mm pipe to carry the 48 kW. The same for the return. Each system boiler had an auto by pass to protect each boiler and a non-return valve on the flow of each. So, each boiler pumped into the flow pipe. Simple, cheap, effective, fast to fit and worked. They were both modulating boilers, so wound down reducing cycling when the place was up to temperature. No boiler sequencing controls were used. In Spring and Autumn only one boiler was on, winter the two
 
Doctor Drivel said:
mehran said:
get a combi that can take inlet water at 60'c then daisy chain the boilers.
eg cold inlet from one boiler from the hot of the other.

remember to remove the flow restrictor, i believe alpha boiler take 60'c inlet for there solar thing

Two 12 litres/min combis can deliver 24 l/min Having them in series I doubt will.

what? why? are you saying:
a, that you can't get 24l/m through the boilers volume wise
b, that it cant heat up 24l/m at rise of 35'c
 
mehran said:
Doctor Drivel said:
mehran said:
get a combi that can take inlet water at 60'c then daisy chain the boilers.
eg cold inlet from one boiler from the hot of the other.

remember to remove the flow restrictor, i believe alpha boiler take 60'c inlet for there solar thing

Two 12 litres/min combis can deliver 24 l/min Having them in series I doubt will.

what? why? are you saying:
a, that you can't get 24l/m through the boilers volume wise
b, that it cant heat up 24l/m at rise of 35'c

Connect in parallel and you will get 24 litres/min. Connect in series, no.
 
right... i taking that as volume wise, maybe i have never done it so can say if the restriction through the boiler even with out flow restriction would be to great.

but in serials i would image you could get some efficiency gains where the first combi would be in condensing at full flow, so instead of two not condensing at full flow draw, it could be one not condensing but the other is?
 
mehran said:
right... i taking that as volume wise, maybe i have never done it so can say if the restriction through the boiler even with out flow restriction would be to great.

It is possible it would work, but highly unlikely. It would take setting up. The first would need to take 24 litres/min, which raises the temperature, as would the second. which take it to setpoint.

but in serials i would image you could get some efficiency gains where the first combi would be in condensing at full flow, so instead of two not condensing at full flow draw, it could be one not condensing but the other is?

With a combi that condenses when producing DHW, having them in parallel is not an efficiency issue. In series, you are right. the first would be condensing 100% of the time and second little at all, if ever.
 
still... at least one is doing it, where the other way both are not.

i not really saying my way is any good, just if every i feel the need to do it, could it work. one way of doing it maybe to upgrade the dhw plate heat exchangers(if it has enough room),

for example i know the biasi m110.32 boilers use heat exchangers rated for 18l/m, then place then in the 24kw version, so have two 24kw boiler with over sized dhw hx. good for a 20l/m cold water supply

now i clearly see this as a lot of work for putting them in series but you never know, maybe the efficiency gains in places with high dhw use my pay for it self.

just pie in the sky stuff maybe but you never know.
 
at full flow, not may combi can condenses in dhw mode when it is giving its full 35'c rise.

i am sure you are going to in lighting me now tho :LOL:
 
mehran said:
at full flow, not may combi can condenses in dhw mode when it is giving its full 35'c rise.

That is because the temperature return from the plate back to the main heat exchanger is too high - above 54C. A larger plate extracting more heat dropping the temperature somewhat, and raising the flowrate can solve much of this.

Many combi makers don't want condensing on DHW as their main heat exchangers can't cope with a large delta T. The better models can. I believe the W-B Greenstar range condense on DHW. Other may confirm.
 

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