Linking two Combi Boilers

S

sturharv

Can anyone see a problem with linking the hot water pipework together from 2 no. condensing combination boilers, we need 2no. boilers for the heating demand and rather than splitting the HW between different WC areas I want to link the HW pipes together, therefore there will be HW available should one boiler be out of use and presumably the HW flow rate will improve too?

Pipe sizes, there is 15mm. connections on the boilers, would they they common into a 22mm. pipe or would 28mm be more sufficient? The HW demand is approx 5 or 6 wash hand basins.

By the way, the clients definitely don’t want any HW storage!
 
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If you put the second one on the wall upside down it will help the flow rate.
 
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There is at least one practical problem. If say just one tap is opened a little the flow rate when divided between the two combis may not be sufficient to trigger the flow switch / diaphragm.

This might be resolved by supplying one combi via a PRV so that there would only be flow through that combi when the other combi was approaching capacity. However it might be better to have just one combi at a time supplying the taps but with a switchover mechanism so that either could supply all.

As for the common supply pipes, the combined flow of two combis might be around 20 litres/min which could be withing the capacity of 15mm pipework, subject to available mains pressure.
 
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Put 'em in series or use a Brittony II before the combi, take out flow restrictors.
 
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D

Doctor Drivel

sturharv said:
Can anyone see a problem with linking the hot water pipework together from 2 no. condensing combination boilers, we need 2no. boilers for the heating demand and rather than splitting the HW between different WC areas I want to link the HW pipes together, therefore there will be HW available should one boiler be out of use and presumably the HW flow rate will improve too?

Pipe sizes, there is 15mm. connections on the boilers, would they they common into a 22mm. pipe or would 28mm be more sufficient? The HW demand is approx 5 or 6 wash hand basins.

By the way, the clients definitely don’t want any HW storage!

This is general relating to domestic:
The best way is to divide and rule. Have one combi say, do one bathroom, one the other. One does the kitchen tap, one does utility. One does one shower, one does the other - one does not affect the other in use. If one is down you have DHW somewhere in the house.

Now the high flows are only needed for the baths. This is where you join the DHW outlets. Have a non-return valve on each combi outlet before the joining tee. After the tee have a shock arrestor. Voila, high flows into the bath. Using two 12 litres/min combis 24 litres/min will come out. This is a highly cost effective route to take. Price up one water heater, unvented cylinder, high flow combi and the price is usually less for the two combis. Combis are mass market products and are competitive in price. Look at the price of two Heatlines (cheaper buying two), the up market models with the stainless steel heat Xs and pre-mix burners, Grundfos pumps etc. Compare to a system boiler, zone valves, unvented cylinder ect. No contest in price and what the two combis can give.

An e.g., is the Atmos Multi stored water high flow combi (floor mounted) at approx £2,500 and two Atmos Intercombis at less than £2,000 (wall mounted and can go side by side or stacked, saving space. They will deliver the same flowrates, except the two combis will never run out of hot water at maximum flow while the Multi will. The two combis can be used to heat separate parts of the house, upstairs and downstairs and have one zone off during the day saving money on fuel bills. The Intercombis are the simplest design around and the most reliable combi of any - cue Dan.

To your case, If something simple like 6 wash basins, just split the basins between the combis. If linking the two combis, if one is down the water may be tepid.

28mm pipe is too much. Two 15mm pipes can merge into one 22mm pipe to the basins.

You could put in a secondary circulation loop. See the current thread on this. It would be setup just like a heat bank would be. A pipe stat on the loop brings in both combis to heat the loop. The DHW on the combi would need to be flow switch acivated not water pressure. Just movement through the com,bi is enough to switch in the burner(s)
 
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The simplest solution is to split the property into two zones each with its own heating area seperately controlled.

That makes the zoning cheaper and easier to control from each boiler without any zone valves.

If its a school or similar then a combi feeding the kitchen with DHW and a HW cylinder for the kids basins.

Tony
 
D

Doctor Drivel

Agile said:
The simplest solution is to split the property into two zones each with its own heating area seperately controlled.

That makes the zoning cheaper and easier to control from each boiler without any zone valves.

If its a school or similar then a combi feeding the kitchen with DHW and a HW cylinder for the kids basins.

They don't want stored water.
 
S

sturharv

Thank you all for you indepth advice. Its a fair point to raise the point that if one boiler is out and the HW is linked then you are going to get cold water mixing with the HW from the working boiler, meaning the HW will be very tepid.
I think splitting the HW beteen certain areas is the best option.

Finally, would there be any issues with: say only 1 zone is calling for heating (consisting of 3 rads or so) will this be too little a demand on the boilers particularly for the pumps??
 
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As long as the boiler is range rated for the actual heating zone then there is no problem as long as correct installation proceedures have been followed which might well include an auto bypass valve.

Tony
 
D

Doctor Drivel

sturharv said:
Thank you all for you indepth advice. Its a fair point to raise the point that if one boiler is out and the HW is linked then you are going to get cold water mixing with the HW from the working boiler, meaning the HW will be very tepid.

Having a 1/4 turn full bore valve on the combi inlet, or outlet, to isolate it if it is down is a good option.

Finally, would there be any issues with: say only 1 zone is calling for heating (consisting of 3 rads or so) will this be too little a demand on the boilers particularly for the pumps??

Many flats have only a few rads.Make sure it modulates down low enough and an anti-cycle function in the pcb.
 
S

sturharv

Presumably if there is an isolating valve on the combi outlet, should one boiler go down then the valve could be closed and the HW wouldnt be tepid?

Are there any regs. on putting IV's on the inlet or outlet from a combi?
 
S

sturharv

What size header would you suggest for linking the 22 mil f & r's into.... 42mm?
 

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