two cheap combi's connected in parallel

[jayjay]

Hi there

I'm considering changing from a vented system to a combi pressuriesed system (4 bed, 3 bathrooms). The thing is - even the biggest combi's cannot cope with potential HW demand (heating not a problem). I was just wondering if anyone had considered installing two cheaper combi boilers in parallel, simply teeing the inlet and outlet pipes. Supply pressure (water and gas) as well as bore diameters would need careful consideration, but could be quite a good solution, including redundancy, so that if one packs up you still have heating and hot water until it gets fixed. Should be fairly straight forward.

Also, can you fit a single controller in parallel as well?

Neat idea, huh?

 

[doitall]

jayjay your both nuts

And just love the Should be fairly straight forward.

 

[fitz1]

have married two combis together to heat unvented cylinder and do heating.but never tried with water side of combis.

 

[croydoncorgi]

It's a very bad idea because a lot of the time one or both combis will be providing hot water at close to their minimum flow rate. This wastes maximum energy and is the most likely condition to create fluctuating HW temperature, boiler overheat trips, etc.

 

[Biggles..]

Why not install one combi and convert to s-plan to heat unvented cylinder. Use DHW from combi for kitchen and then unvented cylinder for bathrooms. Considering some drench head showers now require 30 litres per minute flow you'll never finr a combi to provide that. In commercial premises combi's are tee'd together for heating but have seperate controllers but dhw normally serves different circuits, ie upstairs off one and down off the other.

 

[croydoncorgi]

Often wondered why makers don't make a 'heavy duty' storage combi for commercial sites. OK - the Ariston Genus 27 gets close - but it's OLD!

 

[Biggles..]

Last pair of combi's i touched the pipe sizing for the gas was too large for a domestic installer anyway (max 28mm domestic) two combs would come in around 55kw, and then the meter is commercial so domestic installers cant drop test it. All a nighmare.

 

[doitall]

Have a look at the Alpha 50 CroydonCorgy

 

[doitall]

Last pair of combi's i touched the pipe sizing for the gas was too large for a domestic installer anyway (max 28mm domestic) two combs would come in around 55kw, and then the meter is commercial so domestic installers cant drop test it. All a nighmare.

You can go up to 35mm on domestic lcgs.

Biggest is problem would be exceeding the 6cu/m of a standard U6 meter

 

[ChrisR]

There are loads of successful installations of two combis. If you have a almosta anything else gas though, not on a domestic meter!! If you can separate the HW outputs fine, otherwise a savvy plumber is going to have to consider the low flow issues.
One inst I know has 2 x Turbomaxes, quite happy when the hw outputs are combined, though they're designed to feed separate bathrooms.

Plus, you'd be wasting your time unless you have at least 30 litres a minute from the mains. The one above has 35mm copper .
There are several issues to consider, like booiler sequencing, the electrics and backflow. If you think its a way to go don't move until you and your installer have got info from the manufacturer.

There are things ike the Ideal Istor... , Rinnai...

 

[jayjay]

Thanks for the replys folks, but some people are being a little negative... start thinking a little more creatively.

Water flow rate is not a problem (for me), but have we established that a domestic gas supply is simply not up to the job? This would obviously be a show stopper. As for low flow rates if you simply tee the demand side, aren't modern combi's capable of modulating right down to very low flow rates, and clever enough to handle it. If not, what about a simple pressure differential actuated butterfly valve.

2X24cdi's - someone must have tried it?

;o)

 

[Biggles..]

24CDi requirews 2.9m3/hr, so for the pair that 5.8 add a cooker and gas fire your way over the 6m3 domestic meter. I'd say thats a show stopper, sorry for being negative. If dhw flow isn't an issue why bother joining two combi's or is this just a theoretical could you.
Lee

 

[doitall]

24CDi requirews 2.9m3/hr, so for the pair that 5.8 add a cooker and gas fire your way over the 6m3 domestic meter. I'd say thats a show stopper, sorry for being negative. If dhw flow isn't an issue why bother joining two combi's or is this just a theoretical could you.
Lee

No he want to supply 3 bathrooms as well as the kitchen/utility

 

[Biggles..]

Read the first thread but didn't take it in. the way forward surely should be unvented.

 

[Agile]

There was a discussion on this topic on another forum recently.

Not everyone agreed ( as they dont ) but my view is that thats fine but its better if each combi supplies its own DHE outlets ( with a manual cross connection in case of boiler failure ).

Its better if the heating is controled with non return valves and a simple pipe stat which only brings on the second combi if the CH temperature is say below 65°C.

Of course the mains flow rate is likely to be the critical factor and in many properties a storage cylinder would be a good feature.

Tony