3 port valve or separate pumb for HW?

[Malc]

It would seem that the price of a pump is about the same as a 3 way valve and an alternate Y plan is to use 2 pumps. So which is best? Are there any problems with the 2 pump arrangement? What about when both pumps are on could the suction on one cause problems to the other?

I see you can buy twin pumps but since these much more expensive than 2 pumps, plus when one goes you have to replace both, effectively, then I can't imagine why anyone would buy one.

 

[oilman]

Pumps are more reliable than valves, I would fit them. If you have the right pipe sizes and the right flow rates for the pump, there shouldnt be a suction problem. You don't need loads of heat into the cylinder, so the pump could be set to the lowest speed.

 

[Malc]

Thanks for the info. I was think that I'd need a valve inline to make sure the flow was not too much. Of course I could always use the pump isolation valve and as you say set the speed to the lowest.

What about gravity circulation? If the pump is off but the CH is on so the water is hot will there be gravity circulation through the pump and HW tank making the HW over hot? Perhaps with twin pumps both side should have a check valve to stop gravity flow as there could be the same problem with summer HW heating radiators? I fitted one of these to my solid fuel system as the bedroom radiator were always warm due to gravity flow. The spring in the check valve was enough to stop gravity flow.

BTW any good URLs on two pump systems?

 

[croydoncorgi]

Various heat stores use 2 pumps, one to circulate water through the boiler, the other to feed the rads. Dunno of a document showing exactly what you describe.

The trouble with using 2 pumps instead of a pump and motorised valve(s)will be gravity circulation when you don't want it (ie. when the water in the cylinder is up to temp). Also, controlling the boiler will be tricky - remember that it's the 3-port or zone valves that control the pump AND the boiler on a standard system. How will you wire it so that the tank stat fires the boiler and switches on one pump, the room stat fires the boiler and the other pump? You'll need a couple of relays and extra wiring, at which point some of the cost benefit disappears.

 

[namsag]

Grunfoss do a pump with two pump heads and a Y plan body with the necassary anti gravity valves built in cost about £80 that will do exactly what you want think they call it the pump plan.

 

[ChrisR]

It is!
Also you don't have to change the whole shabang if one pump fails, you just change the pump body.