Positioning pump and zone valves

[crippo2]

Is there a concensus about where the circulator and zone valves should be fitted?
On the output from the boiler where everything is very hot, or on the return to the boiler, where everything is supposed to be some 20C cooler
Our existing system has everything on the boiler return, but all books etc seem to show this stuff on the boiler output.
My logic would presume that because it will be cooler, and thus easier of the equipment, bearings etc, the boiler return is better.
Simply put is suck or blow best

 

[bcp]

This positioning can lead to a 'negative pressure' scenario and draw in small amounts of air. Consideration for positioning needs to take into account issues about positive pressure, pumping over (if vented), neutral zone. Flow side for circulator is what I was taught.

 

[justlead1]

Hi The return was the place in days gone by, however pump design has developed since then and heat is not a problem. Making system design simpler. Good Luck

 

[migoplumber]

I still like to use the return for as much as possible, in domestic installations.
that said on the commercial side most everything goes on the flow side just because of different design and control systems.
why make life harder for the pumps and valves, even if they are a lot better designed nowdays.

shaun

 

[Mike Jackson]

Not many boiler manufacturers will support the use of pumps on the return any more. The only boiler I'm aware of that allows this at present is the Baxi Solo HE.

Mike

 

[stevetheplumber]

Not many boiler manufacturers will support the use of pumps on the return any more. The only boiler I'm aware of that allows this at present is the Baxi Solo HE.

Mike

strange thing about this is every combi i know has the pump fitted in the return anyone know any with pump on flow?

 

[Mike Jackson]

I think it's mainly to do with the pressure zones in an open vented system, more chance of air ingress with the pump on the return. Same reason you used to have to bleed systems with the pump off. As most Combis are sealed systems this is unlikely to be an issue.

Mike

 

[crippo2]

Thanks to all answers so far. The new system will be fully pressurised and have hot water storage (so not combi). It will also have 2 boilers - don't seem to be able to get modulating oil boilers at a realistic price.

unless there are real benefits to putting pump & valves on hot side, it seems that connecting them to the (cooler) return should give longer life. also using a pump which varies its push/suck depending on pressure (eg Grundfoss Alpha) should overcome the possible air ingress issue.

Or is this still nonsense?

 

[Bernie2]

Hmm!

The problem with todays boilers, are that most have high flow resistance whereas the great big old fashioned cast iron ones had low flow resistance.

So on an old system with the pump on the return you could easily pump through the boiler and have bags of pump energy left to pump around the system rads.
Together with the generally larger pipe sizes in old systems the systems did not have so much resistance as the modern one with micro and mini bore pipework.

The job of a pump is to overcome system resistances. Its why good system design is important.

On a modern open vented system, if you put the pump on the return to force the water through the boiler then the chances are it would vent over before it went around the system.

In other words, you would have to turn the pump speed right up to overcome the resistance of the boiler and in doing so, push the water up the free flowing vent. A high speed flow may also cause excess wear in the heat exchanger.

If you did manage to turn the pressure down without it pumping over, then the chances are that the return water flow would drop at the pump inlet to such an extent it started to cause pump cavitation.

Its partly why the 3 "T" rule is used. Have a look at Honeywell or Danfoss sites.

Hope this helps!!

Of course these are only a few things and many of them can be overcome by good system design. It all depends. On balance I would perhaps stick the pump on the flow, if you use long lengths of pipe and millions of elbows, fin rads or its just awkward to pipe up, but I don't know what you have planned and in certain circumstances it may be better to place the pump on the return.

But then I don't like problems, so generally put the pump on the flow.