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Green Building Bible, Fourth Edition
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General: ASHP Circulating Pump on Flow or Return?

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    It looks like I will need to move our circulating pump for the ASHP as it may be interfering with the flow meter. I could either put one on the flow and the other on the return or vice versa. Currently the pump is on the return as that was the design specified by a supplier.

    I'm wondering whether there is a benefit in efficiency one way or the other. The system is open vented, so it's pretty low pressure anyway. My feeling is that it would be better for the pump to be on the flow so that it is pushing the water through the ASHP rather than sucking it, but I don't have an obvious logical reason to justify the intuition, other than it would mean the water going through the ASHP is under slightly higher pressure.

    Any thoughts?
    • CommentAuthorEd Davies
    • CommentTimeNov 8th 2021 edited
     
    Posted By: ComeOnPilgrim…on the flow so that it is pushing the water through the ASHP rather than sucking it, …
    Isn't the “flow” side normally considered to be from the heat source to the emitter (radiator, UFH, DHW coil, whatever)? In which case, wouldn't a pump on the flow side suck from the ASHP?

    One consideration is that you have to be careful of the position of the pump relative to the fill and vent pipes from/to the header tank. What you don't want is for the pump to be sucking water down the fill pipe then blowing it back up the vent pipe. The pressure difference across a circulation pump is not normally that great but then the head between the surface of the water in the header tank and the high point in the vent pipe is not that great, either.

    edit: “heat” → “head”
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      CommentAuthordjh
    • CommentTimeNov 8th 2021
     
    Wot Ed said, plus if the pump is on the flow side (where that means on the output from the ASHP or boiler) then it's operating at a higher temperature and is likely to wear fatser
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    Good points, thanks both!

    Sorry for the confusion. I meant flow to and from the ASHP rather than to and from the emitters.
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    Posted By: djhWot Ed said, plus if the pump is on the flow side (where that means on the output from the ASHP or boiler) then it's operating at a higher temperature and is likely to wear fatser

    Going back a few years when the flow temp. was 70+ deg and return about 50 then the higher temp might have accelerated wear whereas with an ASHP with a flow of perhaps 40 or 45 deg and a return of about 30 - 35 deg I doubt that there will be much wear difference between flow and return placement.

    IMO it probably makes little difference where the pump is placed save that it should not interfere with adjacent equipment say overflow / fill pipe or as in this case a flow meter.

    My system has a heat meter (flow and temp. measurement) and the fitting instructions gives minimum distance of clear pipework to avoid any disrupting turbulence.
  6.  
    As an update, I moved the pump to the other side of the circuit so it is now 'pushing' the water to the ASHP external unit. Before, it seemed to be 'pulling' the water round and, given the relatively low pressure, there were moments when the loop was under negative pressure. Neither the ASHP nor the heat meter seemed to like this. With the revised arrangement, the efficiency seems to be much better.

    Thanks for all your helpful advice!

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