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Discuss S plan - pump config causing problems for UFH in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

B

bjirwin

Hi everyone

Other posts on secondary circulating pumps have been partially useful but grateful for any guidance on the following:

S-plan system, UFH, upstairs rads and HW in 4 story town house. 8 rads and 4 towel rails. Worcester Bosch Greenstar 30CDi system boiler. In order: boiler in garage, UFH manifold tees off flow and return (no zone valve) in utility room, then an inline Grundfos super selectric circulating pump (speed I), then zone valves for each of rads and HW circuits. I guess the layout is what would ordinarily be S plan plus but without a zone valve for the UFH.

Last winter we had problems with the flow temp on the UFH. Couldn’t reliably get anything > 40 degrees and it serves a suspended timber floor in a lossy room with lots of glazing. Manifold pump is fine and have just replaced the thermostatic mixer with little improvement.

The secondary circulating pump fires when there is any call on the boiler including just the UFH circuit. As part of my fault finding I disconnected the secondary pump and got an immediate 10 degree improvement in the UFH flow temp with no other changes. Because the secondary pump is downstream of the flow and return tees serving the UFH manifold, I think the secondary pump was drawing the boiler flow away from the UFH circuit with the effect that there was insufficient flow (75 degrees) available to the manifold to produce a decent flow temp after mixing at the manifold for the UFH.

My questions as follows:

  • I think the inline pump before the zone valves is bad practice anyway. Shouldn’t any secondary pump be specific to a zone? Leave disconnected (i.e. electrically isolated)?
  • Will the electrically isolated pump present a significant inline resistance to the boiler flow, to an extent that might damage the pump in the system boiler? The system bypass is after the secondary pump and before the zone valves for rads and HW.
  • In a 4 storey house should I worry about the system boiler’s ability to pump through the rads on all floors? TRVs and decent gauge piping fitting throughout. (I appreciate this a question of impedance/flow not ability to pump a head of water against gravity.)
I don’t know what guidance Worcester Bosch would offer but I imagine any secondary pump should be very near the boiler, not downstream of the tees for the UFH manifold. If the upstairs rads warm quickly enough I’m inclined to leave the system “as is” with the secondary pump isolated or (if advisable) removed.

Long post – sorry – but any thoughts gratefully received.
 
EvilDrPorkChop

EvilDrPorkChop

Gas Engineer
Hi everyone

Other posts on secondary circulating pumps have been partially useful but grateful for any guidance on the following:

S-plan system, UFH, upstairs rads and HW in 4 story town house. 8 rads and 4 towel rails. Worcester Bosch Greenstar 30CDi system boiler. In order: boiler in garage, UFH manifold tees off flow and return (no zone valve) in utility room, then an inline Grundfos super selectric circulating pump (speed I), then zone valves for each of rads and HW circuits. I guess the layout is what would ordinarily be S plan plus but without a zone valve for the UFH.

Last winter we had problems with the flow temp on the UFH. Couldn’t reliably get anything > 40 degrees and it serves a suspended timber floor in a lossy room with lots of glazing. Manifold pump is fine and have just replaced the thermostatic mixer with little improvement.

The secondary circulating pump fires when there is any call on the boiler including just the UFH circuit. As part of my fault finding I disconnected the secondary pump and got an immediate 10 degree improvement in the UFH flow temp with no other changes. Because the secondary pump is downstream of the flow and return tees serving the UFH manifold, I think the secondary pump was drawing the boiler flow away from the UFH circuit with the effect that there was insufficient flow (75 degrees) available to the manifold to produce a decent flow temp after mixing at the manifold for the UFH.

My questions as follows:

  • I think the inline pump before the zone valves is bad practice anyway. Shouldn’t any secondary pump be specific to a zone? Leave disconnected (i.e. electrically isolated)?
  • Will the electrically isolated pump present a significant inline resistance to the boiler flow, to an extent that might damage the pump in the system boiler? The system bypass is after the secondary pump and before the zone valves for rads and HW.
  • In a 4 storey house should I worry about the system boiler’s ability to pump through the rads on all floors? TRVs and decent gauge piping fitting throughout. (I appreciate this a question of impedance/flow not ability to pump a head of water against gravity.)
I don’t know what guidance Worcester Bosch would offer but I imagine any secondary pump should be very near the boiler, not downstream of the tees for the UFH manifold. If the upstairs rads warm quickly enough I’m inclined to leave the system “as is” with the secondary pump isolated or (if advisable) removed.

Long post – sorry – but any thoughts gratefully received.
If there's a secondary pump then it should be straight after the boiler, as otherwise you experience what you are experiencing. However I would probably question if it even really needs one, and in your case if the system works fine without then I'd remove the existing second pump.
 

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