Auto Bypass Valve OR Proportional Pressure pump mode – how to choose?

[StephenL]

I have read that an Auto Bypass Valve (ABV) is not compatible with a pump set to Proportional Pressure (PP) mode.

I have a recently fitted Vaillant EcoFit Pure 430 heat-only boiler + Vaillant SensoComfort weather sensing controls + 15 rads (12 with TRVs) across 2 CH zones + a static bypass valve set slightly open (no ABV) + pressure vessel. I find PP mode works well (it reduces CH circulation noise and, I believe, it has improved the Flow/Ret delta due to the pump modulating, enhancing the chances of condensing). However, I am advised to install an ABV which is more efficient than a static bypass valve (maintains minimum flow through the boiler as TRVs close, reduces noise and opens the bypass only when necessary).

I would like to use both (ABV & PP mode), but given an enforced choice, which is the better option to select or are there other options?

Thank you

 

[Chuck]

I think it's fair to say that pressure-sensing ABVs work best on constant flow systems and don't play nicely with proportional pressure mode pumps.

People sometimes fit a pressure sensing ABV in series with a static bypass valve but I suspect that it's mainly to allow them to tick the 'ABV fitted' box on their checklist.

A better approach, IMO, is to limit the flow through a static bypass valve with an automatic balancing valve (flow regulator), which you set to match the minimum bypass flow requirement of the boiler. The Altecnic Caleffi Autoflow 127 is the sort of thing I have in mind.

https://www.altecnic.co.uk/591

These can be set to between 2 and 26 litre per minute.

Taking a step back, however, PP circulation is not a good match to a system with weather compensation. In these systems the heat delivery should be controlled by modulating the water temperature and keeping the flow (or in a zoned system, pressure) constant. Hence, you should never get to the situation where a majority of the TRVs are closed. PP circulation is more appropriately used to control the heat delivery in systems without WC, where the flow temperature is constant and the flow rate is controlled by the TRVs and so changes significantly in the course of normal operation.

You can used a pressure-sensing ABV on a system with a CP (constant pressure) pump because the pressure will rise a small amount as the flow is reduced. The ABV has to be set quite carefully so it opens at just the right point.

 

[ShaunCorbs]

With can you tie in a rad in before the port valves eg bathroom rad etc what pump do you have ?

 

[StephenL]

I think it's fair to say that pressure-sensing ABVs work best on constant flow systems and don't play nicely with proportional pressure mode pumps.

People sometimes fit a pressure sensing ABV in series with a static bypass valve but I suspect that it's mainly to allow them to tick the 'ABV fitted' box on their checklist.

A better approach, IMO, is to limit the flow through a static bypass valve with an automatic balancing valve (flow regulator), which you set to match the minimum bypass flow requirement of the boiler. The Altecnic Caleffi Autoflow 127 is the sort of thing I have in mind.

https://www.altecnic.co.uk/591

These can be set to between 2 and 26 litre per minute.

Taking a step back, however, PP circulation is not a good match to a system with weather compensation. In these systems the heat delivery should be controlled by modulating the water temperature and keeping the flow (or in a zoned system, pressure) constant. Hence, you should never get to the situation where a majority of the TRVs are closed. PP circulation is more appropriately used to control the heat delivery in systems without WC, where the flow temperature is constant and the flow rate is controlled by the TRVs and so changes significantly in the course of normal operation.

You can used a pressure-sensing ABV on a system with a CP (constant pressure) pump because the pressure will rise a small amount as the flow is reduced. The ABV has to be set quite carefully so it opens at just the right point.

Hi, thank you for your insightful feedback.

My pump (Grundfos UPS3 15-50/65 130) has 3 modes, Constant Speed/Constant Curve 1-3, PP 1-2 & Constant Pressure 1-2.
I have 2 CH zones [(1) 1 floor with 3 rads & (2) 3 further floors with 12 rads. No UFH]. It’s a 2 pipe system.

I will disable PP mode, as you propose, and let the weather compensation do its work, but which of Constant Speed or Constant Pressure modes should be selected?
You mentioned that a pressure-sensing ABV would work with Constant Pressure mode.

Compared to my current static bypass valve, will the addition of an automatic balancing valve (flow regulator):

• Improve the Flow/Ret delta?
• Reduce unnecessary bypassing (= more efficient)?
• Reduce noise (CSpeed mode is noisy/hissy v PP)?

Also, might it solve an intermittent problem I have with the DHW cylinder heat up [sometimes the boiler will not start and is paused with a Vaillant code S53 “water deficiency (flow/return spread too large)”]?
Background: Recently installed new DHW cylinder (positioned next to boiler) with a 22kWh coil which works very well (fast), but S53 occurs intermittently when selecting instant heat up – mostly it’s fine, but occasionally it’s blocked with S53. This is a Priority DHW system. The work-around when this occurs is to select CH for a minute and then request DHW heat up again, which then works.

Would the automatic balancing valve be fitted on the bypass leg, before or after the static valve (sorry, I am just a novice)?

Thank you

 

[PETE 123]

I find a gate valve as a bypass works fine for me ive had no problems.

 

[StephenL]

With can you tie in a rad in before the port valves eg bathroom rad etc what pump do you have ?

Hi,
My pump (Grundfos UPS3 15-50/65 130) has 3 modes, Constant Speed/Constant Curve 1-3, PP 1-2 & Constant Pressure 1-2.
I have 2 CH zones [(1) 1 floor with 3 rads & (2) 3 further floors with 12 rads. No UFH]. It’s a 2 pipe system.
See attached picture of boiler, pump, bypass and 3 motorised valves.

Can you further explain your bathroom rad suggestion?

Thank you

 

[Chuck]

Hi, thank you for your insightful feedback.

My pump (Grundfos UPS3 15-50/65 130) has 3 modes, Constant Speed/Constant Curve 1-3, PP 1-2 & Constant Pressure 1-2.
I have 2 CH zones [(1) 1 floor with 3 rads & (2) 3 further floors with 12 rads. No UFH]. It’s a 2 pipe system.

With the system as it is at the moment, I'd start by trying each of the constant pressure modes. If it works okay with one these you'll be able to replace the static bypass with a standard, i.e. pressure sensing, automatic bypass valve, which will need to be set quite carefully but should work okay. Consult the manufacturers instructions which, inter alia, state that the bypass valve must be at least 1.5m from the boiler.