Search the forum,

Discuss Constant flow automatic bypass valves in the Plumbing Jobs | The Job-board area at PlumbersForums.net

Messages
36
My earlier post entitled Honeywell ABV setting raised several issues and a number of well-informed comments (thank you). One topic in particular seems worth pursuing on its own, hence the present post. Your comments will be appreciated.


Widely-used domestic CH pumps such as the Grundfos UPS2 15-50/60 have selectable modes of operation: either a constant head for most of the flow range (including no-flow) or modulation of the head inversely to the system resistance (as TRVs warm up and shut, or DHW cylinders hit their temperature set-point, the head is decreased – proportional pressure).


Concurrently, modern gas boilers typically require pump overrun in order to cool the HX when demand for heat ceases. When overrunning with motorised valves shut (or, conceivably, with all the TRVs shut), the pump needs to bypass the heating circuit via an ABV*. Typical ABVs are pressure driven: they open when the head-loss across them exceeds the pressure set by the installing engineer. But since (qv above) pumps either have a constant head for at least the lower part of their flow range (including no-flow) or have a head that is automatically reduced at low-flow or no-flow, it appears impossible to use such an ABV successfully. Worse, when the pump is in proportional pressure mode, a pressure-driven ABV works the wrong way and may shut off the flow just when it is needed for pump overrun!


Unless the boiler itself can provide a signal that pump overrun is about to be enabled – which could then operate a motorised bypass valve, the obvious solution is a constant-flow ABV to achieve a flow rate of say 3 or 4LPM.

And there lies the problem.


The Swedish Maric constant-flow valves are mostly limited to 60degC; the 100degC exceptions have a head-loss starting at 1.4bar (say 14m head – beyond the typical domestic CH system). There is a low-pressure version starting at 0.4 bar (4m head) but this is limited to 60degC.


The alternative I have found to date is Cottam & Preedy’s CP961 (aka ‘Wondervalve’), an isolator ball-valve containing a restrictor cartridge. The low-pressure version can have a 4litres/min restrictor, covers 0.2–4 bar head-loss and is limited to 85degC; and these criteria may well suit the typical domestic CH system. C&P’s literature does however only describe the CP961 as a restrictor (or filter) for DHW, and I am wondering what the snags are for using one as an ABV: perhaps the downstream end must be open (like a tap)? I realise that to avoid flow restriction, gate-valves are preferred to ball-valves in CH pipework, but in this case restriction clearly isn’t problem!
I rang C&P last week them and then emailed, and await a reply.

How do others get round this problem?


* - unless the boiler flow/return are hydraulically decoupled from the heating circuit flow/return by a low-loss header or closely-spaced tees
 
It’s the zone valve meeting it’s open limit switch they sends a demand signal to the boiler. If you allowed the boiler to open the zone valve how would it know not to fire?
 
My earlier post entitled Honeywell ABV setting raised several issues and a number of well-informed comments (thank you). One topic in particular seems worth pursuing on its own, hence the present post. Your comments will be appreciated.


Widely-used domestic CH pumps such as the Grundfos UPS2 15-50/60 have selectable modes of operation: either a constant head for most of the flow range (including no-flow) or modulation of the head inversely to the system resistance (as TRVs warm up and shut, or DHW cylinders hit their temperature set-point, the head is decreased – proportional pressure).


Concurrently, modern gas boilers typically require pump overrun in order to cool the HX when demand for heat ceases. When overrunning with motorised valves shut (or, conceivably, with all the TRVs shut), the pump needs to bypass the heating circuit via an ABV*. Typical ABVs are pressure driven: they open when the head-loss across them exceeds the pressure set by the installing engineer. But since (qv above) pumps either have a constant head for at least the lower part of their flow range (including no-flow) or have a head that is automatically reduced at low-flow or no-flow, it appears impossible to use such an ABV successfully. Worse, when the pump is in proportional pressure mode, a pressure-driven ABV works the wrong way and may shut off the flow just when it is needed for pump overrun!


Unless the boiler itself can provide a signal that pump overrun is about to be enabled – which could then operate a motorised bypass valve, the obvious solution is a constant-flow ABV to achieve a flow rate of say 3 or 4LPM.

And there lies the problem.


The Swedish Maric constant-flow valves are mostly limited to 60degC; the 100degC exceptions have a head-loss starting at 1.4bar (say 14m head – beyond the typical domestic CH system). There is a low-pressure version starting at 0.4 bar (4m head) but this is limited to 60degC.


The alternative I have found to date is Cottam & Preedy’s CP961 (aka ‘Wondervalve’), an isolator ball-valve containing a restrictor cartridge. The low-pressure version can have a 4litres/min restrictor, covers 0.2–4 bar head-loss and is limited to 85degC; and these criteria may well suit the typical domestic CH system. C&P’s literature does however only describe the CP961 as a restrictor (or filter) for DHW, and I am wondering what the snags are for using one as an ABV: perhaps the downstream end must be open (like a tap)? I realise that to avoid flow restriction, gate-valves are preferred to ball-valves in CH pipework, but in this case restriction clearly isn’t problem!
I rang C&P last week them and then emailed, and await a reply.

How do others get round this problem?


* - unless the boiler flow/return are hydraulically decoupled from the heating circuit flow/return by a low-loss header or closely-spaced tees

If you want meaningful answers I’d try rephrasing your question. I’ve read your post several times and I’m still not entirely sure what you’re asking.
 
Rephrased as asked: “If you had to install an ABV to permit pump overrun, which one would you use given that:

- the pump either had a constant head at low-flow (or no flow) or had a head that decreased with low flow

- typical ABVs open at a set pressure
 
Pumps don’t generate pressure they generate flow. It is the opposition of flow that results in pressure. Closing all the zone valves shuts down the boiler resulting in the over run. The pump runs against closed valves leding in greater DP.

I don’t see why an ABV needs to be anything fancy. Simply set it to open at a slightly higher DP than normal operation.
 
If you want meaningful answers I’d try rephrasing your question. I’ve read your post several times and I’m still not entirely sure what you’re asking.

Rephrased as asked: “If you had to install an ABV to permit pump overrun, which one would you use given that:

- the pump either had a constant head at low-flow (or no flow) or had a head that decreased with low flow

- typical ABVs open at a set pressure

Where is this pump located, internally in boiler or externally?
 
I don't have any such problems as my Viesmann 100-W heat only boiler had the option of W/C which I enabled. If I was installing it from new I would have used a diverter valve (rather than mid position), as it was I had to work with the existing S plan but re-wired so heating valve only closes when there is a demand for HW in that way there is always an open pathway for pump overrun even in the summer. My Grundfos Alpha 2 works fine in variable pressure mode (sorry meant variable flow rate mode:D).
 
Re Constant flow automatic bypass valves reply: interesting that you have W/C installed on a Viessmann 100-W S-Plan system. I also ordered W/C. I got a far as having the outside sensor fitted and cabled, then we (me, electrician and GS engineer) realised that the cylinder demand box's role was in effect to override W/C 's recommended flow temperature when giving DHW priority - which becomes a little more difficult when the system has more than one DHW cylinder as I have. Viessmann were blunter: you cannot use W/C with S-Plan.
Back on thread: I currently use a towel-rail (no TRV) as bypass as it is fitted on the boiler side of the MVs and thus get some benefit from the few minutes of overrun (it is also the usual fail-safe against all the TRVs shutting). But intend to try out a C&P 4l/m low-pressure constant flow isolator valve since I cannot (yet...) see any reason why it shouldn't work - and it is half the price of an adjustable pressure-driven ABV!
 
I would say your choice of faux abv will void any boiler warranty
 
Viessmann were blunter: you cannot use W/C with S-Plan.
Oh yes you can & I sent them the wiring diagram to show them how it should be done. Would you like a copy?
The call signal to the cylinder demand box is from the HWS two port end switch so adding a second cylinder should not be a problem but you may need a relay to separate the cylinder satisfied's to power the heating, looks do-able & would save all this with the by-pass.
 
As requested
 

Attachments

  • Notes on Viessman S Plan wiring.pdf
    262.4 KB · Views: 35
  • Vitodens 100-W Wiring for S Plan.pdf
    283 KB · Views: 26
Cunning! So as soon as DHW cylinder demand is satisfied – but not before, CH can heat if (and only if) the room stat demands, and this is independent of W/C. This leaves W/C to ramp the flow temperature up or down in line with the output from the external sensor.

In effect, this creates DHW priority without e.g. needing a diverter valve – just the additional wiring link back to the programmer from the cylinder stat’s “satisfied” terminal and a link in the programmer itself from DHW 'off' to CH selector. It also allows W/C and room stat to coexist rather than the W/C superseding the room stat which seems to be Viessmann’s intention.

Have I understood correctly?
 
Yes very good, in basic terms if there is a demand for hot water the boiler flow temp is ramped up to 80 degC & the heating is disconnected, as soon as the HW is satisfied & there is a demand for heating the flow temp is controlled via the W/C & the particular slope selected on the boiler sat. I use 4 as I have an old 1930's house, although I have done my best to insulate the dumb W/C would not on it's own give tight enough control (unless heating was left on) so I kept the digital stat & it all works well.
I have the HW set for 4omins before the heating is required (AM only) & timed heating morning & night. The only time there is a problem is on the odd PM heating when it is a cold winters early evening as the internal temp has dropped but the outside is still relatively warm but dropping fast. It just takes too long for the house to get to temp but it isn't a problem we just move the boiler stat up to increase the water temp, as the boiler is in the kitchen this isn't a problem.
For what the W/C kit cost it was a no brainer to go with it.

If you have 2No. cylinders it could still be made to work by using a small 240V relay so if there is a demand from either the satisfied from the stats is only allow on to 5 when both cylinders are up to temp.
If you need any help with that let me know.
 
Perhaps CW could kindly clarify one point re “wired so heating valve only closes when there is a demand for HW in that way there is always an open pathway for pump overrun even in the summer”.

If in the circuit diagram DHW demand is satisfied and then either the programmer or room stat turns heating demand off, both MVs would appear to be shut. The boiler will be notified of this no-demand via the link from terminal 12 of the wiring centre but overrun would appear to start with closed MVs.
 
You are quite right re the heating valve but you have to remember what the over run circuit is for, in this case it is only to dissipate excess heat from the heat exchanger (not for the pump), I believe it only comes into operation if the water temp rises in the HX & the high limit stat is in danger of going to lockout.
If the programmer or stat turn off the heating it is likely that the system is already up to temperature, with the boiler running on its lowest modulating setting. As it is under W/C control the water temp is 50 - 60deg C max.
The pump is under pressure control so likely to be running at reduced speed as TRV's are closing a rooms get up to temp.

I have been thinking back to when I first put it in, the system had an ABV installed for the old boiler but I wanted to use the speed control of the new pump so I closed it to see how it all would work.
All I can tell you is it has been working fine for >7 years now.
 
After some thought as to whether the towel-rail substitute for an ABV was the right long-term solution, I decided to replace the Honeywell DU144 pressure-driven ABV by a Caleffi Autoflow 127 from Altecnic that limits bypass flow to 0.2m3/hr (the boiler’s recommended minimum).
The Grundfos UPS2 25-80 pump was then switched into the lowest of the three proportional-pressure modes. With all the TRVs (now including the towel rail) almost shut on hot days the pump should be running at a head of just over 2m and the system is very quiet, but it heats up as quick as ever on cold days.

The Caleffi valves are about £10 more (plus a couple of BSP-to-compression adaptors) than Honeywell ABVs but should be the right solution, whereas in my view the Honeywell (and any other pressure-driven) ones aren’t.
 
I decided to replace the Honeywell DU144 pressure-driven ABV by a Caleffi Autoflow 127 from Altecnic that limits bypass flow to 0.2m3/hr (the boiler’s recommended minimum).
So now you have a continuous flow through the bypass, even if it is limited to 0.2m3/hr. o_O

Grundfos recommend a Taconova Setter valve (does the same job as a Caleffi) in series with the ABV. The bypass then only opens when the pressure exceeds the ABV setting and the flow is restricted by the Setter if the pressure rises.
 

Attachments

  • BypassCommissioningAlpha2or2L.pdf
    167.4 KB · Views: 9
IMO, to satisfy a PP1 setting of 2.0 M above then the ABV setting would have to be set as low or lower than this to give any flow if all zone v/vs, TRV's etc are closed, then as the heating demand increases the Tacosetter will restrict the flow rate which would otherwise be enormous using the ABV on its own, also I thought the tacosetter is more or less a adjustable fixed setting with a flowmeter and is not automatic so, for example if its set to flow 0.2M3/hr @ 2M head then if the heating demand increased to require a PP head of say 4M then the flowrate would increase to almost 0.3 M3/hr whereas, theoretically anyway, the Caleffi will maintain 0.2M3/hr even if the head increased to over 20M(200 kpa).
 
Re John.g's reply: spot on!
I was writing identical comments but was beaten to the draw.
The underlying problem is that a pump such as any in the Grundfos domestic range running in proportional-pressure mode has its lowest pressure at lowest flow (or no flow) - typically when all the TRVs shut on a hot day or when there the CH + DHW demand ceases and the boiler tells the pump to overrun for a few minutes. A pressure-driven ABV set to this pressure will simply open all the time.
And when I looked at the Tacosetter's tech spec, it appeared to operate as a fixed valve with a flow meter. Perhaps the flow set-point is resilient to pressure changes but this wasn't obvious from the spec: perhaps someone more familiar with them can correct me.
 
Re John.g's reply: spot on!
I was writing identical comments but was beaten to the draw.
The underlying problem is that a pump such as any in the Grundfos domestic range running in proportional-pressure mode has its lowest pressure at lowest flow (or no flow) - typically when all the TRVs shut on a hot day or when there the CH + DHW demand ceases and the boiler tells the pump to overrun for a few minutes. A pressure-driven ABV set to this pressure will simply open all the time.
And when I looked at the Tacosetter's tech spec, it appeared to operate as a fixed valve with a flow meter. Perhaps the flow set-point is resilient to pressure changes but this wasn't obvious from the spec: perhaps someone more familiar with them can correct me.

Could this be Honeywell's answer to the Caleffi?.
 

Attachments

  • Honeywell Balancing valve.pdf
    2.1 MB · Views: 7
Sorry I’m lost why is this a problem if you don’t want a bypass valve install a bypass rad eg airing cupboard

Depending on the sys it’s more to go wrong and service it’s only a domestic system
 
The Honeywell VT5005T referred to (in a link) by John.g is indeed
a more flexible version of the Caleffi Autoflow 127. The Caleffi valves (the 127 and its big brothers the 121-126) have changeable fixed flow-rate cartridges whereas the flow-rate of the V5005T is set by the installer and can be varied.
It is horses for courses: the V5005T is twice the price of a 127 but you get 'variability' for your money. However, the V5005T is intended for controlling the flow and temperature of its inputs - it is not just a flow limiter, and the temperature control is regulated by its actuator.
Of technical interest - overkill for a domestic installation - are Honeywell’s electrical actuators (e.g. the M4410E/K) that can be bolted on, such that the V5005T operation can be overridden proportionately (i.e. not just on/off) by an external controller with variable (PWM) output. In an ideal world the V5005T would be normally off (no flow), and the boiler would have a suitable PWM output that would enable it to signal to the V5005T actuator when input flow rate was falling towards the minimum.
(Whether the VT5005T can be set to ‘no-flow’ until remotely actuated I don’t know and the documentation I have seen doesn't say)
 
Last edited:

Reply to Constant flow automatic bypass valves in the Plumbing Jobs | The Job-board area at PlumbersForums.net

Creating content since 2001. Untold Media.

OFFICIAL SPONSORS

OFFICIAL SPONSORS

Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock