Using HW Circuit for Pump Overrun?

[DaveB1959]

Hi,
I have a Y Plan system with Glowworm HX system boiler vented system. The system as originally fitted has one radiator without TRV (Landing) but also has a gate valved controlled by-pass. The pump is controlled by an output from the boiler.

Many years ago I added an Honeywell ABV and this has worked OK up to a recent change for pump to a UPS3 which I have set to PP2 as this seems to be the best overall setting in practice. I've shut the by-pass gate valve as in this mode (or any UPS3 pump mode for that matter) the ABV will no longer work as it needs to (see other forum posts). This has left the landing radiator as the "by pass".

Last year I changed to the Wiser control system and this works very well apart from the fact that the landing radiator is always on when any wiser controlled radiator is demanding heat meaning that the landing gets too hot. Having recently retired and so now have the CH all day this is becoming more noticeable and so I'd like to add a Wiser TRV to the landing radiator but obviously need to reinstate a by-pass.

The Wiser system is such that when the TRV's are all closed the CH demand is removed so the question is could I fit a relay into the 3 port valve wiring as per the attached diagram (along with appropriate surge protection) such that when the is no demand (CH or HW) and the pump may want to overrun, the power is cut to the valve and it returns to the HW (Port B) under spring control and so open the HW coil as a "by pass)?

Hope that makes sense. Any help greatly appreciated.

Many thanks
Dave

 

[Chuck]

Which exact model of boiler do you have? What do the installation instructions say about the bypass requirements? That's what you need to do.

If your boiler needs an ABV, which is probably the case, then you can't use one of of the UPS3 PP modes. The UPS3 instruction book says this explicitly and gives advice about choosing the correct mode depending on your system.

 

[John.g]

Hi,
I have a Y Plan system with Glowworm HX system boiler vented system. The system as originally fitted has one radiator without TRV (Landing) but also has a gate valved controlled by-pass. The pump is controlled by an output from the boiler.

Many years ago I added an Honeywell ABV and this has worked OK up to a recent change for pump to a UPS3 which I have set to PP2 as this seems to be the best overall setting in practice. I've shut the by-pass gate valve as in this mode (or any UPS3 pump mode for that matter) the ABV will no longer work as it needs to (see other forum posts). This has left the landing radiator as the "by pass".

Last year I changed to the Wiser control system and this works very well apart from the fact that the landing radiator is always on when any wiser controlled radiator is demanding heat meaning that the landing gets too hot. Having recently retired and so now have the CH all day this is becoming more noticeable and so I'd like to add a Wiser TRV to the landing radiator but obviously need to reinstate a by-pass.

The Wiser system is such that when the TRV's are all closed the CH demand is removed so the question is could I fit a relay into the 3 port valve wiring as per the attached diagram (along with appropriate surge protection) such that when the is no demand (CH or HW) and the pump may want to overrun, the power is cut to the valve and it returns to the HW (Port B) under spring control and so open the HW coil as a "by pass)?

Hope that makes sense. Any help greatly appreciated.

Many thanks
Dave

If the boiler has a internal bypass then I would think your proposal is OK.
Functionally it looks OK, the only concern is that the CH won't switch off until the TRVs are fully closed? with consequent no flow and the boiler probably requires a minimum bypass of ~ 5 LPM. On PP2 you are possibly running with a flow rate of ~ 0.9 M3/hr (15LPM) at a head of 2.5M " normally" down to ~ 5LPM at a 2M head and 0LPM at 1.7M head.

 

[DaveB1959]

Thanks for the replies.
The boiler is a Flexcom 24HX and the manual states a bypass is required at least 1.5m from the boiler. In my case any bypass (ie the non TRV radiator, the HW coil or the ABP + Gate valve) are at least 10m away from the boiler. I don't believe the boiler has an internal by pass.

Running with the PP2 setting and by pass as the one radiator (the ABP + gate valve fully closed on direct by pass) I'm not exceeding 20degC delta at the boiler as stated in the manual. In fact even at PP1 it only just exceeded it.

The Wiser system controls the TRV's (they are effectively on/off motor driven valves) so the last one to be commanded to close will coincide (as far as I know) with the CH demand being switched off at the controller. The idea being this would then open the proposed relay and so open the HW circuit as a "by pass" even though the HW thermostat is satisfied

One thought I have had is would the boiler cool down sufficiently as the only heat lost would be on the return from the HW coil (the cylinder being at 60 deg or so) - a 10m 22mm pipe run?

Apart from having the by pass open all the time what would be a solution if this idea isn't workable ?

I'm working on the principle that the ABP won't work properly with any of the UPS3 modes as they all effectively have a flat or negative pressure curve as the flow drops (as John has pointed out in a previous thread.)

I have thought of fitting a 2 port valve in place of the ABP and opening that when CH/HW demand is met. I don't like this as the default (no power) is closed and so would need power to open it so doesn't "fail safe".

Any ideas?

 

[John.g]

22mm piping heat loss ~ 45W/m @ 65/20C deltaT so heat loss through 10m = 450w=0.45kw or 0.015kwh over a 2 minute overrun which equates to a 3C drop in temp assuming 4 litres Hx+pipeline? so probably would keep the Hx temperature from rising excessively in any case.
Why don't you use a NO mot.valve that will fail open?? ie its powered (shut) with either CH/HW demand and springs open with no demand?.

 

[DaveB1959]

Thanks John. I've done some further tests shutting off the non TRV radiator and manually adjusting the by pass gate valve to be open a fraction (having set the ABV to it's minimum setting) and the pump over run / boiler temp were fine. I boosted the max temp of the boiler and again it was all fine. This was with the pump in the "ideal" mode of PP2 - nice and quite mode! I appreciate that as the flow goes up the pressure will rise and the bypass flow increase. Also tested using the standard Pump setting 1 but creates increased noise and I believe is possibly sucking air down the vent.
So at the moment running with a manually set by-pass and PP2 mode having fitted a TRV to the landing rad. Not ideal but seems ok until I can get a NO 2 port valve fitted. As it's usually only one or two rads that open at any given time the flow doesn't increase that much and so the increased By pass (due to PP2 mode) isn't that great.
Thanks for all the help

 

[John.g]

Seems a reasonable compromise alright, assuming PP head at 2M @5LPM then the increase even if circulating 15LPM at full output (2.5M) will only result in a flowrate of ~ (2.5/2)^0.5*5, 5.6LPM (+12%) and more than likely probably only running at ~ 2.3M with only a few rads open so flow rate will only increase by ~ 7% to 5.4LPM so if you set the by pass to its minimum with all TRVs shut then you won't get a massive bypass increase (~ 10%) in flow under normal running conditions.