Mid position valve as bypass

[Spectric]

Hi all

Just seeing if this is a common method of providing a boiler bypass thesedays as there seems so many ideas out there, years ago if we needed a bypass a gate valve was the answer but no longer classed efficient and others not suitable due to the smart pump.
The system is a sealed single zone heating system and indirect hot water cylinder using a Grunfoss Alpha smart pump. The H/W zone valve is controlled by an ST9100C timer, a tank stat and the mandatory thermal cut out whilst the heating by a CM907 programmable thermostat. There is a mid position valve that in its static position provides the bypass for the boiler and when powered by either of the zone valves fully opens to direct boiler flow through the required circuit and providing power to the boiler. There is also the usual frost stats and pipe stat for protection.

Anyone got any comments or feedback ?

thanks Roy

 

[John.g]

Have you got 2 X two port zone valves, one each for the CH and HW and a mid position valve as well for by pass, if so how is this installed in the system?. or have you just got the mid position valve on its own?.

 

[Last Plumber]

Hi all

Just seeing if this is a common method of providing a boiler bypass thesedays as there seems so many ideas out there, years ago if we needed a bypass a gate valve was the answer but no longer classed efficient and others not suitable due to the smart pump.
The system is a sealed single zone heating system and indirect hot water cylinder using a Grunfoss Alpha smart pump. The H/W zone valve is controlled by an ST9100C timer, a tank stat and the mandatory thermal cut out whilst the heating by a CM907 programmable thermostat. There is a mid position valve that in its static position provides the bypass for the boiler and when powered by either of the zone valves fully opens to direct boiler flow through the required circuit and providing power to the boiler. There is also the usual frost stats and pipe stat for protection.

Anyone got any comments or feedback ?

thanks Roy

The basic principle of what you describe sounds OK.
If it works, then fine!

In commercial heating we use a 'similar' idea (valve as a slip/by-pass) on some systems although more involved and more accurately controlled.

 

[SJB060685]

Hi all

Just seeing if this is a common method of providing a boiler bypass thesedays as there seems so many ideas out there, years ago if we needed a bypass a gate valve was the answer but no longer classed efficient and others not suitable due to the smart pump.
The system is a sealed single zone heating system and indirect hot water cylinder using a Grunfoss Alpha smart pump. The H/W zone valve is controlled by an ST9100C timer, a tank stat and the mandatory thermal cut out whilst the heating by a CM907 programmable thermostat. There is a mid position valve that in its static position provides the bypass for the boiler and when powered by either of the zone valves fully opens to direct boiler flow through the required circuit and providing power to the boiler. There is also the usual frost stats and pipe stat for protection.

Anyone got any comments or feedback ?

thanks Roy

Hi.
Can you provide a wiring diagram for your above setup please

 

[Spectric]

Hi there

The CM907 Room stat controls a zone valve for the heating circuit(brown motor wire), an ST9100c 7 day timer controls the hot water zone valve(brown motor wire) via the tank stat which is in series with the tank thermal cut out. Both blue motor wires on the zone valves are neutral. Both zone valves have their grey wires to live (N/C contact) with the orange wires both going to the grey and white wires of the mid position valve, which when powered provides the fully open position. The orange wire on the mid position valve goes to the boiler and is only live when the mid position valve is fully open.

I will get a drawing and post as soon as I can.

 

[SJB060685]

Hi there

The CM907 Room stat controls a zone valve for the heating circuit(brown motor wire), an ST9100c 7 day timer controls the hot water zone valve(brown motor wire) via the tank stat which is in series with the tank thermal cut out. Both blue motor wires on the zone valves are neutral. Both zone valves have their grey wires to live (N/C contact) with the orange wires both going to the grey and white wires of the mid position valve, which when powered provides the fully open position. The orange wire on the mid position valve goes to the boiler and is only live when the mid position valve is fully open.

I will get a drawing and post as soon as I can.

Thats how I was picturing it and the normally HW port is your return back to boiler and two zone valves off of heating?

 

[John.g]

That's quite clever but will not satisfy a situation where a minimum flow rate must be maintained through the boiler which may occur if radiator TRVs are fitted.

 

[Last Plumber]

@John.g Makes a good point there. Do you have a permanently open part of the circuit for water to flow through, which is enough to provide the minimum required flow through the boiler?
If you were installing the system from new, you would be better off with a low loss header of some sort or a plate heat exchanger in my opinion.

 

[John.g]

That's one way of doing it (permanently open circuit) but IMO a reasonably good solution (from a energy saving point) would be the installation of a permanent bypass fitted with a manually adjustable flowmeter like the Tacona Tacosetter. If the "smart" circ pump is running in CP mode (or constant curve mode) then set it to say 5LPM, 0.3M3/hr, (or whatever the boiler maker specify) Worst case economy case then is where the TRVs have throttled in to give say a rad flow rate of ~ 5 LPM as well so you will then have boiler return temp of ~ 60C instead of 50C (from normal flow/return of say 70/50C) which will result in a efficiency loss of ~ 0.5%, not a lot, when the heating demand is higher then that loss becomes negligible. If the pump is running in PP mode then the loss will be slightly greater as the flow rate will be set up with no heating demand in which case the pump head might be ~ 2M but when running with a normal flow rate will be ~ 3M so the bypass flow rate will increase by ~ 22% but again this will have very little effect on the efficiency.
You can get constant flow ABVs as well, but IMO, not worth the extra investment.

 

[doitmyself]

That's quite clever but will not satisfy a situation where a minimum flow rate must be maintained through the boiler which may occur if radiator TRVs are fitted.

Most boiler manufacturers only provide the minimum flow at full output, which is not relevant when the boiler has modulated down to the minimum - which it could do if only one radiator is working.

It was different when boiler output was fixed as the flow rate would reduce when radiators were turned off.

 

[John.g]

Quite a lot of boilers, if not the majority, will not modulate below 5 kw, there two reasons, IMO, why a minimum flow rate should be maintained, one is, to avoid thermal stressing of the HX, if one allows say 25C delta t, then a flow rate of - 3LPM. The other reason is more practicle, if the delta t is too high then the boiler will very quickly cut out on high temperature (SP+5) especially on start up when the boiler output can be at 75% during the ignition phase.

 

[Spectric]

Hi there

Please find attached circuit of wiring. Yes thats right I don't use TRV's on all radiators so there will always be a flowpath whilst the room stat is asking for heat even if the TRV's close. The obvious radiator not to fit a TRV to is the one that is in the same area as the main stat.

 

[SJB060685]

Hi there

Please find attached circuit of wiring. Yes thats right I don't use TRV's on all radiators so there will always be a flowpath whilst the room stat is asking for heat even if the TRV's close. The obvious radiator not to fit a TRV to is the one that is in the same area as the main stat.

Pretty good wiring diagram and exactly how I was picturing it.

 

[John.g]

Could a simpl(er) 2 port valve be used as a straight bypass suitably wired from the 2 zone valves, also using the end switches on all three, the bypass end switch to supply the run signal to the boiler to prove it has fully closed?.

 

[SJB060685]

Could a simpl(er) 2 port valve be used as a straight bypass suitably wired from the 2 zone valves, also using the end switches on all three, the bypass end switch to supply the run signal to the boiler to prove it has fully closed?.

Do you mean another 2 port on flow to CH and HW 2 ports?
Why not have a direct short (return back to boiler) tee'd in to boiler flow before 2 ports?
As interesting as this is I'm curious as to why things need to be this complicated. All these additional electronic valves just increase power consumption

 

[John.g]

Yes, that's exactly what I mean, a straight by pass between flow & return. I wonder was the system first conceived with the mid position valve only which I think is not allowed with a unvented HWC as it will power fail to hot water.

 

[SJB060685]

Yes, that's exactly what I mean, a straight by pass between flow & return. I wonder was the system first conceived with the mid position valve only which I think is not allowed with a unvented HWC as it will power fail to hot water.

If using a mid position valve on an unvented cylinder then the only acceptable way is with the use of an additional 2 port after 3 port on flow to cylinder, with its own high limit stat wired in series to motor feed. As you said this is to prevent heated water being circulated through cylinder in the event of fault and boiler left on constantly.
If you were to install new then S plan is far simpler and compliant

 

[John.g]

It wold need to be a NO valve.

 

[SJB060685]

The 2 port after mid position would need to be a spring return normally closed. Or have I misunderstood you?

 

[John.g]

No, I was thinking of a normal s plan with no mid position valve and using a 2 port spring open valve in a bypass between the flow and return.

 

[Spectric]

Hi there

I can see you could use a V4043B1257 across the feed and return as bypass as it is normally open, so if either of the other valves are powered to open then this valve would close when powered. It could potentially have made the pipework easier in some synarios.

 

[doitmyself]

Quite a lot of boilers, if not the majority, will not modulate below 5 kw, there two reasons, IMO, why a minimum flow rate should be maintained, one is, to avoid thermal stressing of the HX, if one allows say 25C delta t, then a flow rate of - 3LPM. The other reason is more practicle, if the delta t is too high then the boiler will very quickly cut out on high temperature (SP+5) especially on start up when the boiler output can be at 75% during the ignition phase.

I may not have made myself clear. I wasn't saying that a minimum flow was unnecessary, but that it did not need to be the same as that specified for full ouput. A 24kw boiler with a 20C differential has a flow of approx 17 litres/min. If the same flow is maintained when the boiler has modulated down to 6kW, the differential will be only 5C. Whether that is a "good thing", I don't know.

 

[John.g]

I'm not that familiar with domestic gas boilers but I was always under the impression that the minimum flow rate was at minimum output, for example a vokera Vision 20S, a 20kw boiler with a minimum output of 5kw specifies a minimum flow rate of 350 litres/hr or 5.83 LPM, by calculation, this results in a deltaT of 12.3C, if the boiler output increased to 20kw then the deltaT would be 49C so clearly the flow rate at full output would need to be ~ 14/15 LPM for a 20C deltaT.