Gas boiler bypass query

[John Kirk]

My old cast iron gas boiler with pump overrun needs a bypass, hitherto provided by one radiator with 15mm inlet and outlet pipes and 2 lockshield valves. My intention is to install smart TRVs on every radiator, which means I will need to install a bypass pipe immediately after the pump, between flow and return, in order to provide flow when all TRVs are shut. The boiler flow and return are 22mm.

My question is whether I can run this bypass in 15mm pipe, instead of the usual 22mm. I would use the normal 22mm bypass valve but with reducers to 15mm. The current bypass flow via the 2 lockshield valves stangle flow volume to a fraction of the capacity of 15mm pipe, so my reasoning is that I would not be restricting flow any further with the alternative bypass in 15mm, probably even increasing the potential bypass flow.

I'd be very grateful if anyone can tell me if there is any reason why I should not use 15mm pipe for the bypass.

 

[Madrab]

As long as you can still obtain the right flow through the bypass to satisfy the pump/boiler requirements then no reason at all. Why not just do it in 22mm tho?

 

[ReganAndCarter]

As Above. Can't imagine why you would use a 22mm bypass, then reducers down to a piece of 15mm pipe. Would be less work and remove the doubt over flow just by buying a piece of 22mm.

 

[johnkirk]

Bypass pipe length would be about 1.5m long, and need to be routed through awkward angles and restricted space to connect flow to return, hence strong preference for the more flexible and space saving 15mm plastic pipe, rather than 22mm.

As for whether I can still obtain the right flow or not I'm suggesting I will achieve this using the reasoning I've stated. In reality I won't be able to test or calculate flow in advance since there is no stated minimum flow rate given by the boiler manufacturer. Pump flow data is available, but this on its own is unhelpful. (Boiler is Potterton Netaheat Profile 50e installed 1990).

 

[Madrab]

since there is no stated minimum flow rate given by the boiler manufacturer

You need to re-read the manual then - Min 1kw/h dissapation with a min flow of 9 L/min

 

[ianmcd]

15mm will be fine

 

[John Kirk]

Thanks Madrab for pointing out these values. They are indeed quoted on page 8 of the installation instructions.

 

[fixitflav]

My old cast iron gas boiler with pump overrun needs a bypass, hitherto provided by one radiator with 15mm inlet and outlet pipes and 2 lockshield valves. My intention is to install smart TRVs on every radiator, which means I will need to install a bypass pipe immediately after the pump, between flow and return, in order to provide flow when all TRVs are shut. The boiler flow and return are 22mm.

My question is whether I can run this bypass in 15mm pipe, instead of the usual 22mm. I would use the normal 22mm bypass valve but with reducers to 15mm. The current bypass flow via the 2 lockshield valves stangle flow volume to a fraction of the capacity of 15mm pipe, so my reasoning is that I would not be restricting flow any further with the alternative bypass in 15mm, probably even increasing the potential bypass flow.

I'd be very grateful if anyone can tell me if there is any reason why I should not use 15mm pipe for the bypass.

Why not leave one rad without a TRV, and install a room-stat in that room? Will give better control, and depending where the room-stat is relative to the boiler, could be cheaper to install. Saves 1 TRV and the bypass.

 

[johnkirk]

Why not leave one rad without a TRV, and install a room-stat in that room?

This is exactly the current setup, which is unsatisfactory. If this radiator, which is in the hallway, has no smart TRV when all the other rooms have them then it will heat up when one or more of the other radiators is calling for heat, irrespective of the wall thermostat which is also in the hallway. With smart TRVs every room becomes an independent zone, so no room ever has to warm up unnecessarily, which is the reason I'm changing all the manual TRVs to smart TRVs.

 

[D_Hailsham]

My intention is to install smart TRVs on every radiator, which means I will need to install a bypass pipe immediately after the pump, between flow and return, in order to provide flow when all TRVs are shut.

Where is the interlock if you have smart TRVs on all radiators?

 

[Madrab]

As @D_Hailsham mentions, you really should keep the interlock (stat) active. The room with the stat shouldn't have a TRV on it as the stat will always battle with the TRV and call for heat if the hallway doesn't heat up and it will just keep the boiler running even if all the TRV's are closed, unless they are all interlinked back to a central control unit (like the evohome) that then acts as the interlock. You could always change the stats position or change to an RF programmable room stat

 

[johnkirk]

Where is the interlock if you have smart TRVs on all radiators?

As @D_Hailsham mentions, you really should keep the interlock (stat) active. The room with the stat shouldn't have a TRV on it as the stat will always battle with the TRV and call for heat if the hallway doesn't heat up and it will just keep the boiler running even if all the TRV's are closed, unless they are all interlinked back to a central control unit (like the evohome) that then acts as the interlock.

I have the Tado system with all trvs being smart ones. The wall thermostat in the hallway is Tado's smart thermostat, but the smart TRV which is also in the hallway will not battle with the wall thermostat because I can select the temperature control in the hallway to be dictated by the wall thermostat instead of by the TRV. I think this is the exception Madrab is referring to. (Where there are multiple thermostats in a NAMED room, Tado cleverly allows the user to choose the one that will be used to command the temperature for that room, so avoiding any clashes. In my hallway the smart TRV would be downgraded via the Tado setting to just an on/off control since the wall thermostat alone would dictate when the TRV should open and close).

So the boiler will never fire when there is no demand for heat somewhere in the house - the interlock issue is taken care of.

I still however need a bypass between flow and return because of pump overrun. If every radiator happens to be shut off when the system water is very hot the pump will continue to run for a while in order to dissipate residual heat from the heat exchanger. Without this, the boiler would be forced to activate the shut-down thermostat, which would then have to be reset manually after cooldown. The boiler is a heat only one (Potterton Netaheat Profile 50e, with mid position diverter valve for the HW cylinder).

 

[fixitflav]

With smart TRVs every room becomes an independent zone, so no room ever has to warm up unnecessarily, which is the reason I'm changing all the manual TRVs to smart TRVs.

OK, it hadn't sunk in that they will be smart TRVs, I was assuming standard old-fashioned ones.

In my hallway the smart TRV would be downgraded via the Tado setting to just an on/off control since the wall thermostat alone would dictate when the TRV should open and close).

If the wall thermostat is existing that's straightforward, but wouldn't you get same result by putting same TRV in the hall as everywhere else?

Tado cleverly allows the user to choose the one that will be used to command the temperature for that room, so avoiding any clashes.

Is the Tado system clever enough to know when any TRV is the only one calling, stopping the boiler but staying open when its temperature setpoint is reached? That would avoid the need for a bypass. It would stay open (with boiler off) until any TRV calls for heat again. If it's that TRV it stays open and starts the boiler, if it's a different one it knows another TRV is open so it's OK to close.

 

[johnkirk]

If the wall thermostat is existing that's straightforward, but wouldn't you get same result by putting same TRV in the hall as everywhere else?

Yes, good point, I could dispense with the existing smart thermostat that's on the hallway wall and use just a smart TRV in the hallway radiator. This would give the same result, as you say. The smart wall thermostat (which I've used for over 2 years before acquiring any smart TRVs) would then become redundant until every room except the last room acquired a smart TRV. The last room could then get the smart wall thermostat and not need any TRV because the wall thermostat would trigger the heat command, and the heat command would affect only that room since all the other ones are controlled by smart TRVs. Since I'm installing smart TRVs in stages, it must be the final radiator that gets the smart wall thermostat if I want to avoid other radiators heating up when they should stay off.

Is the Tado system clever enough to know when any TRV is the only one calling, stopping the boiler but staying open when its temperature setpoint is reached? That would avoid the need for a bypass. It would stay open (with boiler off) until any TRV calls for heat again. If it's that TRV it stays open and starts the boiler, if it's a different one it knows another TRV is open so it's OK to close.

No, I don't think it has that level of sophistication. When the set temperature is reached the TRV shuts fully and stays shut, irrespective of whether it is the only one calling for heat, so I would still need the bypass. (I will check though with Tado's excellent technical helpline).

Thanks fixitflav for raising these points!