Baxi boiler pump overrun using true off delay timer

[Joel Allen]

Hi I have had the same problem with my boiler since it was installed over 10 yrs ago, its the last of the Baxi back boilers which still works ok. I have 13 radiators all have TRV's apart from two, one towel rail and a very small rad near downstairs toilet. All the TRV's are adjusted down, the boiler reduces down but does not stop until the timer program runs out. The system also has a room stat linked to the boiler from the hall. If I try to control the temperature with the stat it works not bad around 21 degrees. It seems like anything higher or lower is a problem. Once the room stat is satisfied the boiler cuts along with the ch pump. The heat stays in the heat exchanger and can be heard still boiling after boiler is stopped. I do have a thermo valve fitted for expansion of hot water but it seems if the boiler fires agin within a few minutes it locks out on OH and trips. The boiler has no facility for pump overrun and read about true off timer delay switches (to keep the pump running for a minute or so after boiler has stopped) I have installed it but not 100% with the wiring, the pump runs consistently, I am sure I have the live in the wrong connection? The timer switch has L 1 2 N, does anyone have any experience of wiring these switches?

I am proper stressed out now!!

cheers Joe

 

[Simon35]

Photos are good.

Off delay timers usually need a permanent live. At a guess, you need permanent live and neutral on L and N, signal from boiler to run pump on 1, and signal to whatever runs the pump from 2.

 

[Joel Allen]

Photos are good.

Off delay timers usually need a permanent live. At a guess, you need permanent live and neutral on L and N, signal from boiler to run pump on 1, and signal to whatever runs the pump from 2.

Thanks Simon Ill give that a go tomorrow and see what happens, the instructions for the switch timer are very poor.

 

[Joel Allen]

Thanks Simon Ill give that a go tomorrow and see what happens, the instructions for the switch timer are very poor.

Thanks Simon Ill give that a go tomorrow and see what happens, the instructions for the switch timer are very poor.

Just another question do I connect the pump live to signal for pump to work?

 

[Simon35]

Just another question do I connect the pump live to signal for pump to work?

I'm assuming that there is a wire going to the existing pump or pump control to tell the pump to run? That wire needs to go in 1 and out of 2 if my earlier assumptions are correct, so that signal is held on to give you a pump run on time.

If that wire is going directly to the pump, meaning it is supplying the power to the pump, then you need to be sure that the off delay timer is suitable for the load.

What's the part number of the timer?

Post a photo of the data sheet if you can, it's all a bit guessworky otherwise.

 

[stem]

You don't give any information about the type of heating system that you have, and this is very relevant.

If your system is the old design type with pumped central heating and gravity circulation of hot water [these type of systems don't allow the heating to be 'on' without the hot water being 'on' at the same time] like the diagram below

In this case, the hot water time control only switches the boiler 'on' and 'off', and the heating controls only switch the pump 'on' and 'off'. In which case what you suggest won't work because the pump will run on for a while (the system would see it as if the room thermostat was still calling for heat and running the pump) and if the hot water was still selected 'on' the boiler will carrying on heating up. Then when the pump does stop after the time delay, you will still have exactly the same scenario as you do now.

Only with a system that has independent control of the heating and hot water with a 3-port motorised valve as below (or two 2-port motorised valves and a by-pass) that allow totally independent control of the heating and hot water would what you propose work.

 

[Joel Allen]

I'm assuming that there is a wire going to the existing pump or pump control to tell the pump to run? That wire needs to go in 1 and out of 2 if my earlier assumptions are correct, so that signal is held on to give you a pump run on time.

If that wire is going directly to the pump, meaning it is supplying the power to the pump, then you need to be sure that the off delay timer is suitable for the load.

What's the part number of the timer?

Post a photo of the data sheet if you can, it's all a bit guessworky otherwise.

I used this as on the reviews someone had already used and said it worked fine

 

[Joel Allen]

You don't give any information about the type of heating system that you have, and this is very relevant.

If your system is the old design type with pumped central heating and gravity circulation of hot water [these type of systems don't allow the heating to be 'on' without the hot water being 'on' at the same time] like the diagram below

In this case, the hot water time control only switches the boiler 'on' and 'off', and the heating controls only switch the pump 'on' and 'off'. In which case what you suggest won't work because the pump will run on for a while (the system would see it as if the room thermostat was still calling for heat and running the pump) and if the hot water was still selected 'on' the boiler will carrying on heating up. Then when the pump does stop after the time delay, you will still have exactly the same scenario as you do now.

Only with a system that has independent control of the heating and hot water with a 3-port motorised valve as below (or two 2-port motorised valves and a by-pass) that allow totally independent control of the heating and hot water would what you propose work.

My system is similar to the one above, a back boiler, fully pumped sealed system with a two port motorised valve, thanks for your reply.

 

[stem]

My system is similar to the one above, a back boiler, fully pumped sealed system with a two port motorised valve.

OK that's good. With 2-port motorised valves if you keep the pump running on a delay, you will need a 'by-pass' so that the water has somewhere to go when the central heating thermostat has closed the valve supplying the radiators...or did you mean a 3-port motorised valve?

 

[Joel Allen]

OK that's good. With 2-port motorised valves if you keep the pump running on a delay, you will need a 'by-pass' so that the water has somewhere to go when the central heating thermostat has closed the valve supplying the radiators.

 

[Joel Allen]

Its fitted with a bypass loop at the cylinder with a sprung valve which releases pressure as the hot water expands, also two rads with no TRVS

 

[stem]

OK if you've a by-pass that's fine. You can't count the the rads without TRV's though as they normally are connected after the motorised valve so won't be receiving any flow once it has closed.

 

[oldbutnotdead]

In your system (typically) live to the thermostats is switched by the timeclock. When a thermostat calls for heat it energises the relevant 2 port valve. When that valve is open the microswitch inside it energises the pump and the boiler.

So your off delay relay needs to be fitted (electrically) between the microswitches and the pump. But this won't work unless your non-TRV rads are before the 2 port valves (pump will run but no circulation cos valves are closed).

You might think ok then, put the off delay between thermostat and valve motor. Won't work, pump AND boiler will fire during the off delay.

Do you still have the installation manual for the boiler- on my old Baxi there had to be either a bypass or a radiator not controlled by the motorised valves or TRV) to address exactly the problem you're having).

Your control issue (by the way)- the radiator where the stat is should NOT have a TRV- if it does then the thermostat is unlikely ever to reach satisfied (the TRV gets to that point before the stat does & shuts down the rad, thermostat continues to call for heat)

 

[winston1]

If your system is the old design type with pumped central heating and gravity circulation of hot water [these type of systems don't allow the heating to be 'on' without the hot water being 'on' at the same time] like the diagram below

They do if they are properly designed. Many are not however.

 

[Joel Allen]

I'm assuming that there is a wire going to the existing pump or pump control to tell the pump to run? That wire needs to go in 1 and out of 2 if my earlier assumptions are correct, so that signal is held on to give you a pump run on time.

If that wire is going directly to the pump, meaning it is supplying the power to the pump, then you need to be sure that the off delay timer is suitable for the load.

What's the part number of the timer?

Post a photo of the data sheet if you can, it's all a bit guessworky otherwise.