How to add pump overrun delay?

[richi]

We have an old but reliable oil boiler. The system is effectively a one-zone S-plan, but with a gravity-fed hot tank (i.e., the hot water rises from the boiler to the tank with no pump).

 

The boiler has no pump overrun circuitry. The room stat is wired to the valve, which then energises the pump. The boiler is switched separately by a timer/programmer.

 

My plan is to use a modern room stat that also has a timer (the wireless one @JSHarris uses), wiring it to fire the boiler and the valve/pump. But I want to have the pump run on for a couple of minutes after the boiler switches off.

 

Surely there's an off-the-shelf timer module I can wire in. Any recommendations?

[dpmiller]

you want a timer module with the function "true off delay".

That's what I did anyway...

[Nickfromwales]

@richi

Why do you need the pump overrun ( POR )? Any residual heat will just go via the gravity hot pipes. POR is only required where lifting demand / call for heat closes any water path from the heat exchanger. 

[Jeremy Harris]

As Nick says, the gravity feed to the tank will take away any residual hot soak. 

 

The only reason for having a pump over run built-in on a modern boiler like a combi, with switched heating and hot water circuits, is because the main heat exchanger has a very low water volume and the water in it could boil (or come close to it) from residual heat soak if both the burner and the water circulation stopped at the same time.  Even if it didn't boil, the water may well exceed the temperature at which which scaling starts, so keeping the heat exchanger cool extends the life of the boiler.

[richi]

Right, yes, of course I forgot to say why. We're renovating the house in two phases, which means we won't be using that tank for a while. So I've added a manual valve to the rising hot feed to it. But I don't want to close the valve until I've worked out a pump overrun. Make sense?

 

And of course, I want to avoid the situation where the room stat doesn't demand yet the boiler cycles because the exchanger has cooled down a bit.

 

@dpmiller would you recommend the part you used? 

Edited November 30, 2017 by richi

[Jeremy Harris]

Makes sense to just add a delay relay, then .  Easy enough to add, you'll need a suitably protected mains supply for it, the switched supply to the boiler and a suitable delay relay and box.

 

If you get a small DIN rail box (the smallest are the ones intended to house double pole isolator switches) and then fit something like this: https://www.rapidonline.com/finder-80-01-0-240-0000-time-delay-relay-timer-1-changeover-contact-ip20-50-5328 with a blanking plate over the unused single module space you should be OK. 

 

If you don't have a suitably protected switched mains feed for this, you could add a 6A MCB in the same enclosure, run a spur from the boiler supply in 2.5mm²  (assuming that the boiler supply is rated at more than the relay rating) just to get protection plus a means of independently turning off the over-run relay if needed.

[ProDave]

It sounds to me like you want some re plumbing. What you have sounds a but wrong.

 

I would be inserting a 2 port motorised valve into the heating and hot water circuits.  The heating one will be energised by the programmable room thermostat you are fitting.  The hot water one will be energised by a time switch and the hot water tank thermostat.  The feedback contact from them both in paralllel to fire the boiler when either is demanding heat.

 

No need for an over run.

[dpmiller]

@richi  I used a din-rail timer that I had kicking around and true off is generally the more expensive timer variety, but something like this would do-

 

https://www.aliexpress.com/item/Power-Off-Delay-Timer-Time-Relay-0-3-Minute-3M-ST3PF-with-socket-base-AC-220V/32607096370.html

 

permanent feed to it and the current pump supply signals it.

[Nickfromwales]

2 hours ago, ProDave said:

It sounds to me like you want some re plumbing. What you have sounds a but wrong.

 

I would be inserting  a 2 port motorised valve into the heating and hot water circuits.  The heating one will be energised by the programmable room thermostat you are fitting.  The hot water one will be energised by a time switch and the hot water tank thermostat.  The feedback contact from them both in parallel to fire the boiler when either is demanding heat.

 

No need for an over run.

You will still need a pump overrun ( PoR ) with that setup as it's a heat only boiler  When the cylinder is hot the DHW ZV will be shut, and when the demand for heating is lifted that ZV will shut too, therefore creating a sealed loop where residual heat in the exchanger will be free to boil and knock the OH stat out. Any arrangement with PoR needs a path to a heat loss circuit ( which EVER that is ) which is open in the standby state or worse a power failure.  

 

Ok. 

 

Assuming the hot tank is staying as is long term and is still plumbed in and remains in commission.  

 

What I'd do ( as a long term / end solution ) is do the 2 x 2-port ZV's so you you have an s-plan, and energise the DHW valve from the 'heating off' terminal in the time-clock.

 

NOTE : 28mm ZV on the hot circuit if it's 28mm pipe

 

So, when the heating runs, the DHW ZV shuts ( from stored  energy ) as it de-energises, and the heating runs through the heating ZV as per normal. 

When the heating demand is removed the heating ZV will close and the DHW ZV will open ( these will operate nigh on simultaneously so you'll never get a closed circuit as far as the pump circulation is concerned ) and and residual heat will then be free to naturally ebb away via the gravity circuit to the tank. The return water in the DHW gravity loop will be ambient ( cool / cold ) so will cool the boiler down nice and quick. 

As for normal DHW production, when you recommission to use DHW again, you just leave as-is but then connect the power for the DHW ZV to a simple timeclock for timed DHW. A nice side effect of this is you get to have a gravity DHW system but retain the option of having the DHW off whilst running the heating.  It would need a tweak to the  wiring to achieve that. 

No complex timers / non-standard controls too so ongoing maintenance / repairs are easy.  

[ProDave]

Sorry I am confused. It's a system boiler feeding heating and HW. Where is this heat exchanger? You have those in combi boilers but then you would not have a HW tank, and every boiler I have seen takes care of pump over run with it's own controls internally.

[Nickfromwales]

6 minutes ago, ProDave said:

Sorry I am confused. It's a system boiler feeding heating and HW. Where is this heat exchanger? You have those in combi boilers but then you would not have a HW tank, and every boiler I have seen takes care of pump over run with it's own controls internally.

The heat in any boiler always goes to the PRIMARY HEx ( what the burned fuel actually heats ) and then from there if it's a combi it gets diverted to the plate / secondary ( DHW ) HEx.  

The oil boiler here will be treated as a heat only / open pipe setup as the huge amount of residual heat in an oil boilers HEx needs to be dissipated after the flame is extinguished. Every oil boiler you buy these days has a pump control terminal on the PCB which fires the pump and does PoR, likewise with any modern system or heat only unit. It's infinitely more problematic with oil and more again with an old oil boiler as it'll likely have a manual resettable high limit stat. If left to boil, then cool, with no PoR, inevitably that safety device will nuisance trigger. 

[richi]

@Nickfromwales's analysis matches my intuition

[Nickfromwales]

20 minutes ago, richi said:

@Nickfromwales's analysis matches my intuition

My head is up my arse at the mo with work, so I'll nudge this later, but this could be simplified and a bit safer with a ZV that is 'normally open' eg energised to close for the DHW ZV. 

[ProDave]

2 hours ago, Nickfromwales said:

The heat in any boiler always goes to the PRIMARY HEx ( what the burned fuel actually heats ) and then from there if it's a combi it gets diverted to the plate / secondary ( DHW ) HEx.  

The oil boiler here will be treated as a heat only / open pipe setup as the huge amount of residual heat in an oil boilers HEx needs to be dissipated after the flame is extinguished. Every oil boiler you buy these days has a pump control terminal on the PCB which fires the pump and does PoR, likewise with any modern system or heat only unit. It's infinitely more problematic with oil and more again with an old oil boiler as it'll likely have a manual resettable high limit stat. If left to boil, then cool, with no PoR, inevitably that safety device will nuisance trigger. 

 

Isn't that what an automatic bypass valve is installed for, to give the boiler somewhere to pump water to if all the valves are closed and the boiler has an internal POR timer.

 

[Nickfromwales]

15 minutes ago, ProDave said:

 

Isn't that what an automatic bypass valve is installed for, to give the boiler somewhere to pump water to if all the valves are closed and the boiler has an internal POR timer.

 

No. that's there to protect the pump in a sealed system boiler typically. It will only open if say you turn the heating on and every radiator valve has inadvertently been shut, and the hot tank is satisfied.

That arrangement would not use a PoR. 

If the automatic bypass were used for heat dissipation it wouldn't be very good as it would just circulate the very hot water back around the primary HEx, and  you'd then be solely reliant on the latent loss of that and the short connective pipewort to dissipate the heat. ?

That all depends, of course, where the pump is, as the pump protective bypass valve needs to be at the pump or before any downstream control valves. 

Thats the reason you always see these true setups with 1 x 3-port mid-position valves rather than 2 x 2-port ZV's as in the de-energised / park state a 3-port MPV always leaves a fully open path to the heating circuit so the pump overrun only needs to control the pump ( as opposed to the complex solutions first discussed here ). 

Proper heat loss PoR setups are designed to draw cool water back into the boiler to be effective. With gravity it needs to work in the event of a power failure, hence my further suggestion ( above ) of a normally open ZV on the DHW circuit ?

Edited November 30, 2017 by Nickfromwales

[ProDave]

I don't have TRV's. I have UFH and DHW each fed by 2 port valves, 3 of them.  The Automatic bypass is fitted at the furthest point from the boiler, in my case at the HW tank.  It has "come to the rescue" when the microswitch in a 2 port valve failed, so it was calling for heat when all 3 valves were in fact shut. At least the boiler had a few metres of pipe loop to pump around so it didn't boil, just circulated boiler temp water round a loop, only firing occasionally to make up the small heat loss.

[Nickfromwales]

35 minutes ago, ProDave said:

I don't have TRV's. I have UFH and DHW each fed by 2 port valves, 3 of them.  The Automatic bypass is fitted at the furthest point from the boiler, in my case at the HW tank.  It has "come to the rescue" when the microswitch in a 2 port valve failed, so it was calling for heat when all 3 valves were in fact shut. At least the boiler had a few metres of pipe loop to pump around so it didn't boil, just circulated boiler temp water round a loop, only firing occasionally to make up the small heat loss.

In your situation that's exactly why it there. 

Remeber though that my comments here are case specific to @richi's setup  

[richi]

Thanks, gang. I've ended up ordering one of these from RS: Broyce Control True Delay Off Single Timer Relay, Screw, 0.5 → 10 min, SPDT, 1 Contacts, SPDT, 230 V ac, 24 V ac/dc

[Nickfromwales]

2 hours ago, richi said:

Thanks, gang. I've ended up ordering one of these from RS: Broyce Control True Delay Off Single Timer Relay, Screw, 0.5 → 10 min, SPDT, 1 Contacts, SPDT, 230 V ac, 24 V ac/dc

Neat little unit. How much was it ?

[richi]

£45, including VAT and "next-day" delivery.