Pump overrun for heating

Wearb

I need to wire in a pump overrun on a heating system, that hasn't any motorised valves. What type of relay would I need? Is there a better way to do it?

Bruthal

The room stat can bring on the boiler direct. And also bring on the relay. Relay contact supplies pump. And pipe stat also supplies pump.

So when room stat or timer etc opens, switching off boiler, the relay opens. This isolates the pump from boiler, and so the pipe stat can keep the pump going until it cools down.

meercat

think youre over complicating it

switch the boiler as normal through the timeclock and room stat
run a permanent feed(from same spur)to pipestat that controls pump

Bruthal

Oil boilers etc will probably want the pump to come on immediately the boiler starts. If not, its overheat trip in a short time.

meercat

Bruthal wrote: »

Oil boilers etc will probably want the pump to come on immediately the boiler starts. If not, its overheat trip in a short time.

to prevent this
place the pipestat as close to the boiler as possible on the top pipe

Wearb

Thanks Bruthal. That sounds a bit simpler. I will draw it out when I get a chance, so that I can study it better.

Wearb

meercat wrote: »

think youre over complicating it

switch the boiler as normal through the timeclock and room stat
run a permanent feed(from same spur)to pipestat that controls pump

Thanks, but that won't work in my case as the feed to the pump will also fire the boiler. Unless I misunderstand your suggestion.

Wearb

meercat wrote: »

think youre over complicating it

switch the boiler as normal through the timeclock and room stat
run a permanent feed(from same spur)to pipestat that controls pump

I get you now. I have seen a few done like that. I don't like them. You can have the water very hot before it comes on, otherwise the stat needs turning down and that can have a cooling effect on heating system. I am talking standard efficiency here. Lower temps would be ok in HE.

Also I want it pre room stat. I only want to grab some of the boiler heat when it shuts down for the night etc. I am not having any hi limit tripping.

meercat

Wearb wrote: »

Thanks, but that won't work in my case as the feed to the pump will also fire the boiler. Unless I misunderstand your suggestion.

no.
wire the pipestat that feeds the pump independent
then the pump only turns on when there is hot water in the pipe

most oil boiler systems are wired this way

Bruthal

Wearb wrote: »

Thanks, but that won't work in my case as the feed to the pump will also fire the boiler. Unless I misunderstand your suggestion.

It does work alright. The boiler starts. The flow pipe out has pipe stat. When this heats enough it brings on the pump. Pump doesn't stop until pipe cools to the stat setting.

The relay method means the pump starts immediately, but doesn't stop until pipe with stat cools.

Older heavier heat exchangers on oil boilers didn't really need overrun.

Wearb

meercat wrote: »

no.
wire the pipestat that feeds the pump independent
then the pump only turns on when there is hot water in the pipe

most oil boiler systems are wired this way

Yes I got that ...eventually. See above.

In addition, boiler is outside and pump is in hot press, so don't want to run another pair of wires to boiler

Bruthal

meercat wrote: »

to prevent this
place the pipestat as close to the boiler as possible on the top pipe

Coming in here talking perfect sense as always:)

meercat

cheers bruthal
op.you may still have to run new wires to boiler for pipestat to control your pump

Wearb

meercat wrote: »

cheers bruthal
op.you may still have to run new wires to boiler for pipestat to control your pump

No. If I use the relay method, I can put stat beside the pump.

Btw I meant to add in a previous that the simpler method increases the stress on the boiler due to thermal shock.

Bruthal

Yes the pipe stat can go anywhere when not depending on it to start the pump.

Wearb

Bruthal wrote: »

The room stat can bring on the boiler direct. And also bring on the relay. Relay contact supplies pump. And pipe stat also supplies pump.

So when room stat or timer etc opens, switching off boiler, the relay opens. This isolates the pump from boiler, and so the pipe stat can keep the pump going until it cools down.

Here is what I think you are talking about.


While this works great and simply as a regular pump overrun, what I need is an overrun that only operates when the timer isn't calling for heat. I don't want it to operate between cycles.
When cycling, I want the heat to have a running start, so don't want overrun cutting in then.

Bruthal

Wearb wrote: »

Here is what I think you are talking about.


While this works great and simply as a regular pump overrun, what I need is an overrun that only operates when the timer isn't calling for heat. I don't want it to operate between cycles.
When cycling, I want the heat to have a running start, so don't want overrun cutting in then.

The overrun doesn't cut in as such. Timer starts boiler. Pump starts with it. Pump keeps running even between cycles. It only stops when boiler stops for long enough for pipes to cool below the pipe stat set point. So in effect the pump stops when the heating is off for a while. Ye hardly want the pump to stop each time the boiler cycles off?

My description was supply the relay contact from a permanent supply, and the timer or room star supplies relay coil. Timer preferable. The way you have it will still work though.

Wearb

Bruthal wrote: »

The overrun doesn't cut in as such. Timer starts boiler. Pump starts with it. Pump keeps running even between cycles. It only stops when boiler stops for long enough for pipes to cool below the pipe stat set point. So in effect the pump stops when the heating is off for a while. Ye hardly want the pump to stop each time the boiler cycles off?

My description was supply the relay contact from a permanent supply, and the timer or room star supplies relay coil. Timer preferable. The way you have it will still work though.

The way I have it, even when the boiler stat cuts out the burner, the room stat will still keep the pump running until room temp is satisfied. So the pump doesn't cycle off with the boiler.
After that, I don't want the pipe stat to play any part until the timer turns off the heating. So really what I want is when there is no power to the room stat, I want power to the pipe stat. When there is power to the room stat I want no power to the pipe stat. Doing it that way means that the boiler will be hot when the room stat calls for heat and it would also allow the heating to be turned off 10 or 20 minuted earlier, due to utilizing the built-up boiler heat, rather than having it just dissipate during the night.
I can work out ways of doing that, but I am wondering if there is an elegant way of doing it with one relay. Or a special type of relay.

Bruthal

I don't think you are fully grasping the concept of what the pipe stat is doing, when you say you want it to play no part until timer turns heating off. Or I could be missing what you're trying to do.

You want the pump to start with boiler, only stopping a while after the heating goes off via timer, yes?

If answer is yes, then relay supplies pump immediately the heating starts because the relay will be switched on by the timer. And when pipe stat closes when it heats up, it also supplies pump. So pump is supplied by 2 sources at once.

This has no affect on the operation of pump, except it keeps going after time clock switches heating off (which switches relay off), until the pipe the stat is on cools down. It's simple. Thinking of switching the pipe stat out of the equation and only switching it back in after time clock goes off is completely unnecessary. It will still work exactly the same.

But your method very briefly disconnects the pump and reconnects it as the relay contacts switch from one source to the other. Which is slightly rough. The method I outline removes the relay supply when timer turns heating off, and a while later removes pipe stat supply when it cools. No brief interruption.

Wearb

Bruthal wrote: »

I don't think you are fully grasping the concept of what the pipe stat is doing, when you say you want it to play no part until timer turns heating off. Or I could be missing what you're trying to do. Not missing anything. That is correct so far, except for the grasping the concept part.

You want the pump to start with boiler, only stopping a while after the heating goes off via timer, yes? Almost a yes. It will also stop from time to time with the room stat and I don't want the pipe stat kicking it on at that time.

If answer is yes, then relay supplies pump immediately the heating starts because the relay will be switched on by the timer. And when pipe stat closes when it heats up, it also supplies pump. So pump is supplied by 2 sources at once. Correct, but I don't want the pipe stat to operate the pump in that scenario.

This has no affect on the operation of pump, except it keeps going after time clock switches heating off (which switches relay off), until the pipe the stat is on cools down. It's simple. Thinking of switching the pipe stat out of the equation and only switching it back in after time clock goes off is completely unnecessary. It will still work exactly the same. It has the effect of cooling the boiler during times when the room stat isn't calling for heat and therefore boiler is slower to respond to room stat call for heat.

But your method very briefly disconnects the pump and reconnects it as the relay contacts switch from one source to the other. Which is slightly rough. The method I outline removes the relay supply when timer turns heating off, and a while later removes pipe stat supply when it cools. No brief interruption.

I agree, but you are missing what i am trying to achieve.

I know that this is starting to seem a bit too pedantic and it is. Calling it a pump overrun isn't accurate in the normal use of that idea. It is an overrun with a difference. That difference is that I don't want (or need) the overrun during normal timed operation. I only want to use it at the end of the heating period, so that the hot water that is in the boiler is utilized. Otherwise it would just sit in the boiler all night and be cold by morning.

I know the I can't utilize all of that heat, as there would come a stage when the water temperature in the heating system would be equal to the water temp in the boiler. I would still expect to be able to get an extra 10-20 minutes extra at shutdown.

Sorry if I am doing your head in. It isn't something that I really need to do, but something that I would like to be able to do if I wanted to. I can do it with a couple of relays, but was just wondering if there was a better way, or another type of relay that would work better.

Bruthal

Wearb wrote: »

It has the effect of cooling the boiler during times when the room stat isn't calling for heat and therefore boiler is slower to respond to room stat call for heat.

You are definitely over thinking it.

Yes the water will remain hotter in boiler if pump stops each time room stat goes off. But the majority of water which is out in rads will become cooler than if pump keeps going. So suddenly cooler water returns to boiler each time room stat comes on, so overall, little is achieved apart from the thermal shock you mentioned earlier. Pump running keeps everything more uniform.

As meercat said, its often done simply using a pipe stat close to boiler on the feed out. Gas boilers all have pump running from start until a while after finish, with them having lighter heat exchangers, this stops overheating. No need to be stopping flow each time room stat cycles.

If the boilers own stat is turned up to a good temperature, pump stopping on each room stat cycle might increase the chance of tripping the high limit stat

If you are going to go with cycling pump, your original diagram will work.