Rads taking ages to warm. CH pump only runs when boiler is heating.

[richardindorset]

We have an old Potterton Profile 80e.

Now that the cold nights have started, I've just set the programmer to run the CH.

I've just noticed that the CH pump stops when the boiler cycles to off. Rads take forever to warm up!

The selector switch in the boiler is set to Fully Pumped, so the pump should run all the time the CH is set to be on.

When the boiler cycles on, the pump runs.

The manual shows the selector switch set to Fully Pumped and the pump connected to the Gravity Hot Water or Pump Over Run Thermostat. Maybe this is faulty?

Looking further at the manual, when the selector switch is set to fully pumped, power for the pump comes from a terminal marked switch live, via the Pump Over Run Thermostat. It says that this is for an external control, which I assume might be a wall thermostat, and if there is no external control, this terminal should be connected to a permenant live.

Checking the wiring, the switched live is connected to a the main 4 core cable that heads up to the airing cupboard where the pump, valves and water cylinder are. This is where the main CH system terminal box is too. The switched live wire is present there in the terminal box but it's not connected to anything else. It's just looped to two other terminals marked Boiler and Pump.

So is it possible that the system has never been wired correctly and the rads will only warm up if there is demand on the boiler for hot water or the underfloor heating system?

Any tips on what to check?


Cheers.

(Boiler schematics below)

 

[Mottie]

I've just noticed that the CH pump stops when the boiler cycles to off. Rads take forever to warm up!

If the boiler is off, what heat benefit do you think will be gained by pumping unheated water around the system?

Try turning the boiler temperature control up. On our ould Potterton Profile, the plastic control knob was broken so we needed a new knob to turn it up. Perhaps yours is too and is stuck on too low a setting?

 

[richardindorset]

If the boiler is off, what heat benefit do you think will be gained by pumping unheated water around the system?

Try turning the boiler temperature control up. On our ould Potterton Profile, the plastic control knob was broken so we needed a new knob to turn it up. Perhaps yours is too and is stuck on too low a setting?

If the boiler has just turned off, the pump stops and the hot water that is in the boiler piping just sits there, cooling down instead of going out to the rads. At the moment all that happens is the pipes near the boiler get hot and none of the rads do. They just get tepid. The rads do warm eventually as the water does circulate when the bolier is actually heating. The system is supposed to be fully pumped. The pump is supposed to run after the boiler burner turns off, for a while. It doesn't.

The boiler thermostat works fine, turning it up might make the rads warm a bit quicker but it may also create issues with the boiler overheat trip due to the over run not working.

 

[Madrab]

The boiler overrun works in conjunction with a bypass ( Auto or rad with 2 lockshields) and is designed to run the pump on after the boiler has shut off to ensure there is enough flow through the boiler to draw excess heat away from the HEX reducing wear, especially when either both valves in an S plan shut or TRV's on the rads close down. So it's not really designed to pump it around the system getting that last heat from the pipes to the rads.

That being said, by the sounds of it your pump may not be wired to an over run thermostat, need to check if there is one internal to that boiler. The switched live (SL) into the main board (not out of) would be coming from an external source that would 'switch' the boiler on, e.g. a thermostat/programmer, if there are no external controls then that SL needs wired to a permanent live (bridge) to complete the switching circuit and energise the boiler controls and fire it up.

So the SL must be connected to a SL somewhere that is energised when the programmer send the SL to the boiler, chances are the pumps is connected to the same SL so when the programmer shuts off the boiler it shuts off the pump too.

 

[richardindorset]

The boiler overrun works in conjunction with a bypass ( Auto or rad with 2 lockshields) and is designed to run the pump on after the boiler has shut off to ensure there is enough flow through the boiler to draw excess heat away from the HEX reducing wear, especially when either both valves in an S plan shut or TRV's on the rads close down. So it's not really designed to pump it around the system getting that last heat from the pipes to the rads.

That being said, by the sounds of it your pump may not be wired to an over run thermostat, do you know if there is one installed, there isn't one internal to that boiler I don't think. The switched live (SL) into the main board (not out of) would be coming from an external source that would 'switch' the boiler on, e.g. a thermostat/programmer, if there are no external controls then that SL needs wired to a permanent live (bridge) to complete the switching circuit and energise the boiler controls and fire it up.

So the SL must be connected to a SL somewhere that is energised when the programmer send the SL to the boiler, chances are the pumps is connected to the same SL so when the programmer shuts off the boiler it shuts off the pump too. it would only be wired wrongly if there is an over run stat there and the pump hasn't been wired to it properly.

Hi Madrab, thanks for the detailed reply! Yes, there is a bypass.

There is a pump over run stat in the boiler but for some reason the feed to it (Switched Live) isn't live. I traced the wiring from the boiler back up to the main heating terminal box and it comes in on terminal 1, then that terminal is linked over to two other terminals, 12 and 13. Neither of which are connected to anything. I suspect it's never been wired in!

As the potterton manual states "if no external control is fitted, wire switched live to a permenant live", I think I will do just that and see how the pump runs.

The manual states that the overrun stat will turn off the pump as soon as the heat exchanger drops below 80c. I assume this is the metal casing of the exchanger and not the water temp! I see that there are two mounting options for the over run stat bulb depending on if the boiler is set up for fully pumped or gravity feed.

 

[Madrab]

Yes, after checking the book of words, that boiler has an overrun stat in a dry pocket on the HEX, therefore it will switch off once the vial temp drops below 80DegC and activates the switch.

That should be wired internally and as you say the output from that stat should run to the pump. If the stat has gone faulty then rather than replace the stat the pump may have been short circuited over to the programmer's SL, therefore when the programmer no longer call for heat that SL is de-energised and the power is cut to the pump.

If the pump is connected that way and you then switch that to a permanent live then the pump (and boiler) may run all the time. It needs a little more checking to see where the live feed from the pump runs to, that'll give you an idea of where it's currently connected to.

If you're handy with a multimeter then it would be time to check the overheat stat too, check it for a resettable cut out or adjustment, you will also be able to replace it if it is found to be faulty.

 

[stem]

The boiler has a dual function internal 'Gravity Hot Water or Pump Over Run Thermostat'. It is factory set to approx. 80 degrees C with a single changeover contact. In your circuit diagram 'NO' is connected to 'C' when the temperature is under 80C and 'NC' is connected to 'C' when the temperature is over 80C.

How it works depends upon the type of system you have and the position of the 'Gravity/Fully Pumped System Selector Switch'

In Gravity Hot Water Mode. The pump will not operate when only DHW is running, because the hot water cylinder is heated by natural (gravity) circulation of hot water from the boiler. In this position the Gravity Hot Water or Pump Over Run Thermostat is in series with the Overheat thermostat and will cut out the boiler if the water exceeds 80C

When CH is running the top of the two L terminals becomes live and the pump will run under the control of external controls [CH timeswitch / thermostat] connected to it. The position of the 'Gravity/Fully Pumped System Selector Switch' takes the 'Gravity Hot Water or Pump Over Run Thermostat' out of the pump circuit so it will not provide a pump overrun.

In Fully Pumped Mode. When either CH or HW are operating the setting of the switch means that the pump is controlled by the 'Gravity/Fully Pumped System Selector Switch' so the pump will continue to run after the boiler has switched off provided that the water is over 80C.

Bottom Line. If the boiler control thermostat is set to less than 70C, it is unlikely (even with residual heat) to get hot enough to operate the overrun thermostat set at 80C, so the pump will go off immediately with the boiler and not continue to run.

It would help to be more specific if you said what type of heating system your boiler is connected to ie. Fully Pumped, or Pumped CH with Gravity HW and what external controls / thermostat are connected.

 

[Madrab]

The selector switch in the boiler is set to Fully Pumped, so the pump should run all the time the CH is set to be on.

 

[stem]

Hmm. Maybe, but how many times have we come across installations that are plumbed up as one system but the controls set for something else. I tend to check now just to be sure, especially as the OP is changing things.... been caught out too many times... Although I have to admit, I've not come across a system with gravity hot water in conjunction with UFH....Yet....So it's likely fully pumped.

So is it possible that the system has never been wired correctly and the rads will only warm up if there is demand on the boiler for hot water or the underfloor heating system?

The boilers sole job is to produce hot water. It simply gets a signal from the external controls telling it to 'run'. and that's it. Where the water goes once it's produced depends upon what the boiler is connected to externally. It would be normal for a system like yours to have two (or more) heating zones each with a separate motorised valve. One for the UFH and one for the Radiators. Each motorised valve then being controlled by a separate room thermostat. The motorised valves have a small switch inside that provide the control signal to run the boiler.

EDIT
Just thinking about this.....

I've just noticed that the CH pump stops when the boiler cycles to off

......I assume that you mean the pump goes off when the boiler goes off, and not that the pump is literally cycling on and off with the gas burner.

 

[richardindorset]

Hmm. Maybe, but how many times have we come across installations that are plumbed up as one system but the controls set for something else. I tend to check now just to be sure, especially as the OP is changing things.... been caught out too many times... Although I have to admit, I've not come across a system with gravity hot water in conjunction with UFH....Yet....So it's likely fully pumped.



The boilers sole job is to produce hot water. It simply gets a signal from the external controls telling it to 'run'. and that's it. Where the water goes once it's produced depends upon what the boiler is connected to externally. It would be normal for a system like yours to have two (or more) heating zones each with a separate motorised valve. One for the UFH and one for the Radiators. Each motorised valve then being controlled by a separate room thermostat. The motorised valves have a small switch inside that provide the control signal to run the boiler.

EDIT
Just thinking about this.....

......I assume that you mean the pump goes off when the boiler goes off, and not that the pump is literally cycling on and off with the gas burner.

The pump IS cycling with the boiler. As soon as the boiler stat fires up the burner, the pump starts. When the burner cycles off, the pump stops. I will double, double check this today, with the UFH and the HW turned off and just the CH programmed to be on.

I have never changed the switch for Gravity/Fully Pumped, in fact I only found it yesterday when I read that there is one!

I read that the pump over run stat bulb is fitted slightly differently for fully pumped, so I will check and see if I can see that too.

I don't think I will be risking anything by wiring the switched live to permenant live in the terminal box just to see what happens, wil I?

The manual says to do this.

 

[stem]

So we'll go for it being a fully pumped system then

In which case, according to the wiring diagram you supplied; in fully pumped mode the pump will run whenever either the CH or DHW are calling for heat. So whenever the boiler is 'on' the pump will also be 'on'. But It shouldn't physically cycle 'on' and 'off' with the gas burner itself. You can see from the diagram the only places that the pump should get a live from are the externally supplied 'permanent live' and 'switched live'. It's in no way linked to the gas burner.

Under normal 'running' circumstances, there should be a permanent live on the top L terminal, and a switched live present on the bottom L terminal, meaning the pump will receive power as per the diagram below, regardless of the position of the pump overrun thermostat.

When there is no demand from the external controls, the switched live is removed, but, the pump will run on, provided that two criteria are met:
1. The overrun thermostat is sensing a temperature in excess of 80.C and
2. The permanent live is present on the top L terminal to provide operating power

If either the permanent L isn't present, or the thermostat detects less than 80.C the pump will stop with the boiler. Once the temperature drops below 80.C the overrun thermostat changes position (to NC) and removes power from the pump.

You can see from the above diagram that the permanent live is required to keep the pump running when the switched live has gone. Which means that you won't get a pump overrun if the permanent live is missing.

Rads take forever to warm up!

After all of this, the presence or lack of a pump overrun won't make any difference to how quickly the radiators heat up, as the overrun only come into operation at the end of the heating cycle. And if the radiators do have their own motorised zone valve it would be closed and stopping water flow whenever the pump overrun was in operation anyway. Which begs another question. The pump overrun requires a flow route somewhere to allow water to flow from and back to the boiler do you have one? Edited as you do have a by-pass

 

[Madrab]

Hmm ... the only way that control board would turn the pump on when the boiler itself (stat) called for heat is if the pump was wired into the fan output. That control board is really simple and I would guess that if that was the case the pump and fan combined would probably draw to much current from that output and blow the board.

The pump should be connected to it's designated terminals beside the main power connections in the control box (post 1, pic 2), either directly or through the wiring centre. If it's not then it's not wired correctly as per the MI for fully pumped, that will ultimately be where the Over Run stat feeds back to power the pump. As suggested, you need to track back from the pump and see where it's drawing it's power from. If it's a remote system pump then it should be pretty easy to trace the connection back. That would be the starting point.

Apologies if you're getting 2 sets of info that are ultimately coming from the same angle

 

[Madrab]

yes, there is a bypass.

 

[richardindorset]

OK chaps, looks like I have been chasing a red herring!

No that I have the UFH and HW completely off, I found that the microswitch in the CH valve was not making contact.

Last night I had I was sure that the moment the boiler burner turned off, the pump also stopped. The rads weren't getting very warm either.

So I started looking down the rabbit hole of the pump over run stat and the wiring...

I guess that as the CH demand wasn't actually working but the valve was open, when the UFH demand came on, the rads got some of the heat too.

Am I right to say that the boiler doens't know what the demand is, it just gets a mains signal from the one of the three valve switches thar are in parallel when it closes?

It's odd as the pump defo stopped while I had my head in the airing cupboard, when the boiler burner stopped.

Anyway, I have fixed the dodgy microswitch and the pump does stay on when the boiler reaches temp and the burner turns off. I guess it should stay on while any one of the demand valves is open?

There is 240V on the Switched Live terminal when the boiler burner has turned off, even though the wiring doesn't go anywhere in the terminal box. I assumed that this was an input to the boiler circuit but maybe its an output for something? I'm not sure of its purpose.

Thanks ever so much for all the info. I have learned some useful stuff and I hope we will be a bit warmer tonight!

 

[stem]

Am I right to say that the boiler doesn't know what the demand is, it just gets a mains signal from the one of the three valve switches that are in parallel

Yes.

the pump does stay on when the boiler reaches temp and the burner turns off. I guess it should stay on while any one of the demand valves is open?

Yes

There is 240V on the Switched Live terminal when the boiler burner has turned off, even though the wiring doesn't go anywhere in the terminal box. I assumed that this was an input to the boiler circuit but maybe its an output for something? I'm not sure of its purpose.

The switched live signal will come from the switches of the motorised valves. When any of the valves are open the switched live should be present and the boiler will 'run'. the boiler's internal circuits will switch the burner on and off as required to regulate the temperature of the water leaving the boiler.