CH Circulation pump (?) fault diagnosis

[Just4Fun]

I am trying to diagnose an intermittent fault with our heating, which started a couple of weeks ago and has happened every 4 days or so since then. Long story short, I have 2 circulation pumps, typical Grundfos pumps. When the fault occurs both the pump bodies are hot but the pipework isn't. On both pumps I tried removing the central vent screws on the pumps to check if the shafts were rotating. On one pump it was. On the other I could not check because when I remove the vent screw water spews out under pressure and I had to replace the vent screw immediately. I did not want that amount of water spraying around electrical equipment.

The two pumps are on different electrical circuits. The one with the confirmed shaft rotation sometimes trips the trip switch on its power supply. This is not the cause of the fault: it only happens if I power down and power up again when trying to identify the fault. When I reset the trip switch and turn everything back on, the heating works correctly.

The heating is not having to work hard at the moment. The temperature outside has only been a few degrees below freezing. It can get much colder here, and no doubt will do so before too long.

The heating is a single zone system with approximately 1 million radiators.

So my questions:
- Does water from the vent screw indicate a fault on that pump?
- Does the fact that the pumps get hot indicate a pump fault? On both pumps?
Or could a fault on one pump block circulation causing both pumps to overheat?
Or would the same symptom result from blocked circulation elsewhere in the system?
- What should I do next to trace the fault?

 

[hunter27]

A few answers at least.
The large screw cap on Grundfos pumps allows you to turn the rotor under the screw and to vent the pump, there is not usually too much water comes out but they all vary a bit. If a pump is not running it will not stop water from flowing through it as its only got an impeller inside.
You can usually tell if a pump is running by feeling it's casing or hearing it. It is normal for the motor to get warm when it is working and will obviously get hotter when hot water is flowing through it.

 

[Bingy man]

The vent is exactly that a vent and will emit water when removed. A shaft spinning does not prove the pump is working as the impeller at the rear of the pump may no longer connected to the shaft . If the pump is hot but pipe work is cold then the pump is most likely faulty or shot. As for the other pump tripping the electric id be inclined to change both pumps . Are they original pumps or the new energy efficient ones ? how old is your system? and what type of boiler do you have . As a get out of jail card turn the electricity off , remove the vent screw with t
towels under / in front of the vent and spin the shaft if possible. Re seal pump and restore power and try again. If both pumps are old then replace them . Are both pumps for heating or is one for hot water?

 

[Spectric]

I am trying to diagnose an intermittent fault with our heating, which started a couple of weeks ago and has happened every 4 days or so since then. Long story short, I have 2 circulation pumps, typical Grundfos pumps. When the fault occurs both the pump bodies are hot but the pipework isn't.

You need a logical approach and have not mentioned if you are using S plan zone valves or a Y plan three port valve or if the pump is in the flow or return. In simple terms with the heating on the thermostat operates a zone valve that once fully open sends the demand signal to your boiler which then controls the pump. This is because you need the boiler to keep the pump running when it shuts down to keep circulation through the heat exchanger. So why do you think the fault is with the pump, the pipes will be cold if the zone valve has not opened and is the boiler firing up to produce hot water ?

 

[Richard_C]

o why do you think the fault is with the pump,

I think thats a fair point, stalled pump might be a symptom not a cause. Are your pumps in parallel? I guess so otherwise why have 2 on a single zone system so one should run even if the other has failed. It would be a massive coincidence if both pumps failed at the same time, so maybe a zone valve or control circuit as suggested, massive airlock*?, mag-filter bowl blocked if you have one, if you have TRVs and they all go to 'off' when the rooms are warm enough is there a bypass circuit or a radiator that is always on? Does the bypass have a regulating valve and is that open properly, has your always on radiator been switched off,

* has the system been opend up/drained at all in recent months, was it refilled and bled properly?

If you check all of that and its OK, then maybe they did both fail at the same time but it stikes me as highly unlikely.

 

[Just4Fun]

Thanks all for your help. I can see I need to provide some extra details, which I was hoping to avoid because this could get very involved without adding much of real use.

The pump which sprays water out when I remove the vent screw is producing a LOT of water. More like a garden hose than the small dribble I usually get when opening the vent on such pumps.

The pumps are simple, old-style pumps, not modern energy-efficient models. One is a Grundfos UPS 25-80 180. I cannot see the details on the other; it is a similar style no-name pump, and I suspect lower rated.

When we moved here the heating was an oil burner and there was a circulation pump. That pump (or rather a later replacement, maybe 5 to 10 years old now) is still in use and is the one that spews water when I remove the vent screw. It runs continuously during the heating season.

We fitted a ground source heat pump about 15 years ago and internally that has a circulation pump that in effect is in series with the other pump. It is a similar (probably higher-rated) pump that just happens to be part of and controlled by the heat pump.

The system is currently working OK and I have just measured the temperatures of the pumps and the pipework. All quite similar. The pumps are a few degrees hotter than the pipes, but not by much. My measurements confirm the flow temperature (44C) shown on the heating system display panel.

There is no S plan zone valve or a Y plan three port valve. This is a central heating system only, not involved in the hot water supply. The pumps are both in the flow, not the return.

There is a thermostat but it only serves as a means of temporarily calling for extra heating. In reality it is never adjusted and never influences the operation of the system. Instead, the heat pump adjusts the flow temperature based on a temperature sensor fitted outside the house. It has a sort of graph that says if the outside temperature is X then the flow temperature should be Y. Actually there are a number of such pre-defined graphs I can choose from, or I can define my own. I don't think that is really relevant to this fault though.

I think the fault may be in the pump(s) because:
- the pumps are hot when the pipework isn't
- the heat pump says it's circulation pump is on
- the heat pump says it's compressor is running
- the heat pump says both of it's backup resistive elements are on
- the radiators get cold

So it looks like the heat pump is doing everything it can to provide heat but hot water is not being circulated around the system. To me, the pump(s) look like the guilty party.

I agree that it is unlikely that both pumps failed at the same time, but I don't think it is impossible that the one outside the heat pump failed earlier without that being noticed until now. I see no reason why the problem should be intermittent though.

I also agree that it is odd to have 2 pumps in series like this, but that is just the way it is. The system was never designed like that - it has just evolved as the heat source was changed from a wood burner to oil and now a GSHP, and the house was extended.

The system has not been drained recently. I have bled the radiators - I check them all as a matter of course when I turn the heating back on at the end of the summer. There was a bit of air in a couple but nothing significant.

I could just change the pumps but the local supplier says these pumps are about 400 euro each. I am reluctant to lay out the cash just to check.

 

[Myfordman]

Presumably the pumps have isolating valves? close these either side of the suspect pump. remove the 4 socket cap screws (5mm allen key IIRC) which should release the motor and impeller. You can then check that the motor/impeller starts and is nice a clean and turns freely. Also an opportunity to check the pump body for sludge.

If perchance the motor is free turning and yet starts only after giving a spin by hand, then suspect the run capacitor in the connection box. It's 3uF approx from memory.

 

[Just4Fun]

Presumably the pumps have isolating valves? close these either side of the suspect pump. remove the 4 socket cap screws (5mm allen key IIRC) which should release the motor and impeller. You can then check that the motor/impeller starts and is nice a clean and turns freely. Also an opportunity to check the pump body for sludge.

That is a good idea. I think I will try that when (or to be optimistic, IF) the fault repeats. As it is currently working OK I will leave it alone. The last failure was Thursday night so another is maybe due soon.

 

[Spectric]

Having two different pumps in series does seem odd even if they are just circulators. The best place normally for your pump is in the return to the boiler because it runs cooler and also if after the mag filter is also protected.

 

[Richard_C]

I wonder if ... if the pumps are accessible, have isolators and are the same size between flanges, could you buy one pump, change that which you suspect is the problem, if that doesn't solve it then swap the one you took out (which should be 'good' if nothing changed', with the other one ?

 

[Thingybob]

Quick test to see if pump is running put a large flat head screw driver on the pump any where and put your ear on the end of the handle of the screwdriver you will hear the pump running

 

[Fergie 307]

Quick test to see if pump is running put a large flat head screw driver on the pump any where and put your ear on the end of the handle of the screwdriver you will hear the pump running

Good call, was going to suggest the same but you beat me to it

 

[Just4Fun]

Quick test to see if pump is running put a large flat head screw driver on the pump any where and put your ear on the end of the handle of the screwdriver you will hear the pump running

Ah, I should have thought of that. Good idea.

 

[pe2dave]

Unless the two pumps (in series) are truly identical (unlikely?) surely you'd get them fighting one another? Thermal trip as one overheats? Any other protection mechanisms in the system? Just a (non plumber) thought.

 

[andy48]

1. If the pumps are hot but the water is not I'd suspect faulty capacitor(s). With this problem the rotor won't start, and the pump acts like a coil and gets hot.
2. If you are comfortable working with mains voltage electricity and have appropriate experience, try
2.1 Disconnect the pump wiring to the heating system.
2.2 Wiring each in turn to a mains plug and see if it spins.
2.3 If you can feel a mains hum from the winding but no turning it will be:
2.3a. The capacitor faulty.
2.3b. The pump blocked (see points in previous replies).
3. If the capacitor is faulty, try and find a MAINS rated motor capacitor of around 8 microfarads which will physically fit inside the case.

 

[Spectric]

Have you thought of just having one single new pump fitted that would be more energy efficient, two in series just does not make sense.

https://www.screwfix.com/p/grundfos-alpha-1-15-50-60-domestic-heating-circulator-230v/483fj
I fitted a Grunfoss Alpha and it only uses a maximum of 7 watts, only issue with these so called smart pumps is your bypass because the pressure relief type just don't work because as the load increases the pump backs off. One solution is a Taco setter bypass valve but for maximum efficiency I use a three way Y plan valve.

 

[Just4Fun]

Unless the two pumps (in series) are truly identical (unlikely?) surely you'd get them fighting one another? Thermal trip as one overheats? Any other protection mechanisms in the system? Just a (non plumber) thought.

That is an interesting theory. Unfortunately I don't know enough to comment on whether it makes sense. Certainly the 2 pumps are not the same. The only thing is that the heating has worked this way for 15 years so something else must have changed to trigger the current problem.

1. If the pumps are hot but the water is not I'd suspect faulty capacitor(s). With this problem the rotor won't start, and the pump acts like a coil and gets hot.

Thanks for the suggestion. However, if the capacitors only have a role when the pumps start I think I can rule out the capacitors because:
A) The pump outside of the heat pump is powered from a normal electrical socket and runs continuously. It doesn't stop & re-start so the capacitor should not matter. Thingybob's screwdriver test suggests that pump is working.
B) My earlier tests confirmed that the pump inside the heatpump cabinet is rotating, so that pump's capacitor must also be OK.
Does my logic look reasonable?

 

[Just4Fun]

Have you thought of just having one single new pump fitted that would be more energy efficient, two in series just does not make sense.

https://www.screwfix.com/p/grundfos-alpha-1-15-50-60-domestic-heating-circulator-230v/483fj

hmmm ... no, I have not really considered that. Indeed, until now I had not thought about the 2 pump arrangement in any depth. It is another interesting thought though. I guess I need to consider a few things here.

1. Why do we have 2 pumps?
   I think this is just a historical accident. There was an existing pump in place when the GSHP was fitted and that pump was simply retained, adding to the pump within the GSHP cabinet.
   
   
2. Why was the original pump retained? Why was it not removed?
   Could have been an oversight. Or it could have been because of the size of the house and the head required, given that the GSHP is at floor level in the basement, suggested the GSHP's pump might struggle. I cannot tell. I could test it by unplugging the original pump.
   
   
3. Would a Grundfos Alpha pump be a suitable replacement?
   As I understand it these pumps are primarily designed to cope with different flow rates as zone valves or thermostatic radiator valves restrict circulation. That doesn't happen with our system. There is only 1 zone and there are no thermostatic valves. The load is always the same. Would there be any advantage to fitting such a pump?
   
   
4. Would a Grundfos Alpha pump play nicely with the GSHP controller?
   I really have no clue.

 

[pe2dave]

(next question if one pump used).
Capacity of each pump? n l/s + m l/s = ....
I'm OK with batteries, but pumps? Is the upstream pump arguing with the downstream one? I can't see that they are simply additive? Or are they?
Anyone?

 

[Just4Fun]

Or it could have been because of the size of the house and the head required, given that the GSHP is at floor level in the basement, suggested the GSHP's pump might struggle.

Thinking more about it, this makes no sense. Before the GSHP was fitted there was an oil burner and one circulation pump. That pump was able to circulate water around the whole house. The GSHP added a second pump, which I am sure is more powerful than the old pump and so must also be capable of circulating water around the whole house. There was therefore no need to supplement the GSHP's pump by retaining the original pump.