Hot Water Does Not Heat Up Unless Central Heating Is On

[dwarfer16]

I'll try and be as succinct as possible but apologies in advance for all the text! Here is the order of events:

1. In the beginning…whenever our water was being heated up the radiators always came on a bit. I assumed this was normal.

2. In June, our boiler started to not heat water all the time resulting in the overheating light switching on. I would turn the boiler off and then on again to clear the light after which it would work for a day or two and then it would happen again.

3. I called a plumber who thought that it was a problem with our pump. He said he could replace the pump or do a power flush through the system as he thought the cause could be gunk in the pipes. We opted for the power flush only. (He also said that this pump configuration was unusual and not used anymore).

4. After the power flush the overheating light never come on so I presume this problem was fixed. The plumber said that lots of gunk came out the system (he showed me) and thought it possibly hadn't ever been done (the boiler has been in the house for ~18 years). He was confused that the radiators were heating up when the water was on as he said this shouldn't happen. I said it has always happened to a degree.

5. After a couple of days I noticed the radiators were much hotter now when the water was on. They were at the same temperature as when the central heating was on now even though it was off. I called the plumber. He said that there must be some kind of piping issue where the pipes have been connected incorrectly. I asked him for a quote to fix but never heard back. In the meantime, on his advice. I turned off all the radiators values which solve the issue in the short term.

6. Fast forward to this Monday. My wife tells me the pipes were making lots of banging noises in the night when the water should have been heating up. There is also no hot water. Since then I have experimented a couple of times with turning the water on and when I do the boiler does fire up but turns off after a short time as, presumably, it isn’t getting any water. However, if I turn the central heating on I DO get hot water! I have tested this the last two evenings and this does indeed seem to be the case.

Strange: That originally the radiators would get hot when the water was heating up but now I only get water when the radiators are on.

Cause: I am thinking this must be a pump issue. When I turn the hot water on it is as though there isn’t any water to pump but I guess there must be for the radiators to work. The pump in use is a UPP 15/50 Pump Plan (http://www.plumbcenter.co.uk/en/archive/heating-archive/pumps-archive/grundfos-upp15-50-pump-plan-152613). I have spent the evening trying to figure out the central heating/plumbing system in general as it is the one part of the house that I have never fully understood. Please excuse my laughable drawing attachment. The items inside the green square represent stuff I cannot trace but is how I assume it to be.

Water/Central Heating System Diagram

• dwarfer16
• 8 Oct 2014
• 1

So I guess my questions are:

1. Could the pump be faulty for the hot water but not for the central heating?
2. Surely it can’t be faulty if it still pumps water when the central heating is on?
3. Could it be air in the system?
4. Is it possible for radiators to come on even hotter after a power flush when the water is on or did the plumber change/set/do something incorrectly on the pump?
5. Is this pump really unusual or did the plumber just not know about it?
6. I find it tough to believe that the pipework is incorrect. It seems much more likely to me that there is an issue with the pump….?

Any and all advice is appreciated. Thanks in advance for reading this mass of text. Craig.

 

[picasso]

I reckon the hot water pump isin't working and the check valve is stuck open.

 

[dwarfer16]

I reckon the hot water pump isn't working and the check valve is stuck open.

Can you elaborate at all on where I would find the check valve? Is this something in the pump? I searched the manual for that pump but couldn't see anything relating to a check valve.

Thanks.

 

[kbdiy]

The banging in the pipes suggests to me that it is more likely you have an airlock which your plumber may well have introduced as a result of not refilling correctly after the powerflush. The lack of circulation to the cylinder for hot water would also indicate an airlock in that part of the circuit.

Ideally photographs would help to understand the relationship between pump/pipework/cylinder.

PS - ignore everything to the right of the cylinder in your diagram, it has no bearing on the problem you have.

 

[kbdiy]

I searched the manual for that pump but couldn't see anything relating to a check valve.

Thanks.

The manual does refer to a non-return valve in the pump - same as a check valve in this instance.

 

[cross thread]

from the look of your drawing you might have a grundfos twin pump set up ?
get a good eng in who should easily diagnose fault and advise on updating system to stop heating of your rads with hot water only on

 

[stardanny]

It would be better to replace twin pump setup with single pump and 3 port mid motorised valve and can be done with little pipe change and a change in control wiring.

Daniel.

 

[dwarfer16]

The banging in the pipes suggests to me that it is more likely you have an airlock which your plumber may well have introduced as a result of not refilling correctly after the powerflush. The lack of circulation to the cylinder for hot water would also indicate an airlock in that part of the circuit.

Ideally photographs would help to understand the relationship between pump/pipework/cylinder.

PS - ignore everything to the right of the cylinder in your diagram, it has no bearing on the problem you have.

Thanks for the info. I have taken a photo.

Photo of pump

• dwarfer16
• 9 Oct 2014
• 1

I have had a further thought. The radiators used to come on when the water was heating but now the water only comes on when the central heating is switched on. Before this problem started I reckon that it didn't matter whether the water or the CH was on, either way both were heating up. I just never noticed the water being on with the CH because why would I unless I felt the pipe going to the cylinder. Not sure whether this makes a difference.

I don't understand how this could happen unless the water was being passed through both pumps and exiting via both output pipes even when only one pump was on. Is this possible?

Thanks.

 

[dwarfer16]

from the look of your drawing you might have a grundfos twin pump set up ?
get a good eng in who should easily diagnose fault and advise on updating system to stop heating of your rads with hot water only on

That's exactly the pump that is installed. The previous plumber seemed to suggest that this type of pump was uncommon. He had to look up the instructions for it and thought the problem could have been the pump.

 

[picasso]

Chances are the hot water pump has packed up, your getting hot water because the ch pump is pushing the water round the rad circuit through the cylinder return and back through the stuck check valves, the check valves are a well known problem on the pump plan, you can either replace the nrvs in the outlet of the pumps, cut new nrvs in the pipework in front of the pumps or as already said change for a three port, you will need to replace the hot water pump head anyway.

 

[dwarfer16]

It would be better to replace twin pump setup with single pump and 3 port mid motorised valve and can be done with little pipe change and a change in control wiring.

Daniel.

Okay so as suggested it seems like the Hot Water pump is knackered. My next question is what should it be replaced with? As suggested above should I opt for a different pump setup or change it for same pump model? The benefit of using the same pump is I think I could give it a go myself as the pipework would not need to change. Although this pump seems hard to come by. There is a second hand one on eBay for £120 + PP (http://www.ebay.co.uk/itm/GRUNDFOS-...arden_Hearing_Cooling_Air&hash=item4add962541).

Is there some negative to using this type of pump (as suggested by the plumber originally) and therefore would a 'single pump and 3 port mid motorised valve' be better?

Thanks all. Craig.

 

[kbdiy]

May still be available new here it seems http://www.pumpsalesdirect.co.uk/gr...g-primary-hot-water-pump-circulator-230v.html
Shows as out of stock but no harm in phoning to see if they can supply?

 

[Agile]

I actually like the pump plan and feel it should certainly be retained!

As I recall there is a non return valve in each outlet.

They are pretty good and unlikely to jam under normal conditions but if the system was very dirty its possible they have become jammed somehow.

I am not convinced that its a stuck non return valve but its always possible.

They can be inspected of course.

Tony

 

[dwarfer16]

May still be available new here it seems http://www.pumpsalesdirect.co.uk/gr...g-primary-hot-water-pump-circulator-230v.html
Shows as out of stock but no harm in phoning to see if they can supply?

Thanks, I have emailed them but unfortunately they don't stock it anymore because of this: http://www.allaboutpumps.co.uk/new-european-union-legislation/

Given this, perhaps it isn't the best idea to replace like for like?

 

[Agile]

I would never suggest replacing the whole pump assembly and particularly when there is no direct evidence that there is any fault in it.

Its easy to see if both pump heads are turning. See FAQ.

Its still possible to buy that type of pump head and replace just that.

If so then it needs removing so the non return valves may be checked.

Tony