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Discuss Boiler Greenstar 30CDi return pipe cool but display says 70 - 80 degrees. in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

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6
Vented system, Worcester Bosch 30CDi, NEST control.
Radiators not getting hot, or takes hours.
Wanted to ask if below is a good indicator of a technical problem, rather than looking for blockages etc etc.

The boiler is cycling from 45 degrees, climbing quickly (a degree a second) to 50 where it alternates between the temp & H -¦-. Gets to 80 degree's & stops firing, quickly drops to 45 degree's, fires again & repeats all in the space of a few minutes, mostly allowing for cool down.
The return pipe is luke warm, even when the display says 80 degrees.
On the most base level, Is this incorrect?
I assume this is meant to be the return temp or is the display something else.

If not normal behavier (i.e. temp & pipe not similar) either the Boiler has developed a fault or as below...
.. 2 years ago had NEST installed to replace standard stat. Was a "NEST engineer" but I think had no idea as was messing with wiring inside the boiler! I said to him sure he only needed a relative simple rewire & did not need the case off, I had read the instructions. In the end the boiler was going bonkers. He went away & sent a friend who I assumed fixed but ever since not great but to be honest is a warm house and was in the summer so come winter & not great but just OK.

Revisiting now & just think not correct.
The flow out is super hot & the pump is working, just changed thinking it might have been that but no better.

HW gets hot but pipe close and I image flow & return helps.

3 port value works fine, moves to select the correct flow direction.

Does this cycling on HW only, CH only & both.
Most obvious on CH only as flow pipe v.hot & return tepid.
With HW on I think as pipe close natural return does get bit hotter.

I imagine could do with better balancing, checking for trapped air & even power flush but just think this is wrong.

Could there there be another explanation for high temp reading & cold return.
If there were a blockage on flow would the boiler show high temp, typing this I guess there would be some fail safe to stop overheating but is that the display.

I will get an engineer but trying to work out myself in advance.
 
Messages
6
You’ve changed the pump ?
Yes, I now think the old one was OK but put a new Grundfos UPS3 15/15/65 (replaced a UPS2)
What I am trying to work out is if the behaviour could be normal.
The basic question of, the return pipe being cool but the boiler saying 80.
My thinking is
a) if the rads were taking all the heat the boiler would keep running.
b) if there was a blockage, yes the return pipe would be cool but would the boiler say 80 degrees?
I wondered about this, if there was a blockage I guess there is an over heat failsafe but is that linked to the actual read out out a different temp sensor & just throws up an error I am assuming.

I did read that the cycling I detail before, with the rapid increase in temp, "is not a over heat as such" but a warning that the return is getting hot to quick.
 

ShaunCorbs

Staff member
S. Mod
Plumber
Gas Engineer
Messages
26,076
No it’s not normal

flow pipe should be hot sounds like you don’t have any flow pump valves open ? Any gate valves on the system?
 
Messages
6
No it’s not normal

flow pipe should be hot sounds like you don’t have any flow pump valves open ? Any gate valves on the system?
As before
.. The flow out is super hot & the pump is working, just changed thinking it might have been that but no better.
HW gets hot but pipe close and I image flow & return helps.
3 port value works fine, moves to select the correct flow direction.

it is just the return that is cool, does get hot when just HW is on but that I think is just the fact it is close to the tank.

Don't worry I don't think I can explain the problem well enough.
Should the temperate display on the boiler be directly related to the return pipe temp when actually touching the pipe, just before it goes back in to the boiler. because it is not.
 

ShaunCorbs

Staff member
S. Mod
Plumber
Gas Engineer
Messages
26,076
The return will be cool if the rads are doing there job eg heating the rooms, the return will only get hot when the rooms are upto temp the temperature on the boiler is the flow temperature

so I guess it’s all working
 
D

David Goodwin

As before
.. The flow out is super hot & the pump is working, just changed thinking it might have been that but no better.
HW gets hot but pipe close and I image flow & return helps.
3 port value works fine, moves to select the correct flow direction.

it is just the return that is cool, does get hot when just HW is on but that I think is just the fact it is close to the tank.

Don't worry I don't think I can explain the problem well enough.
Should the temperate display on the boiler be directly related to the return pipe temp when actually touching the pipe, just before it goes back in to the boiler. because it is not.

The display on the boiler is showing the flow temperature, not the return, which is fairly unhelpful. Ideally it should show both. I have recently had exactly the same problem but replacing the pump seems to have fixed mine.
 
D

David Goodwin

I have recently had exactly the same problem on the same boiler. The temp on the boiler shows the flow temperature, which is not that helpful. Ideally it would show both so you can see if it is condensing. I replaced the pump and that seems to have fixed mine but I am now struggling to get the radiators anything like balanced.
 

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