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Two radiators not getting hot

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Hi,

I have a sealed system with 18 Rads/towel rads which is run by a greenstar boiler with a sealed hot water tank. I'm using a tado thermostat and the Pump is a 15/50 Groundfos.

The issue is there are two radiators that are cold.

So I closed down every radiator except for these two, apart from the two towel radiators which are connected to the hot water loop, and this got the radiators nice and hot quite quickly. But when you slowly add the other radiators back into the system they have gone cold again. I then presumed that there must be air in the system as there was no air to bleed from the rads. To remedy this I attached a drain cock to the offending rads, closed down the rest of the rads and closed off the trv side and opened the return lockshield. letting water out through the draincock there was some air, I then closed the Lockshield side and repeated the process on the trv side, once again releasing some air from the system. Once the heat was back on once again the rads were very hot and once again when the other rads came on the offending rads became cold. Following on from that the next phase was to go through and balance the rads with a thermometer, this also hasn't had the desired effect, as the rads are still cold. I can normally get some heat, nowhere near as much heat as the other rads, by turning off some of the bigger rads completely around the house, but this does seem counter productive in heating the whole house!
I'm just about to bleed the rads again, just in case there is air in the system, as the boiler man emptied a rad to add inhibitor when he did the service. But the problem was there prior to his servicing the boiler as was all the work I had done.

Any thoughts or ideas would be gratefully received, especially as the rads in question are in the MIL room and she just whacks on the electric fire and unsurprisingly moans about it alot!!
 
Pump is too small for 18 rads / Getting weaker
 
does it heat if you shut the lounge one and I’m guessing there off the same drop ?
 
I have shut the off one of the big rdiators in the conservatory before, which i think is on the same drop, and they get hot.
Would this confirm that the pump needs to be bigger?

The water is taking the path of least resistance you could try balancing the ones on the same leg as the problem two but I highly suspect the pump
 
The water is taking the path of least resistance you could try balancing the ones on the same leg as the problem two but I highly suspect the pump
if the other rads are already balanced will the bigger pump help overcome the water taking the path of least resistance?
I have checked on the grundfos site and the pump they suggest as a direct replacement for mine is the pump that John mentioned earlier. Surely that would give the same output as my existing pump and therefore the issue continues?
 
You may need a 8M pump but more than likely that a UPS3, a very powerful 6.5M pump which will still pump 1.7m3/hr at a 5M head should be quite adequate IMO.

I have checked with the grundfos replace app and it has highlighted the ups3 as a direct replacement for my existing grundfos 15-50x18. if the outputs are matched wouldn't I still have the same problem?
 
Yes a new bigger pump would sort it out

Your is a 5m head eg can pump 5 m up
The ups3 can pump 6.5m up

Easiest way to think of it

If yours is going it could be pumping 3 m etc and slowly getting less and less
 
I have checked with the grundfos replace app and it has highlighted the ups3 as a direct replacement for my existing grundfos 15-50x18. if the outputs are matched wouldn't I still have the same problem?
If you post a close up photo of your pump then might be able to get the pump curves but as pointed out above it will only pump the same volumes of water at a much lower head than the UPS3 so far less need for balancing/tweeking lockshield valves if UPS3 installed.
The UPS3 is a direct replacement but you would have to choose different pump curves to give the lower head that your pump is pumping presumably on speed 3?.
 
If you post a close up photo of your pump then might be able to get the pump curves but as pointed out above it will only pump the same volumes of water at a much lower head than the UPS3 so far less need for balancing/tweeking lockshield valves if UPS3 installed.
The UPS3 is a direct replacement but you would have to choose different pump curves to give the lower head that your pump is pumping presumably on speed 3?.
This is the Pump I have at the moment and you're right it is on speed 3
 

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You have done well Kevin, that pump was built in week 34 2000 I think so 23 years, not bad but only a minnow compared to the UPS3., immediately below, which will pump 1.7m3/hr at a 5M head if required, the selectric will only pump that at a 3.3M head.

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You have done well Kevin, that pump was built in week 34 2000 I think so 23 years, not bad but only a minnow compared to the UPS3., immediately below, which will pump 1.7m3/hr at a 5M head if required, the selectric will only pump that at a 3.3M head.

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that could well explain things. the house was actually built in 2001 as a 3 bed with a lounge and kitchen. I purchased it in its current state, 5 bed and 4 reception (includes a big conservatory with two big rads) in 2015. Lots of additions, but no update for the pump.
Looks like I will be replacing the pump and trying that.
thanks for the help, if it doesn't cure the problem don't know what I'm going to do!
Here's hoping
 
You will be lucky if the O rings dont leak on the ball valve spindles. You might find it easier as well to just cut across the securing nuts ( at the two slots) with a junior hacksaw to remove them, if the O rings don't leak you can just buy replacement nuts, they have two slots like in the photo and you can slip them on over the valve flanges, also use rubber gaskets.
 

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Update:

Fitted New ups3 pump, as predicted the valves leaked through the o rings, but luckily I had purchased new ones as that had been expected. All fitted and working perfectly. Considering the house is now twice the size it was when the original pump was fitted in 2001, the old pump hadn't done bad. If only I had asked the question 18 months ago, I could have saved my ears from the MIL :)

Thanks, once again for your help.

Kev
 
It is set to that, max output, CC3 at 6.5M, by default but you may find that constant pressure CP2 at 4.5M or CC2 at 5.2M will be adequate, its only a few button presses.
 

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