Struggling to lower flow return temperature

NannaH

My Vaillant ecotech pro 28kw is currently set to 60° Flow temperature,  on checking the boiler diagnostics,  the flow return is showing 55°,  so it would seem that the return temp is far too high and it won’t be condensing, although the condensate pump runs regularly,  so there’s clearly fluid entering the pump. 
The rads are all balanced.  8 rads on the system.
No radiators are off,  living room trvs on two large rads set to around 19°, one dot below number 4,  only our bedroom has a trv upstairs, currently on max as I have the thermostat in there,  trying to get upstairs within 1-1.5° of our living room,  it’s currently 3° cooler.  

The boiler pump is set to auto.  

My husband is gas safe,  so can tinker with the boiler but he isn’t a ‘heating installer’   He just services and fixes boilers.  

He’s going to fit a much bigger rad in the bathroom next year so that should ease the flow return a bit.
Any tips please? 

chris_n

The rads may be balanced in that they all heat up ok but the flow rate is too high so not enough heat is released from the water resulting in the high return temp. Turn all of the lockshield valves down quarter of a turn and see what difference that makes. If necessary repeat.

Ectophile

55C isn't far too high, it's at the top end of OK for condensing.

NannaH

I’ve just found the lockshield valves on the two largest radiators, downstairs,  3000mm x 500 and 1600mm x 400 fully open.
Daft twit must have done it when he drained our system for a chemical flush a few weeks back 🙄
Need to check the hall one but no trv so I don’t know which end is which till the heating kicks in again.  

lohr500

If closing down the lockshield valves as per @chris_n suggestion doesn't help, is it worth slowing down the boiler circulation pump as well?

I wonder if your boiler is oversized for the number of radiators and the water is flowing around the system so fast that the radiators aren't able to dissipate heat quickly enough. Closing down the lockshield valves should slow down the flow, but a slower pump speed could help as well.

Do you have access to a reasonable quality infrared thermometer so you can check what the inlet and outlet temperatures are on your radiators?

NannaH

I could get one and put it through the books as an expense 😉
Neighbour is a commercial plumber,  if he’s around tomorrow, I’ll see if he’s got one. 

Husband is going on a Vaillant diagnostic course in the new year so he can get advice on the pump from the horse’s mouth .  As it’s set to auto he assumes it adjusts itself accordingly and he has not long fitted an extra radiator in the conservatory, it’s fairly small though. 

It’s a good an excuse as any to get a huge fancy new bathroom radiator,  especially as he gets trade discount now .  My Daughter has one and it really belts out the heat, her main bathroom is slightly bigger than ours and it’s always warm in there, whereas ours is quite cold since we replaced our old system boiler.  

grn99

I think the pump speed is the thing that needs slowing down first if it can be, rather than too fast a flow and then restricted further with lockshields. Just been redoing my system here and spotted only a 10 deg C loss on the return. I reduced  pump speed and now seeing 20 C delta on the return at the boiler. One can get cheap digital thermometers that are amazingly accurate for what is needed and make the job 10 x easier...one therm on each of the flow and return pipes and another two to measure flow and return for each radiator I was checking/balancing.

chris_n

The OP has already found lockshields wide open, no need to change the pump speed. Get the lockshields set right to balance the return temperatures 12 degrees lower than flow across all radiators with TRV's wide open. Then set TRV's to desired settings and leave the pump to do its thing on Auto. That's what it is designed for.

lohr500

With the pump on auto, does it or the boiler control monitor the return temperature and adjust the speed to suit? 
The pump on my oil fired system isn't that smart. It has several settings, but none of them allow me to enter any temperature settings.

chris_n

lohr500 said:

With the pump on auto, does it or the boiler control monitor the return temperature and adjust the speed to suit? 
The pump on my oil fired system isn't that smart. It has several settings, but none of them allow me to enter any temperature settings.

The pump contains a pressure sensor and keeps the pressure constant. When a TRV closes the pump slows down, when it opens the pump speed up. With all of the lockshields open there is little backpressure so the pump pumps at max all of the time keeping return temp too high.

grn99

chris_n said:

The OP has already found lockshields wide open, no need to change the pump speed. Get the lockshields set right to balance the return temperatures 12 degrees lower than flow across all radiators with TRV's wide open. Then set TRV's to desired settings and leave the pump to do its thing on Auto. That's what it is designed for.

Agree re. lockshields not set being set first but the auto pump speed still runs based on the what the installer chose in the diagnostics menu when it is commissioned and if a low loss header is installed it needs a fixed speed.