2 Rads not hot. Flow, return & bonus 3rd pipe?

[WhydidIstart]

Hi, I have 2 double rads that struggle to get more than ‘tepid’ when the heating is on. Boiler is a Vaillant ecotec plus 831, I think it’s probably undersized but these two rads don’t get hot even when most of the other rads in the house are turned off. Boiler flow temp set to 75C.

Lockshield valves were already fully open, I tried replacing them and the TRVs, but that didn’t make any difference at all.

The rest of the house has copper piping for the heating, except these two rads which are done in 15mm plastic (I think they were done with the kitchen...). I had put it down to the slightly smaller bore of plastic pipe and it not being enough to get good flow to the two double rads, however I decided to remove a cabinet and panel and investigate further, see pics below.

I was expecting flow and return, but there’s a bonus 3rd pipe sticking out of the concrete floor! In the pic with the plastic pipes the top pipe is flow, the very bottom pipe is gas and the one above that is return. Then there’s a surprise bonus 3rd pipe that the return of the two double rads is connected to.

Does anyone have any idea why there would be 3 pipes like this? Do you think moving the plastic return to the lower pipe would sort out the two tepid rads?? Any advice very welcome!

 

[RobTi]

The top plastic connection in the last pic is definitely connected to the flow any chance of a couple of pics from the side showing the connections behind the filter (Adey magnaclean)

 

[WhydidIstart]

Hi Rob,

Thanks for your message. Pics behind the filter added.

Return pipes from upstairs and downstairs join, then go into the filter, then out the filter and into the boiler, with a tee off to the expansion tank and gauge.

I'd expect the plastic pipes to conenct between the upper flow and lower return (excluding the gas pipe on the floor). So what's the bonus 3rd pipe in the middle and why might the plastic pipe be conencted there? It must go somewhere as the rads get slightly warm

 

[heatservice]

I would not think your boiler is undersized with a heating output up to 24Kw (110,000 btu's) if all the other are getting hot, have you tried it with all rads turn off except one of the problem ones, if it then gets hot you will have proved theres not a blockage, do the same thing with the other problem rad, if both work on their own then it may be a case of balancing your system as water will always take the route of least resistance unless made to do otherwise, start with turning you upstairs lockshields off then open them just a 1/4 turn, you can do the same to other rads that work well, opening them a fraction more if it looks like their not getting enough circulation.

 

[copea]

How many radiators is that boiler supplying?

Regardless, I'd be tempted to move the return to the pipe you can trace back to the boiler return, and see if that improves matters.
At least that way the return route is a known entity. It's also the most cost and time effective method at this point.
The pipe currently connected could be crushed/restricted under the floor and that would be the easiest way to ascertain whether or not that's the case.

 

[Nige F]

The current connected pipe has nowhere to vent to and could trap air. So go with copea

 

[WhydidIstart]

Hi, thank you for the replies.

There are 20 radiators, mostly double and when they are all on the boiler struggles to keep them all hot and some rooms struggle to get above 17C in winter. Solid walls don’t help.

heatservice – I tried turning off the other rads and the two problem ones get warmer than before, but still no where near hot. 75C ish water is making it to the entrance into these rads, but the flow must be really low as the exit temp is pretty much cold. So could be a blockage, but the water in the system is clear and having seen this weird 3rd pipe setup I’m wondering what the third pipe does and if that’s the problem!?

I did notice something interesting though, with all rads off (they all have TRVs), the flow and return pipes disappearing into the floor get hot. I double checked and all rads were cold, so there must be some kind of bypass under the floor somewhere. Would that have been common with an older heating system?

copea – That makes sense, I assume I’d just cap off the random third pipe?
Having discovered there is apparently a bypass between flow and return under the floor somewhere, do you think it’s worth locating it and adjusting it down, or off completely, would that be ok? I believe this ecotec plus has a built in pressure activated bypass just in case there was demand but no flow. Would it be more efficient to use the built in one rather than have this underfloor one, which given the age of the system is probably just a half open valve – I’m thinking it’s just circulating hot water under the floor and raising the return temp unnecessarily?

 

[D_Hailsham]

I did notice something interesting though, with all rads off (they all have TRVs), the flow and return pipes disappearing into the floor get hot. I double checked and all rads were cold, so there must be some kind of bypass under the floor somewhere.

Could they be the connections to a towel rail? If not, then further investigation is required as heat is being lost.