LOFT CONVERSION done BUT 2 out of 3 loft rads not heating

[RJD68]

Hi again Ric2013. As the 2 offending/cold loft rads have NO air in them (only water comes out when I bleed them) I must assume the air lock is somewhere between the 1st rad on the loft circuit (as this has always worked) and the two others. The pic I posted earlier showing the 90degree elbows taking first floor (copper pipework) into plastic pipe that feeds the 3 loft rads. From the plastic elbows to the first rad (that working one) is about a 5m straight & level run. From this rad to the next rad (which is the first cold one but full of water) is another straight and level 3m run. I'm surprised how the air can always get trapped somewhere on this section and not say in the 90 elbow area. Will an air vent (auto or manual) in the only accessible area (below the 90degree elbows shown on pic) solve this if the air lock is elsewhere along the run ie. over 5m away? Can this air just come from elsewhere in the heating system and migrate up into loft and just happen to always want to 'settle' in this particular area of pipe run?

One other qu. that might be relevant to our loft airlock? We have a tall Bisque rad similar to this
TRB-180-30 - Trubi radiator range - General Radiators
in a first floor bathroom. It's 1800mm tall. It makes a waterfall/running water sound which I understand from Bisque tech people is quite common for these tall rads (so much so that they have a specific tech sheet they send to help customers bleed these style of rads). I can get the air out by way of this convoluted process each year but neverteheless it does eventually start the running water noise a few weeks after having been bled. So might this one bathroom rad on the first floor (which used to be the highest point in the system) be generating air just because of its design and it's this air that's now seeing a higher point to travel to ie. Our loft?

 

[RJD68]

Kop it seems we posted at the same time. We have a Vaillant EcoTec 637 combi boiler

 

[king of pipes]

You most definitely need a auto vent in the tall radiator you may also need to add a extra expansion vessel to the system you could possibly be loosing water when the boiler is heating disscuss this and the above with your installer , 15mmm pipework will only run 2 rads really you may need to sacrifice heat in other parts of the home to improve this hope this helps. regards kop

 

[Ric2013]

Hi RJD,
I agree with you that, if the horizontal run is genuinely horizontal and not dipping due to flex in plastic pipe, it is not that likely to airlock. The new radiators will not be creating air, nor will your bathroom towel rail, although air can sometimes get in through less than perfect joints, valves etc.

3 causes seem most likely (though there are other possibilities):
1. there is insufficient corrosion inhibitor which is creating a build up of hydrogen gas in the system
2. there are various air pockets in the system (plus the air that will always be in fresh water), and the air is migrating to the highest point, and there getting stuck (should be a short-term phenomenon, but can sometimes take months to complete)
3. there is an inherent design flaw in your system that admits air - until now this has settled in the bathroom radiator from where you have removed it, but it has now found another place where it prefers to settle

The point, though, is that it is not acceptable for the air to settle in places in which it gets stuck. If the flow to those rads all comes via those elbows, I wonder if bringing those elbows up half an inch (making an artificial high point) and replacing them with tees with an autovent on the top of the upstand thus created, would allow the air to settle and be removed from here, instead of running further along where it seems to be getting stuck? If it's stupid but it works...

I agree with what others are saying about 3 rads off 15mm pipe (how big are these rads and are they single/double/finned/unfinned?), but would suggest that, even if the pipe sizing were out, I'd expect to see some flow to the radiators served rather than a total blockage, and you said it has worked previously, so it really suggests air rather than lack of flow.

 

[RJD68]

Hi Ric. Installer here now trying to work it out. I agree that the issue is air. As far as the pipe work design is concerned there is an area where the 15mm loft pipework goes over a steel (despite me thinking earlier that we didn’t have that issue) but it’s more a rise and not an abrupt ‘U’ and besides it’s now inaccessible under a wooden floor!! However what’s interesting is that the loft rad that works is fed by a pipe run that comes up from the first floor and goes conventionally/horizontally under the loft floor. Tee’d off this run is the pipe work to the 2 rads (that don’t work) and this pipe run has to rise 2-3” to go over a steel.
So he started by closing down slightly the lock shield on the 1 good operating rad in loft (double finned approx 4000btu) and opened the lockshields on the other 2 in loft (one a towel rail 1800btu and the other a double finned rad 2500btu) and this created some flow and heat to these 2 previously cold rads and also seemed to shift a small amount of air!
So maybe some extra water flow is enough to dislodge the smallish amount of air....if this is indeed possible. (I’m not a plumber but my physics is sound).
However this has been done before and here we are again with the same problem so I am again questioning the design around this wretched steel beam.
From your post it might seem prudent before fitting air values etc and/or an extra CH that we try the easy method of removing air build ups as they occur over the next few weeks/months.

(he was questioning the Vaillant EcoTec pump output but I checked that it is a 6m head pump so man-enough for a loft I’d say.
He was suggesting that with insufficient pump pressure from our ground floor boiler that flow into the loft area might not be sufficient and that any small amount of air in the15mm pipework in the loft might just be enough to throttle the water flow).

When we get fed up with this and decide this hasn’t worked then we move on to the air valve &/or extra pump.
What’s your view of installing a greater capacity pump to ‘push’ the water up to and around the loft with greater efficiency and power?

 

[Ric2013]

From an energy efficiency/environmental point of view, the pump should be sized and set to provide the radiators with sufficient flow to have a temperature drop of approximately 10°C - 20°C from the flow pipe to the return pipe on each radiator. Radiators receiving more flow than this are conventionally throttled down to allow more pressure to be developed for the radiators receiving less flow (i.e. system balancing).

I don't know enough about boilers/gas to know whether changing an internal boiler pump for a different model is considered acceptable practice or not.

Based on my experience and training on pumps that are external to the boiler itself, running a higher head (pressure) pump or running the same pump at a higher speed than required for the above purpose purely to clear airlocks will mean that the temperature of the return water will be higher and this will make a condensing boiler less likely to run as efficiently as possible, and wastes electricity at the pump. It may solve your problem, but I would consider it to be a bodge to consider this as a final solution. On the other hand, as a temporary measure, it may be just the ticket, but bear in mind you'll probably have this problem every time your system is drained or worked on.

For comparison, a Grundfos Selectric UPS 15-60 (1980s-1990s model) is currently running at speed 2 in a house I have worked on. At speed 3 (i.e. max), it is a 6 m pump. The house is two halves of a semi knocked together and heats 13 medium to large radiators (Victorian building, so requires a fair bit of heat). I would be very surprised if the pump were the problem, unless your house is exceptionally large.

What temperature drop has your installer balanced your radiators to? If the drop isn't acceptably large with all TRVS fully open, I would think the pump is okay.

Your radiator sizes aren't so large that I'd really expect 15mm pipe run (even plastic) to be so restrictive as to prevent sufficient flow, and if the lockshields were partially shut down down during balancing, then this suggests that the pipe run and pump selection/setting is sufficient for the radiators served.

 

[RJD68]

I agree Ric, the pump is doing just fine and will only be changed as a last resort. The house bigger than average I’d say: kitchen/diner UFH (60m2 area) + 18 rads (which includes the 3 in loft) so well within the 37kw capacity of the Vaillant.
He’s balanced rads to approx a 12-15C drop. Today he closed all rads off then went through house opening each one, one at a time to try to isolate a rad or a part of the CH pipework that might have been producing air. An area was highlighted where there was definitely an air noise albeit not much...funnily enough it’s this area of the existing pipework that the loft pipework has been tee’d off from!
He managed to push this air up to loft then bleed it out from there so fingers crossed that’s it. If it turns out not to be then he now knows the area most likely producing the small amount of air and will return to fit an auto air valve in the run up to the loft plus extra inhibitor.
So my take on this issue is that in some cases it can just be a waiting game to see if it settles and also a bit of a detective game to work out if there’s an area that’s most likely producing air. We are closer to resolving this than we were this morning.
Really appreciate your views and time taken to contribute to this thread.

 

[Ric2013]

Big house, yes. Still, sounds like you're on track to a solution now.