Cold radiators - not sure why...

For the rad to get hot you hot water to flow in one side and out of the other, and also in enough volume to carry enough heat.

So the fact that you can bleed water from it quickly establishes that at least one side of the pipework is "open", but not both. So you need to check that valves on both sides are open.

The next thing is the system needs to be balances well enough to ensure flow through all the rads. They are connected across the flow and return in parallel, so if some rads have a much easier flow path than others - that is where the bulk of the flow will go.

So once you have checked the lockshield valves on the cold rads are open, you could could throttle down the hottest rads, and see if that causes the cold ones to start heating. If it does then you need to balance the system properly.

This sounds like you have a control wiring problem. With both heating and DHW off, the boiler should not be running at all.

It might be you have a rad connected as a bypass (since with S plan, both valves could be shut, and that would occlude the path of water through the system - say when the room stat is satisfied and ceases demand, the CH valve would close, but the boiler may run the pump for a bit on overrun to disperse the hot water out of the boiler and into the rads).

(some boilers have an internal sprung bypass valve)

If the system is very sludgy, it might cause localised flow problems - but that seems unlikely if it all used to work before the mods.

You could always do a main water flush:

Is this 22/15mm piping or microbore with 8/10mm pipe?

Airlock in the feed or return pipes?

Turn off all radiators except the one you are working on, turn the pump to full speed to force the water around the circuit to the problem radiator. If this works restore original pump speed and radiator settings.

Thanks John - this sounds logical. One other thing is that the radiator that's been replaced was isolated as it had rusted badly so maybe the new one, which is also cold, is out of balance with two others that were put in in the summer and are also now cold. They are in the next room.

There are 17 rads in the system so this could be a slow process...

It's a 2 year old Vaillant condensing boiler and was serviced a few weeks ago. The central heating motorised valve head had failed so that was replaced at the same time and I don't think it's either of the valves or wiring. The boiler staying on has coincided with the new rad last week so this balance issue may be a factor but maybe the plumber has disturbed the wiring.

That's beyond my DIY capability but good to know that we probably don't need an expensive flush.

I think these are 2 separate issues. The radiator which doesn't heat up sounds like a blockage or airlock. If you bleed it with both radiator valves open, you'll get water coming out as long as at least one side is working, so there could be an airlock or blockage on the other side. You need to bleed it first with one valve open and the other one closed, and then vice versa.

The always hot radiator sounds to me like a zone valve problem. I assume that the radiator in question is connected to the HW circuit rather than the CH circuit? If the HW zone valve is stuck open, it may be causing the boiler to fire and run the HW circuit even when there's no call for heat from the programmer and cylinder stat. Does the manual lever on the zone valve move freely when it's supposed to be off?

It's an oldish system so presume the former piping. I think an airlock is probably a good shout.

How would I adjust the pump speed - it's internal to the boiler isn't it (Vaillant Ecotec system/condensing boiler). Not sure this is a DIY job and there are 17 rads.

If it is pushing water around 17 radiators it's probably already running at faster speeds but possibly modulates down when encountering increased pressure such as when TVRs shut down (and possibly an air lock).

By shutting down radiators the water should be forced through the problem ones hopefully taking the air with it. This used to be the problem in my parents house - two radiators would not heat up after a drain down and just closing down the radiators that got hot forced the air in the pipes to move to the problem radiators.

The zone motorised valve(s) should have been put in the manual override open position when the system was refilled. Has the lever on the motorised valve for the water been put back to the auto position?

Although I've only had experience with 3 position motorised values on one that was fitted to my system the lever could be manually moved and latched in the open position (open to both A and b ports). The valve remained in this position until manually moved from this latched position. The replacement valve still has this latched open position for re-filling purposes but reverts automatically to the working position mode when CH is selected by the control electrics.

If the original poster has the former type where the lever has to be manually moved back for normal working then this may explain why the boiler is still firing - the water is always being heated and the boiler only goes off when the water temperature is >60C.

The following video shows how to latch a honeywell valve open and then release the latching.

ok, so checking both valves are open. Depending on what kind of lockshield valve you have you may need to take the cap off before you can see the bit that you can adjust - normally a brass shadt with a couple of flats on it, that you can grab with pliers or an adjustable spanner.

To do a full and complete job yup, but if it is a balance problem you should be able to get some results quickly.

Find the hottest rad, close its lockshield and then open half a turn.

Repeat for next hottest etc. You often need only small tweaks on the valve to affect the performance - say 1/4 turn at a time.

How is the heating controlled - Vaillant specific controls or standard programmer and room stat?

Well if it turns out that it needs a flush, and you don't want to do it yourself, then an "expensive flush" is probably your only option (since any heating installer will use a power flush as the go to option)

The ecotec has an internal sprung bypass valve (with a know to set the opening pressure)

The pump speed on the ecotec pro can be altered, but I don't think it can on the standard ecotec - but it depends on which revision it is. The default pump settings will be fine for the vast majority of installs (I never changed mine and it copes fine with 21 rads and a cylinder on 3 heating zones)

It is a sealed system, so air locks are less common IME. You could try bleeding a problem rad first with one valve shut, and then the other - so you pull water through both pipes.

But possibly a pump with proportional pressure control that adapts to how many radiators are open. When TVRs etc. close down the pump adapts which also reduces its power consumption.

Agreed, I've seen the air lock problem on an open vented system and a pressurised system should have less of these types of problem, but it shouldn't be ruled out.

The OP will have reduced the pressure significantly with the amount of water he is taking from the radiator bleed. Perhaps before bleeding he should be pressurising the system to perhaps 2 bar and then do as you suggest - and re-pressurising back to 2 bar every time before opening a bleed?

As per another post, perhaps the hot water motorised valve has been left in the manual latched open position (for refilling the sysyem). With water taking the path of least resistance it possibly favours the hot water path. Checking that the motorised valve lever hasn't been left in the latched open position for hot water should be the first check before attempting things like balancing/bleeding radiators.

Or it has just failed, and doesn't operate the microswitch any more, the boiler thinks it's needed all the time. Speaking from experience....

So the builder got the plumber back and he and I went round the entire system turning off lockshields and lo and behold the cold rads sprang into life and then opening up the closed ones a bit has brought the whole system into some kind of balance. So thanks for that.

Embarassed to say that the plumber had left the hot water valve latched open - I thought it wasn't but it was... Plumber must have had a memory bypass as he didn't put it back to auto. So that's solved too. Plumber in his 70s so is forgiven and he was very good at getting the system back up.

We've never had it properly flushed so will give this a go next year.

One more thing...

I think the plumber has turned up the flow temp on the boiler from 65 to 75. This has had a dramatic effect on the heating up time and indeed heating of this Victorian semi but I presume some efficiency of the boiler is lost (no condensing?) and gas consumption will go up a fair amount. But I'm going to leave it while it's this cold outside.

Embarassed to say that the plumber had left the hot water valve latched open - I thought it wasn't but it was... Plumber must have had a memory bypass as he didn't put it back to auto. So that's solved too. Plumber in his 70s so is forgiven and he was very good at getting the system back up.

All the motorised valves that I have used (we have 9 in this house) automatically release the manual latch if they are opened electrically. It would likely have closed by itself at some time.

Good, glad that is sorted...

If it has been properly maintained (inhibitor etc) then it may not need it. Normally it is done prior to installing a new boiler, or if you get problems with blocked flow or cold sections at the bottom of radiators.

Sometimes you will need a high flow temp to get the performance required. Very cold days in particular. The effects on efficiency will depend a bit on how it was balanced. For a condensing boiler you normally aim for a ~20 degree C drop between flow and return (compared with 11 for a conventional boiler), so even with the flow at 70, you will still be getting good condensing performance. There is a "knee" in the condensing efficiency graph that shows a steeper line below 54 deg C, but that does not mean no condensing happens above that - it is just the gain less.

If you have a weather compensation control (basically an automated way of turning the flow temp up and down depending on how cold it is

Have a look at the "Heating Curve" chart here:

The gotcha on my 3 way motorised valve is that to release the latched open position it's an electrical control to the heating position that releases it. Once latched open electrically selecting just the hot water leaves it in the latched open position. This fooled me the first time when I drained down the system in summer and didn't need the heating on after the refill.

The two on our system are made by Danfoss and Honeywell. They seem to stay locked open with the manual override although I've found they can spring back out of the latch if not seated well. Certainly the Danfoss stayed open even when the programmer was telling the motor to open and close.

We don't have weather compensation on our Baxi 105HE combi. I've recently installed a room thermostat, improved the insulation in the attic (bungalow) and turned off the radiator in the hall completely. I'm trying to learn how the new arrangement performs (11 radiators, one turned off as above, a couple on low) but with a flow temperature of 60 set on the boiler the lounge today has not reached target temperature Thermostat is set for 19 (0630-1030), 17.5 (1030-1630),

Despite failing to get there the burner still cycles. Approximately:

Pump is operating all the time.

Surely the call for heat should keep it running at 60? It's as if the boiler is saying "I'm hot enough now".

I can't help feel something is not working as it should. I know its cold but we are using something like 4cu ft /day for heating at a cost of £10/day.

I'm now going to up the temp a notch to see if we can take an overcoat off before the evening is out.

The boiler probably cannot modulate low enough to keep it running at

No, its working as I would expect given the previous postings you have made. In general, the way to reduce the flow temperature right down is to correctly size the boiler for the demands of your house heating and increase the radiator sizes by upto 3x.

What you have is a radiator system designed for perhaps a 75C flow temperature and a boiler oversized for your central heating needs.

The reason that the radiators are taking so long to heat the house is the lower flow temperature. Consider, if your CH heating was designed for an overall heat loss of 10Kwh on a day like we have had recently and the radiators have been selected for a flow temperature 75C - your radiators are now 10kWh@75C. If you now reduce the flow to 60C your radiators output less heat thus taking longer to heat the house or maybe not even matching the heat loss of the house.

You have to work with what you have and possibly small tweaks are all that's possible. You may be able to achieve 10% savings but definitely not 20/30/40%

I know its

Drawing curtains in front of DG windows can reduce heat loss. Reducing the temperature of rooms you don't use can reduce consumption BUT only if you keep the doors to those rooms shut.