Single Rad - cold at bottom, hot at top

[XrayDave]

As the title says, I have one radiator where the bottom is cold (or at best mildly warm) whilst the top is hot (too hot to touch).

We recently had a can of inhibitor injected into the CH filling loop.

I'm gussing that this is a build-up of sludge?

Given that the rad is small, I can easily isolate it, drain the rad, flush with a hose & re-install.

Rather not do that if sludge is not the problem but frommmy rudimentary knowledge of CH systems, that's what I think it is.

Cheers and Happy Christmas to all.

Dave

 

[CBW]

It could be sludge, it could also be poor circulation to that radiator from a kinked pipe, valve issue, etc.

 

[XrayDave]

Hi Chris,

Worked fine for the 24+ years it has been there until this October when the CH was started up after a 6-month summer lay-off.

There are two rads in the bathroom and the one opposite gets toasty hot. In fairness, the rad I'm talking about eventually gets hot at the bottom but it takes a long while.

All other rads on the same floor get toasty so I'm thinking that the circulation is okay - and similarly for a valve as both would affect more than one rad?

Dave

 

[JohnD]

are any of your rads fully hot, all over, all the time? Or get hot very quickly? Especially any that are newly fitted, or near the boiler, or have been removed or turned off and on again?

Turn them off with the knob on the valve at the end

Does the cool one now heat up?

Do you know the principle of radiator balancing?

Is the cool one a recent addition?

 

[DP]

In a nutshell, go and feel both inlet and outlet pipes at each radiator and compare overall where the difference of temperature between inlet and outlet is slight (excessive flow through that radiator ) or too great ( insufficient flow). Or too great a temperature difference, open the lockshield, for slight temperature difference, close the lockshield valve.

Carry out a check after the system has settled down to any adjustments.

CHeck applies to hot water cylinder too if it is installed.

Feeling the top of the radiator is pointless

 

[XrayDave]

Hi John,

To answer each of your questions one by one.

are any of your rads fully hot, all over, all the time?

Nope!

Or get hot very quickly?

No again

Especially any that are newly fitted

Last new rad approx 24yrs ago

or near the boiler

Boiler is 2 floors below

or have been removed or turned off and on again?

Nope!

Turn them off with the knob on the valve at the end

Which? There are two radiators in the bathroom (one of which is cold at the bottom but eventually warms, the other warms all over and quickly), and one radiator in each of the three bedrooms on the same floor.

Do you mean to turn off all rads on that floor except the 'problematic' one? Do you mean to turn them off on the flow side or the return (lockshield) side?

None of the radiators have TRVs by the way.

Does the cool one now heat up?

Do you know the principle of radiator balancing?

In short, no. How does one go about this should it be needed? How does one know that the system needs the radiators balancing?

Is the cool one a recent addition?

No, but it was the last one to have been added to the system around 24 years ago!

This radiator worked fine until the 6-month lay-off earlier this year due to the mild spell from late April onwards.

This is why I suspect a build-up of sludge in this radiator.

I hope these answer your questions.

In a nutshell, go and feel both inlet and outlet pipes at each radiator and compare overall where the difference of temperature between inlet and outlet is slight (excessive flow through that radiator ) or too great ( insufficient flow).

Will do.

Or too great a temperature difference, open the lockshield, for slight temperature difference, close the lockshield valve.

Thanks for this.

XRD

 

[JohnD]

the principle of balancing is to get hot water flowing through all the rads at about the same rate, so they all heat up at about the same speed.

As you might not have guessed, this is not done by opening the cold ones, but by closing down the hot ones.

As you have no radiators that heat up particularly quickly, you (probabably) have a general, rather than a particular, problem, of the hot water taking the easiest path and neglecting the longer, tighter or more arduous path.

In that case, you can experimentally, turn off all the hot ones, and it is very likely that your cool one will heat up very fast*

Assuming that each of your rads has one manual valve with a knob on it, and one where the spindle has a plastic cover ("shield") locked into place to prevent it being altered (the "lockshield valve"), after you have tried the experiment, and assuming it works, you can continue the experiment by opening up each of the manual valves by a quarter turn (yes, that's all)

then go and put the kettle on, make some tea, sit down with a biscuit.

After half an hour, go and feel all your radiators, both ends. Are they all about equally hot, with the pipe at one end being noticably hotter than the other? If not, you can open up the cooler ones by another one-eighth of a turn, or close the hotter ones by one-eight of a turn. This is because almost all the flow adjustment on the valves is when they are almost closed, so al adjustments mut be very fine, and must be left long enough for the water to circulate thoroughly before making another adjustment. So allow another half-hour before making another one-eighth adjustment.

Assuming you do not have pipe thermometers, the hotter (flow) pipe should be "too hot to hold" and the cooler (return) pipe should be "too hot to hold for long." This is near enough.

Go and make some more tea, and check again after another half hour.

If* balancing does the trick, you can go on to do a permanent job on the lockshield valves, screwing their caps down afterwards to prevent disturbance to the settings. The reason I suggest starting by closing the knobs on the manual valves is that it is quicker and easier to test my hypothesis that you have a balancing problem, and it will avoid throwing out all the previous adjustments.

*if not, you may have a different cause to your problem

If all your radiators are cold at the bottom, then you probably have a general poor flow, perhaps a failing pump, or sediment blockage, or an open bypass. But balancing is a cheap and easy DIY job, so test that first.

edit
I see now that @DP has made a shorter reply about balancing.

 

[XrayDave]

Thanks John

Dave

 

[BlueLoo]

Treat the symptoms not the cause.

This rad has worked fine for decades previously.

Remove it and flush it out as you say.
A cold bottom is sludge.
You may be surprised at the amount of crud that comes out.
You should look to do all the rads if you do one.
A simple plan may be:
Drain the system down,
Clean the rads out,
Clean the head tank out (if you have one)
Put cleaner in for a few weeks,
drain once more,
Fill and drain again to flush out cleaner,
Fill again,
Add inhibitor.

Like servicing a car, one thing oft leads to another so do a review of the whole system, check for leaks, make sure all valves, etc work fine.

This should cure the illness, not the symptoms.

You can then do a rebalancing (or tuning the hydraulic profile) if you want, knowing that you are doing this to a clean system and not simply pushing flow around the system to mitigate local poor flow caused from corrosion.

 

[Madrab]

If it's small and easy to remove, with the symptoms you describe and for all the time it would take, if it was me I'd isolate, drain, take it out and flush it with a hose.

When you bleed the radiators(s), what colour is the system water?

If it's still behaving the same after that then time to look for other faults.