Air gathering in bathroom towel rail

[CaseyJones1]

I know, yet another one!

My parents have an Ideal Mexico RS60 boiler (about 10 years old, fitted by BG) working a 30-year old pumped C/H system with gravity hot water,
in a four-bed detached house. When the boiler was replaced, the rest of the system was unchanged.

It all works OK, except for air gathering in the bathroom towel rail, about 1 'bar' per week. (It is air, not hydrogen.) This towel rail replaced the original bathroom radiator about 2 years ago. Before that, 1 or 2 upstairs radiators required bleeding about 2 to 3 times per year, but they are now OK, and it's just air in gathering in the bathroom towel rail (it is now by far the tallest).
Otherwise, the system is original, and has been flushed out and re-Fernoxed about every 5 years. (My father is obsessed with this.)

Testing found that when the C/H pump is started, there is a surge up both the feed and expansion pipe and the vent pipe at the same time. (Ripples could be seen in the header tank water, and air was pushed out of the vent pipe but no water came out.) The header tank only has a few inches of water in it, so the surge was easily seen.
The feed and expansion pipe and the vent pipe are as original and connected as per Ideal's instructions, on the gravity hot water circuit return and flow respectively. They are not on a pumped circuit.

The C/H pump is connected on the return pipe just before it enters the boiler. (However, Ideal show the pump in the C/H flow pipe just after leaving the boiler.) How it was connected before the boiler was replaced is not remembered.

The C/H header tank is about 500 to 600mm higher than the top of the bathroom radiator.

Could this air be being drawn in via 'microleaks' as the pump is 'pulling' the water round the C/H, or could it be connected to the 'surge', causing oxygenated water to be drawn in every time the pump starts? It appears as if the pump is causing an increase in pressure in the boiler chamber thus pushing water up both the gravity hot water pipes.
If the pump was on the C/H flow, would it have the opposite effect and suck water down the feed and expansion and vent pipes?

A professional plumber came, and suggested a combined feed and expansion/vent pipe might sort out the problem, but didn't seem too keen on doing anything regarding the pump.

Any (polite) suggestions please?

 

[g20]

bathroom towel rails are generally the place where the air accumalates as its the highest place in the crcuit, however if you are having a major air problem (you shouldnt really have to bleed it more than 2-3 times a year) then it could be the position of the pump, What speed is the pump on (1,2 or 3)

there are kits you can buy though to swap a conventional boilers/systems in to sealed system boilers, these normally solve all air problems, however i'd recommend getting a plumber to do it.

 

[CaseyJones1]

Pump setting is no.2 (no.1 is too slow, rads take too long to heat up, and no.3 is far too noisy)

Question is not why air is collecting where it is, but why it's building up this much in the first place, or where it's getting in.
As I said, apart from the newer boiler and towel rail, all is as originally installed, but the pump position does not seem right somehow. I know it's where they used to fit them (cooler part of system) but why the surge? Surely that's not normal?

 

[GPlumb]

Not sure why air is getting in, in the first place but an automatic air vent could be installed on the bathroom radiator to get rid of the air.

Not fixing the problem but treating the symptoms.

http://www.heatandplumb.com/acatalog/copy_of_Aladdin_Autovent__Vertical_Radiators_.html

 

[Watchmaker]

An amateurs comment but, hopefully, worth considering. If the primary circuit has been bled, and no air remains in that circuit, then any further air in the circuit must have replaced some "lost" water. Is there a small leak anywhere, ie, behind the boiler which evaporates and then is not noticed or, is there a leak, under the floorboards, where the 30 year old pipework feeds the lower radiators? Obviously, the tank valve should open to make up any loss in the circuit but, is the feed tank ball valve in good condition or,is the tank level set far too low, (those few inches which you mention)allowing the pump to overtake the re-fill rate and draw air into the system? Is the feed tank ball valve sticking at certain times or, is the outlet, from the tank, restricted by debris etc ? Anyway, good luck and, hopefully, one of the professionals can identify the proble,

 

[g20]

more likely that when the pump comes on its sucking air down the open expansion pipe and the airs gathering in the towel rail

 

[twgas]

could be gassing, due to corrossion and lack of inhibitor.

 

[CaseyJones1]

could be gassing, due to corrossion and lack of inhibitor.

I did say it has Fernox, and has been re-done over the years. And I said it was air, not hydrogen.

 

[CaseyJones1]

If the primary circuit has been bled, and no air remains in that circuit, then any further air in the circuit must have replaced some "lost" water. Is there a small leak anywhere, ie, behind the boiler which evaporates and then is not noticed or, is there a leak, under the floorboards, where the 30 year old pipework feeds the lower radiators?

No apparent leaks anywhere, and it's all very clean. Unfortunately, the house has solid floors downstairs, so checking for a leak 'under the floorboards' is not an easy job! Upstairs there shows no signs of leakage, at least, the downstairs ceilings show no signs.

Obviously, the tank valve should open to make up any loss in the circuit but, is the feed tank ball valve in good condition or,is the tank level set far too low, (those few inches which you mention)allowing the pump to overtake the re-fill rate and draw air into the system? Is the feed tank ball valve sticking at certain times or, is the outlet, from the tank, restricted by debris etc ?

Header tank valve was replaced for good measure recently whilst I was doing the testing and checks. Don't know quite what you mean by 're-fill rate', as the tank has water about 1 inch above the feed pipe to system, and never drops to the level of the feed pipe (I watched it for a long time, very boring, and it rises on expansion of the water, as you'd expect).
The C/H and hot water system are working fine apart from the air collecting in the towel rail. I understand having the header tank too full is not recommended. The outlet from the tank must be fine, otherwise the system would not have filled from the tank in the first place.Also, there would surely not be the surge up both pipes if they were blocked.

 

[twgas]

could be gassing, due to corrossion and lack of inhibitor.

I did say it has Fernox, and has been re-done over the years. And I said it was air, not hydrogen.

pardon me!

everyone says air - most DIYers don't know about hydrogen