CH boiler backfeed problem

[Catweazel]

Several years ago I had a wall-mounted boiler fitted to replace an old floor mounted one. The subsequent re-organisation of the plumbing has caused a number of problems since, including the production of 'interesting' sounds from the CH system at various points in the heating cycle.

A couple of years ago I noticed, when preparing to drain and flush the system, some clumps of nasty brown growth floating in the CH header tank. Thuis has been observed to re-appear at regular intervals. This phenomena I think is the result of the configuration of the pipe- work, in that, at the end of the heating cycle, when the boiler and pump switch off, I've detected hot water going up the cold-water feed into the header tank, rather than via the expansion/ overflow pipe. I have recently felt this [hot] backfeed by putting my hand in the header tank over the outlet to the boiler. I'm thinking that this has to do with the reduced 'rise' of the expansion/ overflow pipe coupled with a more powerful convection- upflow from the boiler.

It occurred to me that I might use a one-way valve device, which I discovered recently is known as a 'check valve' thus taking away the easier route for the hot water backfeed and hopefully reduce the amount of hot water getting back into the tank, although I think some may still get in via the overflow pipe].

My questions are these: [1] How likely is it that fitting a check valve be successful in stopping the hot water back-feeding up the inlet pipe, and [2] What's the best orientation to fit check-valve, horizontal or vertical ? [there's space for either and I suspect the vertical orientation will work best].

TIA

[John Stumbles]

Convection involves a circulation which isn't occurring in this case since there isn't a return path. It sounds as if it's expansion of the system water that's occurring, and if you put a check valve in there then it's liable to go up by the vent pipe instead.

A normal spring check valve also requires a significant head to overcome the resistance of the spring so your system is likely not to fill properly.

I suggest you clean the muck out of the header tank, disinfect it with bleach or maybe Jeye's fluid and refill it adding maybe an inhibitor with a biocide.

[Andrew Gabriel]

Hang on a mo... That tank is there in order to accomodate expansion of the water in the heating system as it warms up. You therefore expect that warm water will enter the tank from the bottom in normal operation.

Is the tank covered? Could the brown growth be the remains of a bird or squirrel or mouse? It sounds like the system might be in need of a drain down and flush through, and refill with inhibitor.

[David Hansen]

On Wed, 5 Dec 2007 01:42:34 -0800 (PST) someone who may be Catweazel wrote this:-

You don't have a cold water feed pipe. What you do have is a feed & expansion pipe. This allows water to flow into and out of the heating system when necessary. When the heating is turned on and the water in the system expands it flows up the feed & expansion pipe back into the tank. When the heating is turned off water flows back down the f&e pipe as the water in the system contracts. This is dependent on the water temperature, if it really only happens when the pump is turned off then there is a problem with the pipework design.

You also have a vent pipe. This is there as a safety feature, if the f&e pipe is blocked then this will allow the water to expand/contract without causing an explosion or collapse.

Don't be a fool.

Probably 100%. What then happens depends on how lucky you are. If you are lucky you may get away with it for a while. If you are unlucky some part of the heating system will explode, probably a hot water cylinder.