Replacing an Indirect Hot Water Cylinder

[Edward W. Thompson]

Have just found a drip from the bottom of my hot water cylinder so I guess its time to replace it! I need some advice.

The cylinder is probably 15 years old and is lagged with expanded polystyrene, I guess. It is an indirect cylinder with an immersion heater. Looking at the catalogues it appears an identical cylinder is available with the correct configuration of inlets and outlets. So far straight forward to replace ;-).

With respect to replacement. There is a shut off c*ck for the cold water in and a gate valve on the CH boiler hot inlet to the coil (think it is the inlet). There is no isolation on the other side of the coil (outlet).

My question is do I need to drain down the CH system to replace? What crossed my mind is I could put a cork into the outlet from the CH header tank in the loft that should stop drainage from the outlet side of the coil when I disconnected, at least in theory. Any thoughts on this?

With respect to the immersion heater. Do you think I will be able to get it out of the old cylinder bearing in mind it has been there 15 years or so? I know I will need to buy/rent a spanner but are these things in to stay once they are this old? Further, do you think it sensible to replace the immersion heater anyway as I guess they don't last forever?

All guidance will be much appreciated.

[Ian Stirling]

I'd certainly first try to see where it's leaking from - and if it's a place where pipes go through, to give the relevant nut another 1/6 turn or so. If that diddn't cure it, back the nut off, put in some PTFE, clean the surface a bit, and tighten it back up. They rarely - though not never - leak through the wall of the cylinder.

[Doctor Drivel]

When replacing a cylinder go for a quick recovery m,model. Part L is 'not' quick recovery. They go by names of Super Duty Ultra Cal and the likes.

Quick recovery takes all the boilers output heat up very fast and are cheaper to run. You existing boiler may not take full advantage of the coil as the coil may be capable of taking a boiler much larger, but when you change the boiler, in time, it certainly will show even more dividends.

[Andy Hall]

Notwithstanding Ian's comments, which do make sense, I've put a few comments below if you do need to or decide to replace......

Well yes, but will always take longer than you think. Nowadays, they are better insulated so do check external dimensions if the cylinder is a tight fit in the airing cupboard.

There is a technique where you can put bungs in the vent pipe and the feed pipe from the roof tank. Normally this is for radiators swaps etc. I am sceptical that you would get away with it for this because the pipe is 22mm normally and you are likely to get water running out more easily. You might be able to remove the pipe quickly at the cylinder and bung that end as well, but I'd suggest draining down and not having the hassle

If it's the original, then I would dump it along with the cylinder. New ones are also safer since they have an extra safety cutout to reduce the risk of boiling if the thermostat fails.

[Andrew Gabriel]

Don't do this unless you have a condensing boiler, as it will force a non-condensing boiler into condensing mode, and rapidly destroy it.

Also probably not a smart move if you have a mid-position valve, as you might find the cylinder steals too much of the boiler output for the heating to continue working, unless your boiler has an output power well in excess of the house heating alone (which is unlikely as the system was designed to heat a hot water cylinder).

Quick recovery cylinder is one part of a complete heating system design. You can't drop one in without knowing the rest of the system was designed to support it.

[Doctor Drivel]

Nope. You put in a gate valve in the coil return to the boiler and balance the coil to suit the boiler. When a boiler change you open up the gate.

Hence the gate valve in the return pipe. Must cost all of £2-3.

You could insert a quick acting Reliance UFH blending valve (TMV) on the coil flow and return. This ensures the boiler only pumps 80C heat into the cylinder coil and keeps the return temp above the dew-point, and heats up very quickly. Recommended for quick recovery coils and heating direct thermal stores/heat banks.

[Edward W. Thompson]

Thanks for the suggestion but the 'drip' is appearing at the front of the cylinder almost exactly 90° displaced from all pipe connections. While the leakage could migrate under the insulation I doubt whether it would migrate 90°. Further, having been 'tight' for 15 years it seems improbable that a pipe connection should begin to leak at this stage. Regrettably I think the cylinder itself must be leaking for one reason or another.

The complication of fitting a 'quick recovery' cylinder is simply not viable or economical for me. There are only two of us in a small cottage and our heating bills are hardly a major expenditure. Further, being in the geriatric category I simply can't be bothered to modify the pipe work, as will likely be required. Thanks for the suggestion.

The cold water supply to the cylinder is 22mm but the CH connections are smaller (18mm?). I can isolate the cold water but not both sides of the CH boiler connection to the heating coil. I know it makes most sense to drain down especially as I need to replace a few of the radiator thermostatic control valves. I suppose I better 'bite the bullet ad do the job properly.

Thanks for the advice re immersion heater. I suppose I can buy a cylinder complete with immersion heater and thermostat fitted. The present cylinder looks as though it was a fully assembled unit judging from how it is insulated.

Off to B&Q, Wickes and Plumbase this morning!

[Ian Stirling]

It's not impossible. Some sorts of insulation are slightly water permeable, and drips onto one bit can come out elsewhere. First thing I'd do is to give all the connections a little more turn. If it works, it's free.

Other approach is to simply dig out the insulation using a bit of wood, and get down to the bare cylinder. This'll take all of a minute, and will let you see directly if there is a pinhole leak.

[Richard Conway]

Agreed - my "it'll only take a few hours" cylinder swap a few months ago tunrned into a whole weekend - including the usual mad dash to B&Q on Sunday afternoon before it shuts for extra parts!

One other thing - are you absolutely sure that all the inlets and outlets are in identical places? We though they were, but when we got home it seemed that the top inlet for the coil was higher than on the old one - presumably because the coil was larger to comply with Part-L regs.

I agree - remember, you probably won't have to drain the whole system, just to the point below the cylinder.

Definately scrap it - I can't imagine it would be easy to get it out of the old cylinder anyway. We got a new one for less than £15 so hardly broke the bank!