Replacing Indirect Hot Cylinder in a Semi-Gravity System

[BraileTrail]

In another thread

It was established in another thread that I have a leaking heat exchanger coil in my hot cylinder. The system is a semi-gravity system more than 25 years old.

I will at sometime in the future need to replace the aging boiler and with that in mind, if I replace the cylinder myself, I would like to replace it with a good quality fast recovery model with double thickness insulation, etc.

Most of the specification sheets I have read say that this type of cylinder should only be used in a fully pumped system. Is this just to get the best performance out of the cylinder, or is there some real technical reason they wont work in a gravity hot water system? Will installing one in a gravity system damage anything?

If I can't fit this type of cylinder now, then I may as well consider changing the boiler and converting to fully pumped as well.

Thanks, BraileTrail

[YAPH]

No, but the coil will be designed to give a lot of heat exchanger area for fast recovery at the expense of higher thermal resistance than a coil deigned for gravity circulation, so you're likely to get even worse re-heating than the original unless you do convert to pumped.

You could just keep the cylinder, blank off the coil connections, buy a PHE and bits and and use it as a DIY heatbank (see wiki).

[Andrew Gabriel]

My _guess_ is the flow resistance through the coil might be too high. Instead of one 22mm or 28mm coil, they increase the coil surface area by having something like 4 microbore coils. I'm guessing these will have higher flow resistance. No damage, but heat input might be significantly reduced due to low flow. Given that a gravity cylinder is effectively 'on' for ages during the winter, this may not matter in practice, unless you try having many baths or showers in quick succession. If you found this to be the case, and it really didn't work, you could always add a pump to the circuit.

[Doctor Drivel]

AS you are to replace the boiler soon, replacing the cylinder and updating controls, maybe false economy. Consider a high flow quality combi boiler. Look at the Remeha Avantaplus 39C, get the outside weather compensator senor. You will not look back. And lots of space liberated.

[Michael Chare]

And if you are not satisfied with the performance could you not convert your present system to being fully pumped?

[Tim Downie]

And you can join the ranks of those like my brother who spent three weeks with no heating or hot water earier in December thank to the lack of cylinder & immersion heater.

Tim

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[Andy Champ]

This has just happened to a guy at work. Currently living in one room with a fan heater and borrowing friend's showers.

Andy

[Doctor Drivel]

A cylinder does not provide DHW. You can have a small back-up instant electric water heater to give DHW to a tap and shower - cost is around £100. Fine until the combi is back up and running.

If you want full electric CH and DHW backup you go thermal store/heat bank.

Or maybe fit a £300 B&Q cheapo combi as back only, which I have done a few times. Use the same gas pipe and a throw over electric switch so both combis cannot be on at the same time. Very cost effective and still saves space.

[Doctor Drivel]

He could have a small back-up instant electric water heater, the outlet of the combi runs through it when it is off. Fine until the combi is back up and running. He has heat via the fan heater, which is probably fine untill the combi is back up.

[Roger Mills]

It does on the planet I live on! And if it has an immersion heater for emergency use, it *continues* to provide DHW even if the boiler isn't working.

[Doctor Drivel]

Typo should have been CH. Now read again.

[Roger Mills]

No-one was suggesting that a cylinder provided CH - but simply that with a stored HW system you don't have all your eggs in one basket, and you can still have hot water even if there's no CH. The same is *not* true if you have a combi.

[Doctor Drivel]

I know that.

What you are on about is backup if the boiler is down. That can be had cheaply with a combi by installing a small in-line instant water heater that takes little space. I clearly explained this.

In installation costs it will be cheaper having a combi and an in-line electric water heater, than a boiler/cylinder/zone valves/controls/immersion heater/tank in the loft. For heating backup a small, cheap fan heater may be OK for a day or so. Second hand you can pick them up used for a few pounds.

If you are obsessed with having a cylinder ina normal house for some perverse reason, then go for a thermal store/heat bank which gives 100% CH & DHW backup and high pressure DHW.

[Pete Verdon]

Probably slightly more civilised than the outdoor shower I rigged up here while rebuilding the bathroom. The old mouldy shower tray, and a wrap-around of blue poly-tarp suspended from the wall of the house.

Meant to be for a week, ended up being for a month...

Mind you, the actual shower itself was better than before, due to the extra head of being downstairs :-)

Pete

[Tim Downie]

Yeah but, how many plumbers advise their clients about the lack of back up or install any sort of back up system whilst they're ripping out an old cylinder system? Damn few I'll bet. I've never met a combi owner with any sort of back up.

Tim

[Doctor Drivel]

Plumbers are good for drains.

It is easy to do. I just run the bathroom basin and shower through the in-line heater. A kitchen always has a kettle and with dishwashers and washing machines the kitchen is covered.

As I said I have done few with a super cheap combi as backup. Not the sort as the main combi which will be a quality model, but OK to use once every 7 years for a few days, to give whole house backup, with reduced DHW as it will be a low kW model. Just throw a switch and it works. Just an extra £300 or so. the cheapest you can get. Price up a quality combi and a B&Q cheapo using the same gas supply pipe to cut installation costs, to a cylinder/boiler/tank/etc, and it still comes out cheaper and still little space used. Little extra pipe is used if fitted next to the main combi.

Why some combi makers do not have a small electric backup heater inside is beyond me. It must be a good selling point. Gledhill's thermal stores had one if the boiler went down, that automatically came in.

[Doctor Drivel]

The same cost wiring up an immersion. The heater will be 7 to 9kW so not that great of an expense. I have used existing immersion cables as the cylinder was ripped out leaving a large space to put a very small box on the wall. The cables were man enough.

You could fit a small 15 litre 2kW storage water heater using an existing

2.5mm cable. Heated to 60C, or more, it will give a ~5 minutes shower at 5 litres/min. OK for backup. Gets you wet and with hot water.

Fitting in a small instant in-line water heater locally at the kitchen tap permanently switched on, not for backup, can eliminate the cold dead leg delay. Once the cheaper to heat water, heated by gas, comes in from a combi or cylinder the electric heater switches out. This can be cheaper than running a secondary circulation pump for only a remote tap(s), such as a kitchen. If on a water meter it cuts down the water bills as well, so maybe well worth it.

[YAPH]

I'm not - I don't have pressure jetting equipment and CCTV cameras etc. - and I don't suppose many other plumbers do either, any more than a pure plumber has the expertise and tools to do heating.

...

because a small electric backup heater in a combi would be about as much use as a chocolate teapot. For instantaneous water heating you need a large - as large as you can manage - electric heater: 8.5 kW minimum - and that's not going to run off the boiler's 3A fused spur, is it? It's not even going to run off the local ring main, as Gledhill's 3kW immersion heater and the immersions in a bazillion and one gravity hot water systems can do. If the boiler installation is next door to the CU then it's not too much of a problem, but if (to take a wildly improbable and contrived scenario :-|) the CU is in the cupboard under the stairs and the boiler is at the back of a back addition in a terraced house, then you're looking at running maybe 10 or 15m of at least 6mm^2 - possibly

10mm^2 - cable to it. Do-able, but not trivial, and not a trivial extra cost for the day or two per year when, probably, on average, you're likely to need it.

[YAPH]

Well, yes: one sees electric showers (same current demand as what we're talking about) hooked up on 2.5 or 4mm^2 cable, but for a competent, professional installation rather than DIY (in the pejorative sense of the word) one needs to run properly-specified adequately-sized cable.

[ARWadsworth]

Prolly best to fit two combis then:-) Just in case.

Adam