Faster recovery coil

[Sparks]

My hot water cylinder seems to take ages to reheat, can I change to coil for a better one, or do I need to get another cylinder?

I am afraid I have no idea what make or model it is (but it is covered in what looks like sprayed on insulation)

I just measured the temperature of the in, out and the brass immersion plug at the top with a fairly cold cylinder...

In 67C Out 65C Brass immersion plug at top 31C

Now I am no expert in these things, but I would have expected a lot more than 2C difference with a cylinder temperature of 31C!!

It is a fully pumped system with a three way valve (Readings taken with an IR Thermometer, with just the HW on).

We have an ion exchange softener, so scale should not be an issue!

Any suggestions, and thanks for any hints!

Sparks...

[Doctor Drivel]

Yes, rapid recovery coil cylinders are available. Don't buy Part L, they are not rapid recovery. Rapid recovery are named Ultra Cal, Super Duty, etc.

If your mains pressure and flow is good, then consider a heat bank cylinder while you are at it. This will give powerful showers without a pump and very rapid reheats and eliminate the tanks in the loft. It can also go in the loft and free up the airing cupboard.

[Tim S]

No. The coil's big and it goes in during manufacturing in all the cylinders I've seen (which isn't many)

Hmm.

Might have a label, unless it fell off.

Quick back of envelope calc...

Let's say 0.2l/sec = 0.2kg/sec x 4200 J/kg * 2C = about 1.6kW

Seems rather low = but then given I've guessed the flow rate hideously - 0.5l/s = 4kW which is better.

High flow rate though the coil? Imparts more heating for less temp differential.

How long is ages? All the heating systems I've had take about 30 minutes to get the top strata of the water hot and a lot longer to heat the whole tank, like maybe an hour.

Well, if the water is hot to the tank, and the differential is small either the flow is OK or the coil's scaled.

Has it been there as long as the HW cylinder?

Could you confirm "ages" and the size of your cylinder (width and height will do).

Tim

[David Hansen]

On Sun, 26 Feb 2006 00:25:45 -0000 someone who may be "Sparks" wrote this:-

What do you think "ages" is? 60 minutes, 30 minutes?

If possible at all it would be very difficult.

Far easier. Get one with a high recovery coil, which has much more surface area.

They all tend to be much of a muchness. Measure the height and diameter, roughly, then consult a supplier to see what is available in this size. You may also want to consider a larger cylinder, if you draw off a lot of hot water.

[Andy Hall]

It isn't economically practical to change it since the coil is installed during manufacture.

This suggests a fairly poor surface area. I would have said scaling, except that your comment about the water softener eliminates that (assuming it's working).

Is the boiler a conventional one or condensing? If it's conventional and the flow at the boiler is also 67 degrees then it is way too low. With heating running and a lower return temperature, you may well get condensation in the boiler which is acid and will corrode it. Anything below 55 degrees return and than will be happening. If it's a conventional boiler, then the thermostat needs to be turned up.

You could drain the system and have a look through the immersion heater plug point. This would confirm absence of scaling. It would also show that you probably have a rather peewilly coil.

Take a look at Albion fast recovery cylinders. These are better than standard part L1 cylinders, although those are better than the one you have.

[mike]

"Sparks" wrote in news:4400f5a1$0$1169$ [e-mail]:

You can get quick recovery cylinders, like the Albion Superduty that I've got.

I can't reproduce your measurements just now, as I can't make my HW fire without messing with the stat, and after the time it took me to get it right, I'm not touching it for anyone!

But ISTM your temperature observation is correct, but it could also be caused by your circulation being fast, so there's not much temperature drop.However that shouldn't make temperature recovery slow.

"Seems to take ages" is a bit subjective for the hardnosed heating gurus round here, they would probably like to hear about the size of your cylinder in litres, or at least it's external dimensions, and how long it tkaes to recover after drawing a bath, or some such thing.

I replaced my old tank a couple of years back with a superduty, and am very pleased with it. (Basically, all it is is four small pipes in the coil instead of one thick one).

mike

[Andy Hall]

That's not quite right.

Having the flow rate too fast does not of itself mean a small temperature drop.

Remember that heat transfer is proportional to the mass of water flowing and temperature drop.

However......

If you halve the flow rate through the coil, you will increase the temperature drop. You might double it, for example. This does not increase the heat transfer because you have also halved the mass of water flowing through the coil and hence the amount of heat being transferred per unit time. In effect, there is no change.

In practice, it's probably not quite linear like this, but the principle is there to a first approximation.

The issue is the poor rate of transfer through the coil. This is probably because of low surface area (could have been scaling were it not for the softener).

As you say, the Albion has hugely more surface area. In that case, the heat transfer is much improved. So for a given flow rate, you will get more temperature drop.

[dennis]

Why is the input temp so low? Its about 80C on mine. Having a higher temp will speed it up by a bit.

[Doctor Drivel]

It is?

You got that right.

[Dave Liquorice]

The use of an IR thermometer raises a small query flag on the readings to me. See fairly recent discussions about IR thermometers.

[Sparks]

Yes, it may have had one, but I cant se any trace of it!

Well, the water was at about 32C and an hour later there wasn't enough for a bath (and the bath is a normal size, not a swimming pool!)

2 hours later it was hot enough, so somewhere between 1 and 2 hours to heat enough for the bath it seems.

Not this one, but we had a timer model before, this used loads more salt, so I expect it was regenerating far too often.

The size is 53cms around and 98cms high (This is with the insulation, which is about 1.5cms thick

[Tim S]

Ow. That is way too slow.

The IR thermometer may be lying?

About 170 litres.

I'd agree that the feed from the boiler is on the low side at 67C - can you turn the boiler stat up a bit, or try the high setting if it's a switch? Or is it that being cold, that the boiler is having trouble keeping up with the demand from the rads, assuming the system tries to heat rads and HW in parallel?

Might be an idea to buy a pipe thermometer and double check the readings - they are useful for balancing radiators too.

How fast to heat a cold cylinder if you shut the rads off, so all the boiler's output goes to the cylinder?

I've never had this problem even with fairly ancient cylinders (ie not fancy pants fast recovery). The slowest system I've ever had was an antique primatic system (boiler primary fills from HW tank) - and that wasn't as slow as yours.

If everything draws a blank, after getting some more heat out of the boiler, I would consider draining the cylinder and having the immersion heater (or blanking plug) out and have a good look at the coil.

If SWMBO exists, wait till spring, unless you want your knees to develop extra degrees of freedon ;->

Tim

[Sparks]

This was with the CH off (Three way valve, fully pumped system)

The boiler thermostat was at mark 5 if 1,2,3,4,5,MAX

It is a Potterton Profile 80e This boiler was installed, probably 10+ years ago, and has never been serviced :-) Is that bad!? I replaced the PCB a couple of years ago, as it died, but that is all that has been done to it.

Maybe this is part of the problem (although the CH seems fine to me!)

Where would you suggest is a good place to get a couple of these? Screwfix doesnt seem to have them!

Thanks for the help :-)

Sparks...

[Tim S]

Not good.

Hmm.

I'm a bit suspicious of the boiler now. It could really do with a routine service after that long - at least the burner will get cleaned out, the emissions checked and maybe a few other things. Shouldn't cost the earth and well worth it for the peace of mind of a) it possibly being prevented from breaking down at an inconvenient cold moment; b) worse things.

This:

this:

a local plumber's merchant.

HTH

Tim

[Sparks]

Okay, seems a good idea really, I will get a corgie in :-)

Sparks...

[Tim S]

Ask him to check the boiler stat too - it seems to me that if it's set to 5, it really ought to be producing water that is exceeding hot (nearer 80C I would have thought). Probably best to mention the PCB being replaced - might have a bearing, or might not - but if so, saves him wasting time to find it out by himself, then charging you extra!

You could take a temperature reading of the primary circuit outflow pipe near the boiler, just in case something funky is going on further up the pipework near the HW cylinder. If the boiler output is obviously *much* hotter than the pipe feeding the HW cylinder coil, it could be there is a flow restriction somewhere (3 port valve for example) - but it doesn't really concur with only 2 degrees drop across the heating coil - low flow would be expected to show up as a much larger differential.

My money's on the boiler not producing enough heat or the boiler stat - but it's hard to be very certain without being there - the best intentioned but theoretical diagnosis is often riddled with oversights.

The CORGI bloke is the right thing to do - you won't be wasting your money as the boiler needs servicing from time to time anyway (see the thread a couple of weeks ago about the boiler with the yellow flame and soot).

Suggest to get him to check the thermocouple too - they cost very little to change while he's in there and I'm surprised yours has lasted so long - our old Potterton (25 years ago mind) ate thermocouples regularly.

Good luck.

Let us know what it really was - it's of interest and useful for the google archive :)

Cheers

Tim