Author Topic: Immersion heater replacement in sealed system - tips? (Read 1849 times)

TimC · « **on:** 05 December, 2020, 05:19:31 am »

My all-electric house has an unvented hot water system based around a Heatrae Sadia (Santon) PP170D dual-immersion cylinder. I've noticed over the past few months that the amount of hot water available has significantly reduced, and I suspect that the lower element has failed (if it was the top element, I assume that the amount of hot water would be fine, but it might never get to its design temperature in the Economy 7 heating period).

Of course, restrictions permitting, there will be three of us in the house over Xmas rather than just me, so the demand will increase somewhat - I've been coping fine with the reduced supply on my own.

I can source the element, and the associated thermostat (I'll replace both), but are there any things I should know before I attempt the replacement?

aidan.f · « **Reply #1 on:** 05 December, 2020, 07:29:03 am »

Find and read the manual? I think these devices have an air space to allow for thermal expansion and there are (strictly speaking) rules about who is qualified to work on them.

Diver300 · « **Reply #2 on:** 05 December, 2020, 08:06:55 am »

Obviously, isolate power before doing anything to the elements.

Firstly check the thermostats. They are in the dry, and can be taken out by removing the wires and pulling them out. They seem to be more expensive and complicated than the ones that I have seen. They seem to be like this:- https://www.dhsspares.co.uk/product/heatrae-95612719-control-assembly-stat-wire-ind.-1735847

You can check the thermostats in a pan of water with a thermometer and a multimeter. There should be no resistance between the terminals of the thermostat when cold, and they should go open circuit when hot. If a thermostat is open circuit, it may be that it got too hot and the thermal trip inside it tripped. I can be reset, but the fact that it tripped may mean the the thermostat isn't working very well and it may be easiest to change it.

While you have the connections to the elements available, measure the resistance. Each should be around 17 - 22 Ohms for a 3 kW, 230 V system. There should be no conductivity to ground from either end of an element.

If the electrical resistances are correct, it would usually mean that the element is OK. I would not change an element with the correct resistance unless I had some other reason to think that it was the wrong type or there was some other problem.

If you do change the element the instructions are here:- https://mediacdn.heatraesadia.com/-/media/themes/heatrae/literature/installation-manuals/megaflo-eco-installation-manual.pdf?v=1&d=20171113T161519Z on page 26

If you do change the element, I suggest that the first thing to do, before turning the water off, is to get an element spanner and loosen nut that holds element in place. When loosened, it will start to leak, but until you get it more than about 1 turn unscrewed, it will be a very slow leak. Just get the element to move a bit with the water pressure still in the tank. You only need a fraction of a turn. The reason for leaving the water pressure on is to make the tank as stiff as possible. The weight of water also stops the whole tank moving if you have to apply a lot of force. Once it's loose, turn off and drain the water.

TimC · « **Reply #3 on:** 05 December, 2020, 09:53:09 am »

Thanks Diver. Some good stuff there. I think I have everything necessary to follow your guidance. Fortunately the previous owners were very good at hanging on to the manuals and tools, so I have the installation paperwork and the element spanner. The stats I have are slightly different to the one you link to (and a bit cheaper!), but not significantly. I'll have a go at checking it in a pan! I can confirm it hasn't tripped its reset.

Aidan, the expansion vessel is a separate component. I had the delight of repressurising that yesterday (it's very easy once you have access). Owing to its location, it was quite a wet affair! Draining the cylinder if I have to replace the element is also likely to be a bit of a wet T-shirt event, and is probably the most daunting part of the whole process!

Edit: definitely the thermostat. New one sourced for £28. Hopefully I'll stay dry while I arrange proper water heating again!!

Feanor · « **Reply #4 on:** 05 December, 2020, 12:01:17 pm »

The thermostats on a Megaflow unit have 2 separate things:

The twiddly adjustable thermostat, and a second Safety Cut-out wired in series.
The Safety cut-out will sometimes just decide to activate, and can be re-set.

It's a non-obvious small red rectangular recessed push-button right next the twiddly adjuster.
On mine, the word 'Safety' is marked on the body of the unit, with a small arrow pointing to it.

Use a small tool to give it a push in to re-set it.

Worth a check before replacing it.

Oh, never mind. I see that point was covered already.

TimC · « **Reply #5 on:** 05 December, 2020, 01:02:32 pm »

Yes, it was the first thing I checked - and when I got it off the cylinder I made sure that I hadn't made a mistake. It's definitely not tripped. But thanks for the reminder! The new one should turn up on Tuesday, so we'll have to wait until then to see if I've got it right!

Diver300 · « **Reply #6 on:** 05 December, 2020, 01:13:01 pm »

A new thermostat is likely to be one of the cheapest and dryest options.

TimC · « **Reply #7 on:** 10 December, 2020, 07:58:05 am »

Well, that didn't work. The new thermostat is good. The element appears to be good (reads 20.6 ohms, exactly the same as the upper element). I'm aware that the two elements cycle so I don't get a 6Kw load, but with the top thermostat turned right down, the bottom turned right up, no power is going to the lower element. Indeed, there appears to be no supply upstream of the lower thermostat (the upper has a supply whether above or below the demanded temperature). So I'm suspecting the timer unit (a Horstmann Electronic 7 Maxistore). I'll check the mains wiring inside the box (with the breakers off!), but looks like a new one is on the cards.

If I do go for a replacement, any recommendations?

Feanor · « **Reply #8 on:** 10 December, 2020, 10:17:59 am »

<ETA:> There will only ever be power to the lower element during the Off-Peak times. It's not just that you are checking for power at the wrong time of day?
The 'Boost' function will supply power to the upper element only. (When you select the Boost function during the day.)

Anyways, I'd be inclined to try to replace like-for-like if it comes to that.
That way, you can leave the existing backing plate in place.

I've looked at the installation manual for that timeswitch, and as you say, only one element can be active at a time.
So if you are seeing power on the upper element, there will not be power to the lower element.

http://horstmann.securemeters.com/files/9114/7261/7065/Electronic_7_installation_for_web.pdf

Refer to P2 of the manual.
The schematic shows that power is sent out terminal 5 to the lower element during off-peak hours.
Outwith these hours, the power is directed through the Boost function relay to the Upper element via terminal 4.
(It's not clear whether during off-peak hours the upper element also gets an occasional cycle: that's a software thing in the timer, it's possible it does.)

With the timer removed from the back-plate, you should be able to test the wiring/thermostat/element from the back plate.
With the tank cold, ( thermostats calling for heat ), you should be able to 'see' the elements with an ohm-meter.

Measuring between terminals 3 and 4 should show the 20 ohms of the Upper (Boost) element.
Measuring between terminals 3 and 5 should show the 20 ohms of the Lower (Off-Peak) element.

If you can 'see' the 20 ohms here, then the wiring, thermostats and elements are OK, and it's possible the timeswitch is faulty.
If you can't see the 20 ohms of the lower element, then there's a problem with the wiring/thermostat/element that needs diagnosing.

TimC · « **Reply #9 on:** 10 December, 2020, 11:24:24 am »

Thanks Feanor. Yes, took a little searching, but you're right that the lower element is off-peak only. So that explains the lack of wigglyamps. However, the reason I was investigating further was that the amount of hot water available this morning was no more than it was before I changed the thermostat, and that amount is not what I'd expect from a 170L (roughly 40 gal, I think) water cylinder. I live in a notoriously hard water area. There is a Combimate device fitted which I think is intended to reduce the lime problem (and the balls have been changed on schedule), but I think it's probably inadequate for this supply. The cylinder is about 15 years old, I think (before Santon became Heatrae Sadia), so I'm wondering if it's furred up.

Could be an expensive Christmas if I have to replace that and fit a proper water softener!

robgul · « **Reply #10 on:** 10 December, 2020, 01:51:52 pm »

Quote from: TimC on 10 December, 2020, 11:24:24 am

Thanks Feanor. Yes, took a little searching, but you're right that the lower element is off-peak only. So that explains the lack of wigglyamps. However, the reason I was investigating further was that the amount of hot water available this morning was no more than it was before I changed the thermostat, and that amount is not what I'd expect from a 170L (roughly 40 gal, I think) water cylinder. I live in a notoriously hard water area. There is a Combimate device fitted which I think is intended to reduce the lime problem (and the balls have been changed on schedule), but I think it's probably inadequate for this supply. The cylinder is about 15 years old, I think (before Santon became Heatrae Sadia), so I'm wondering if it's furred up.

Could be an expensive Christmas if I have to replace that and fit a proper water softener!

... years ago we had a similar problem with an immersion heated cyclinder that was probably 12-15 years old ..... I took the cylinder out and knocked it all round the top and bottom with a mallet (gently) and then tipped out all the fur debris through the element hole - nearly a wheel-barrow full!!! Water was OK after that when I refitted the tank.

Kim · « **Reply #11 on:** 10 December, 2020, 01:53:40 pm »

I saw a hot water cylinder that had for some reason been split open in a skip in Canterbury once. There wasn't an awful lot of room left for water.

TimC · « **Reply #12 on:** 10 December, 2020, 02:01:10 pm »

Quote from: robgul on 10 December, 2020, 01:51:52 pm

Quote from: TimC on 10 December, 2020, 11:24:24 am
Thanks Feanor. Yes, took a little searching, but you're right that the lower element is off-peak only. So that explains the lack of wigglyamps. However, the reason I was investigating further was that the amount of hot water available this morning was no more than it was before I changed the thermostat, and that amount is not what I'd expect from a 170L (roughly 40 gal, I think) water cylinder. I live in a notoriously hard water area. There is a Combimate device fitted which I think is intended to reduce the lime problem (and the balls have been changed on schedule), but I think it's probably inadequate for this supply. The cylinder is about 15 years old, I think (before Santon became Heatrae Sadia), so I'm wondering if it's furred up.

Could be an expensive Christmas if I have to replace that and fit a proper water softener!

... years ago we had a similar problem with an immersion heated cyclinder that was probably 12-15 years old ..... I took the cylinder out and knocked it all round the top and bottom with a mallet (gently) and then tipped out all the fur debris through the element hole - nearly a wheel-barrow full!!! Water was OK after that when I refitted the tank.

This is a modern-style cylinder with the insulation (60mm) under a metal jacket, so I doubt I'd be able to dislodge the deposits effectively. I think I'd just end up with a lot of dents and some crushed foam!

Bolt · « **Reply #13 on:** 10 December, 2020, 03:35:20 pm »

I successfully cleared a terminally scaled hot water cylinder by draining the header tank and adding 5 litres of industrial descaler. The plumbers merchants I got it from seemed reluctant to sell it to me and told me they hadn't any in stock, which was comical as there was shelf full of it behind the counter!

TimC · « **Reply #14 on:** 10 December, 2020, 05:45:36 pm »

Mine is a direct, unvented system, so no header tanks or any other way of introducing descaler that I'm aware of. Installing a water softener might slowly descale the tank, but I'm not optimistic that there's any method I could practicably employ that would descale it reasonably quickly.

AuldThumper · « **Reply #15 on:** 13 December, 2020, 11:55:19 am »

If you feel competent to do it safely....

Run off the hot water
Isolate the system from mains power
Disconnect and cap off the lower element power supply
Disconnect upper element power and connect it to the lower element
Re energize upper "day use" circuit and see how much hot water you get :-

None - the lower heating element is dead
Loads - the E7 meter is dead
Some but not enough - the cylinder is furred up

TimC · « **Reply #16 on:** 14 December, 2020, 04:53:59 am »

Way ahead of you, AT. I did it a bit more simply by disconnecting the top element and connecting the bottom element to the top supply using the switched plugboard behind the control panel. This confirmed that the bottom element is fine, but that the total amount of hot water available is rather less than it should be. So the cylinder is furred up. I shouldn't be surprised; I have to descale the kettle twice a month! I can survive by setting a timed boost in the afternoon and avoiding baths.

It'll do for now, but early next year I'll be building a small extension with a shower room, and I'll replace the cylinder and fit a water softener (not one of those silly magnetic thingys). The rising main is in the bathroom, so I'll need to fit a new parallel cold water pipe to the kitchen for drinking water (or fit a filter tap), which complicates it a bit but I'll find a way.

rafletcher · « **Reply #17 on:** 14 December, 2020, 09:08:37 am »

I’d fit a separate takeoff for drinking water, rather than rely on filters.

AuldThumper · « **Reply #18 on:** 14 December, 2020, 10:40:58 am »

If you're rejigging the whole system it's worth at least a bit of research on heat bank/thermal store setups

It's, as usual, a swings and roundabouts situation but the big selling point to me is the bits of the system prone to scaling issues are individually serviceable or replaceable

TimC · « **Reply #19 on:** 14 December, 2020, 04:52:26 pm »

I'm not really over familiar with them, but as I live in a tiny, thatched, listed cottage, there isn't going to be any change to the energy source for my hot water. I have no wet heating in the house - indeed, there is no water whatsoever in the original building. The bathroom and kitchen are extensions. I did apply for listed building consent to put PV/solar hot water panels on the tiled roof of the new extension, but Babergh told me 'no way'.