This morning our loft was making a boiling sound and so I popped up to investigate. It seems the thermostat for the hot water hadn’t kicked in and the system was boiling up. The expansion tank was full of boiling water.

It’s an ancient Ideal Mexico Super. The stat in the boiler downstairs was still in the boiler and seems attached. Are they any other things to look for whilst I’m waiting for my plumber to come around?

Thanks

Obviously I can’t see the type of system you’ve got but you could have a failed tank stat. (this should be strapped to the side of the tank).

I have also seen a stuck motorized valve which has prevented the boiler from turning off when demand was removed, thus boiling the water.

Any more descriptions/pictures of the system you could give?

Rich.

This morning our loft was making a boiling sound and so I popped up to investigate

Our system would need two thermostats to fail to boil the HW tank, the HW tank thermostat and the boiler thermostat.

We do get some kettling in the heat exchanger, but that’s mainly as it’s needs it’s 10 year DS-40 dose to dissolve all the limescale….

Mmm, I thought the tank stat might just be used for the immersion.

It’s an old gravity fed central heating system. Boiler downstairs, hot water storage tank, expansion tank and cold water tank in the loft.

I’ll replace that then and get the plumber out.

The immersion has a stat inside it. The tank stat. will be strapped to the body of the tank about 3/4 of the way down.

Multimeter time again! There will be a common terminal in (live) and 2 terminals out. One with a wire attached the other empty. These are the Normally Open, Normally Closed contacts.

Turn the temp on the stat. down as far as it will go. There should be voltage between the common and earth but not between output and earth.

Turn up the stat as far as it will go. The stat. should click and then you’ll see voltage between output and earth. Note that you may not get voltage if the tank is still so hot that the stat won’t call for demand.

If you’ve got voltage between output and earth with the stat. right down and the water hot the stat is faulty. Note that if the water in the tank is cold the stat. may show live between these contacts in this situation.

Some more modern stats have a high limit cutout which will prevent the water getting above about 90 degrees.

Rich.

Cheers Rich…you’re proving rather useful this past fornight 🙂

Sounds like I’ll be running up and down the loft ladder and stairs a lot tonight.

Will there be an obvious earth point to use?

Thanks

I like jobs like this (no dirty hands!).

The stat will look something like this (not my picture) –

I can’t readily find an internal picture but the cable will almost certainly have a yellow/green earth in it. Use that or the metal of the tank should provide a return path.

If you’ve a 3 port valve you might find the tank stat has 3 cores to it as the valve needs to know when there is no demand for hot water to position it properly. If this is the case then let me know and we’ll work out a debug routine.

A simple test is to turn the stat knob and listen for a click. You should hear the contact make/break.

I’m getting out of my electrical expertise area now, but I would guess that if you have a “Y” plan system which utilises a 3 port valve, a failure of the valve to operate properly may result in the hot water being heated when the central heating demand is on.

Debug the tank stat. first and then if that is ok we’ll look into the valve situation.

Rich.

So here’s the thing then.

When I moved in 3.5yrs ago, I replaced the old electro-mechanical controller and wall stat (which was right next to the boiler in a small utility room) with a Honeywell Sundial RF2 pack 3 to move the wall stat and heat the house better. This pack came with a remote hot water stat which I hadn’t planned to use but the controller seemed to require and I attached it to the tank similar to the above. However, I set the water temp value high on this so the boiler stat would just control the water temp as it had been doing previous.

The controller is currently wired up incorrectly as when the CH timer comes on, only the water pump fires (the boiler doesn’t). I’ve compensated for this by just having the hot water timer(s) and CH timer(s) on at the same time when I want the heating on.

The CH wasn’t on this morning when the system was boiling.

Best get all the tank jackets off and find all the tank stats as a starting point. Cheers!

If it’s gravity hot water, you won’t have a cylinder stat

Would the boiler actually boil the hot water tank? Was the immersion heater on? to me even if the boiler is heating water flat out then it still wouldn’t boil as the hot feed from the boiler to the cylinder does not get to 100C

If the immersion heater was on and its stat failed it could boil the water

It will if the boiler stat has failed, which is the only thing that controls the water temperature on a gravity system

So Paulosaxo is correct, no stat on the tank. It’s just a copper tank, no insulation on it other than tank jackets.

Have made sure stat firmly in boiler. Will see what happens in the morning.

Boiler can definitely boil water in this old system. I take the stat out to remove air from the system by boiling it up.