No hot water

[Graham46]

The system is 20 years old comprising pressurised DHW tank in airing cupboard, gas boiler in garage and recently replaced zone valve to the hot water tank. The CH controls are firing the boiler OK but the hot water controls are not. I have read the Wiki and FAQs and it seems likely that the problem lies with the DHW thermostat. Can anyone advise how to test this to see if it has failed, before fitting a new one? It is a Honeywell L641A 1005 also marked with T70. Also a similar looking device with a red button is fitted towards the top of the tank: a Honeywell 6190 2004.

The dial on the thermostat squeaks a lot as you turn it with a screwdriver so it is quite difficult to hear the click of the microswitch activating but I am pretty sure that it is not clicking around the 50 degree mark which is where I would expect it to activate with the water now pretty tepid after a cool shower this morning. But I think I can hear it click when the pointer goes past the six o'clock position ie going round from well past the max to the min again.

Any advice gratefully received.

Regards
Graham

 

[bellairaphon]

why was the zone valve replaced? same problem? was it working when the valve was changed?

 

[Agile]

To have any chance of deciding whats wrong you would need to do electrical tests.

As its an unvented cylinder any work on it should be carried out by someone qualified to work on them.

The zone valve is part of the safety system so that should also have been replaced by a qualified person.

If you cannot do the electrical tests on the thermostat I would say that you will need to call someone.

Tony

 

[Graham46]

Thanks for your comments. I may have misled you somewhat re the zone valve. This was replaced about six months ago (by a qualified plumber) as it was sticking open and the water was getting too hot while the CH was on.

The whole system has been working fine until this morning.

As it is cold here today and the wife is not feeling too well I have put the CH on and, as an experiment while waiting for your expert comments, put the DHW zone valve ito the manual open position. This has given us a tank of piping hot water but the cylinder thermostat still does not seem to click anywhere in the range 50 to 80 degrees so I am now pretty certain that this is where the fault lies.

If there are no simple DIY tests I can do to check this, can anyone give me an idea of the price of a new one and a steer on how to replace it? My main concern is how to isolate the electric supply while doing it. Does this all come from the mains switch that supplied the timer controls? Then I presume it's just a matter of loosening the metal holding strap and replacing the wires on a like for like basis?

 

[Agile]

Locking the valve open is bypassing the safety function which they provide.

These cylinders really can seriously explode!

In view of the safety implications I dont want to risk giving you any advice which might encourage you to attempt DIY repairs.

If you dont know how to safely isolate the supply AND test that its really isolated then I dont think that you are competent to work on it.

Tony

 

[Graham46]

@Agile
Thanks for your concern. However, when the zone valve was stuck open for several days and the water got REALLY hot because the CH was on all the time, the tank did not explode. When I locked the DHW zone valve I knew what I was doing and only left it on long enough to get the water hot. I could have used the "emergency" immersion heater but wanted to make sure the zone valve was opening OK.

I was competent enough to replace just the motor box on the Honeywell zone valve a couple of years ago so I feel competent to replace a cylinder thermostat if it is just a matter of connecting the wires to the new item. I just wanted to double check that the electric supply to all the controls is isolated by the same switch which isolates the timer.

Thanks again for your concern.

Regards
Graham

 

[gas4you]

My main concern is how to isolate the electric supply while doing it. Does this all come from the mains switch that supplied the timer controls? Then I presume it's just a matter of loosening the metal holding strap and replacing the wires on a like for like basis?

If the system has been wired correctly then you are correct, it should all be isolated from the main heating switched fused outlet.

However, in this far from perfect world, it has been known for some 'idiots' not to wire things as they should be, therefore once you think you have isolated the power, you should always check the terminals you intend to work on, that they are actually dead.

I would also recommend removing the fuse in the switch, just so no one else can accidentally switch it back on whilst you are working on it.

 

[steved101]

an Unvented cylinder thermostat cannot be picked off a shelf in b & q orordered from Screwfix! They need to be exact replacements, a) ordered from the MFR ideally and B) Installed by a competent unvented installer. It is a real safety issue here.

 

[Graham46]

@gas4you
Thanks. I thought I remembered correctly from when I replaced the zone valve motor but always just worth checking. And a good tip re the fuse!

@steved101
Well I just Googled "Honeywell L641A" and Screwfix do sell them off the shelf at £19.99. Latest version seems to have 1039 after the part number rather than my old 1005. Instructions leaflet also online shows just two wires to connect at C and 1. If I ensure a good clean firm contact with the cylinder, how difficult/dangerous can it be? And as I understand it these u/v tanks have several pressure release safety valves just in case things do get too hot.

I appreciate everyone's concern for my safety but no-one has answered my original question re a simple test for the thermostat to see if it is working properly or not. Or, given that the zone valve has been proved as working OK, do all the other circumstances point to the stat?

Regards
Graham

 

[steved101]

Graham, it is an easy task for a competent person to check the correct operation of a cylinder thermostat / motorised valve but it is ILLEGAL to work on unvented if you are not trained/qualified to do so! You can replace it if you wish but it is ultimately down to you.

Yes the Unvented cylinder has safety controls (but bypassing the Motorised Valae) eliminates a few of these for a start. If a disaster happened after you did a replacement yourself (touch wood it wouldn't ever happen) and some of your house exploded then you would NOT have a leg to stand on - you would not be able to claim on your insurance!!


Unvented is stored hot water at MAINS PRESSURE. Hence the safety features.

Graham: do you have a multimeter and no how to use one?

 

[Graham46]

I can borrow a multimeter and yes I do know how to use one. Please tell me what readings I have to check to see if the DHW stat is shot or not.

As a matter of interest, if it is, as you say, ILLEGAL to work on an unvented system if not fully qualified, why is this Health & Safety obsessed government/local council not advising me of such a fact. And why is the water board not offering me such frightening advice and offering me overpriced insurance against such an event like they do for leaks in mains pipes between the main and my property?

And what is a definition of "working on"? Is it illegal for me to check the circuits with a multimeter?

This is getting really heavy, Man.

Regards
Graham

 

[gas4you]

I think everyone is assuming you have an unvented cylinder such as a megaflo with a built in cyl stat.

I thought, maybe wrongly, from your description in your post, that you have a very old unvented cylinder, i.e. something like a copper cylinder, that has a standard Honeywell cylinder stat strapped to the outside, as a normal vented cylinder has, therefore the G3 regs have nothing whatsosever to do with you fitting a new cyl stat

 

[Agile]

I can borrow a multimeter and yes I do know how to use one. Please tell me what readings I have to check to see if the DHW stat is shot or not.

As a matter of interest, if it is, as you say, ILLEGAL to work on an unvented system if not fully qualified, why is this Health & Safety obsessed government/local council not advising me of such a fact.

Regards
Graham

Woe betide anyone who lent you their multimeter! But you can buy them from about £6 upwards. You might blow it the first time you tried to measure the current in the mains.

If you really knew how to use a multimeter then you would not need to ask us what to measure.

The HSE does not write to you because they dont know that you are planning to do work which is illegal and you are not qualified to do.

The police dont write to you to tell you not to burgle houses. They just chase you if you do!

Tony

 

[Graham46]

Good morning, gentlemen

The cylinder is an IMI Range Tribune 150 litres manufactured in 1988. Very similar to but not exactly the same as the photo in Agile's profile. The thermostat is a Honeywell L641A and is held in position with a metal strap.

The "typed" comments on the label state: "do not tamper with or adjust the controls". Does the stat count as a "control"? and does this include:
1 adjusting the temp on the stat?
2 replacing a faulty stat?

The printed instructions warnings mention that work should only be carried out by a BBA approved installer. The owner of the multimeter that I said I could borrow is a fully qualified electrician who would have also been "borrowed" (sorry I forgot to mention that) is licenced to certify electrical installations. I have never asked him if he deals with CH controls but I guess he would be able to establish whether or not a microswitch in a stat is bust or not. I was just trying to avoid bothering him at this stage.

I take on board the warnings on the tank label about H&S, Building Regulations and Water Byelaws and appreciate all your comments.

So, can I replace the stat or not without falling foul of the law? Alternatively can my qualified electrician neighbour do it for me? And I would still like to know the details of how to test the stat. Agile initially said that I "would need to do electrical tests". If he would specify exactly what to do I will decide if I am competent to do them. Despite what you might think I am by nature cautious and will only take on what I am confident to do.

And BTW can someone tell me what the other strapped on control with the red button (Honeywell L6190) does and could this have anything to do with the problem?

Thanks and regards
Graham

 

[Graham46]

Well, guys, despite your reticence to help, I found the problem and fixed it. Remember in the first place I only wanted advice on how to check whether the cylinder stat was working or not. With everything switched to “on” for DHW I just used a mains tester (not a multimeter) and found live feeds from the timer, the aquastat and the tank stat and established that the latter did indeed click and switch power on and off correctly. This only left the power head of the motorized valve as suspect. Removing the cover of this it was clear that the motor was not moving and contacting the microswitch to activate the pump and fire the boiler when the tank stat was moved to an on position. I tried the microswitch manually and this did fire the boiler and start the pump. So it was our “usual suspect” the motor in the motorized valve after all. I knew that it could not be the valve itself as this was replaced only last winter and was moving freely.

So a new power head fixed the problem and all is working correctly again. However, just for the record I would still be interested to know how you would have tested the tank stat with a multimeter. Would it have been any simpler that using my trusty mains tester?

Cheers
Graham