New tank thermostat still giving scalding water

erehwon · 12 Oct 2019

A little while ago, the hot water tank on my pumped heating system started producing scolding water. Doing a bit of research, it pointed to an issue with the tank thermostat. It was many years old so I bought and fitted a new one. Same problem - scolding water. I tried adjusting the thermostat but could only get either not quite hot enough water or scolding water.
Could there be another reason for the scolding water, or do you think my new thermostat is also faulty.
Any advice and suggestions gratefully received.

CBW · 12 Oct 2019

Any photos of installation? You may not have the contacts touching the tank, or do you have an immersion heater on as well?

erehwon · 12 Oct 2019

I'll sort something out tomorrow. The thermostat was an exact swap of the old one so should be OK but something is obviously wrong.
Thanks Novice!

ericmark · 13 Oct 2019

If using the old C Plan then the thermostat only controls boiler and water temperature in the summer, once the central heating is running then it does not control the water temperature.

So have you got a motorised valve? if not than what you have is normal and not a fault, you could fit thermostatic mixing valves to reduce temperature but tank temperature should be over 65°C to stop legionnaires.

With old C plan the tank thermostat was just to stop the boiler cycling in the summer.

erehwon · 13 Oct 2019

It always takes me a while to work out exactly what heating system I have.

I had to look up C-Plan, but I think I have an S-Plan, with a motorised valve (pictured below). I can independently control my hot water and heating. Does that sound right?

Everything was working fine with the system. I then swapped the programmer unit for a Hive control unit. It was about this time (I don't know if it was exactly the same time) that the hot water went too hot. So I replaced the tank thermostat with a new unit, a Drayton HTS3, which happened to be the same at the old unit, making it an easy fit.
But the new unit didn't fix the problem. You can see from the photo that I have to have the temperature about 50 degrees. This is the point were minute changes make it too hot or not hot enough.

So I'm thinking that the new unit is also faulty (possible) or it's a different problem, but I don't know what else it could be. Any suggestions?

Mottie · 13 Oct 2019

Isn’t a 3 port valve a 'Y' plan? Perhaps with the heating off, the valve is not shutting the hot water off and is constantly heating it when the heating is on? With the hot water set to minimum and the heating on, do both outlets from the valve get hot and is hot water being pumped into the cylinder coil?

Hugh Jaleak · 13 Oct 2019

You've a got a Y plan system there. I might be being cynical, but you say this problem started about the same time you fitted the Hive? I would wager there's nothing wrong with the Cylinder stat, (new or old), and the problem lies in the wiring.

When you do get it working properly, Cylinder stat needs to be set around 60°C to ensure Legionella bacteria cannot form in the hot water system.

CBW · 13 Oct 2019

Hugh Jaleak said:
You've a got a Y plan system there. I might be being cynical, but you say this problem started about the same time you fitted the Hive? I would wager there's nothing wrong with the Cylinder stat, (new or old), and the problem lies in the wiring.
.

Agree it could be the wiring. Also check to make sure you don’t have the immersion heater on also.

erehwon · 13 Oct 2019

Thanks for all your replies.
@Motman - Yes, I've been told my system has a Y connector. Is that different from an S-Plan. Too confusing for me! I'll check the pipes to see if the valve is getting stuck. I think the valve is quite new (this year) but that doesn't seem to mean much!
@Hugh Jaleak - I'm worried that it could be a wiring problem, which is more difficult to find and fix. I am making sure I keep the water on the scalding side, rather than the not quite hot enough.
@The Novice - The immersion heater is definitely disconnected. I'll add your vote to Wiring! I might try going back to the old programmer to see that still works.

Mottie · 13 Oct 2019

Until it’s fixed, could you not manually move the valve into the heating only position and run your hot water on the immersion heater?

erehwon · 13 Oct 2019

@Motman - if I used the immersion heater, should that heat the water to the correct temperature, ie that set by the tanl thermostat? Is so, I could test if the thermostat was working correctly.

Mottie · 14 Oct 2019

No. The immersion has its own thermostat which is adjustable. It’s usually under the plastic cover where the wiring goes to it.

75pete · 14 Oct 2019

I take it that you have fitted the correct Hive. Just to be clear, yours should be Heating and Hot water and not the Hive for a combi boiler.

stem · 14 Oct 2019

The HTS3 cylinder thermostat has three terminals marked C, 1 & 2. First of all, is there a wire in all three of the terminals?

If you installed the Hive yourself:
Does the wire in C connect to terminal 3 "Hot water on" at the Dual Channel Hive?
Does the wire in 1 connect to the boiler, and the orange wire of the motorised valve?
Does the wire in 2 connect to the grey wire of your motorised valve, and terminal 1 of the Dual Channel Hive "Hot water off"

erehwon · 14 Oct 2019

@75pete - yes, I got the Heating and Hot Water one.

@stem - I think there were three wires on the thermostat. I'll check later. I copied the wiring exactly from the old one.
For the Hive, I had a common backplate so I didn't change any of the wiring. I presume the wiring doesn't need to be changed between units. On the old programmer, I could control the heating and hot water separately and I'm hoping it is still OK.

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New tank thermostat still giving scalding water

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