Hot water cylinder - is this a thermostat?

Page may contain affiliate links. Please see terms for details.

BTB 500

MB Club Veteran
SUPPORTER
Joined
Aug 7, 2005
Messages
23,211
Location
Shropshire
Car
R129 SL500, W639 Vito 120, S205 C300
We moved house recently, so are getting to grips with a new heating system! There's a gas-fired Rayburn (Aga) downstairs for hot water, with a cylinder plus the programmer in the airing cupboard upstairs. The issue is that the boiler seems to heat the water continuously when on. The water gets very hot so the boiler is working OK ... found a thermostat on the cylinder set to 80C, however turning this down doesn't stop the boiler & pump from running (but there's a normal 'click' as you go past the actual current temperature). So possibly a faulty thermostat, however there's a second device on the cylinder which may also be a thermostat - it's not like any I've seen before though so I'm a bit reluctant to start tinkering!

General view of the cylinder:

picb.JPG

The normal thermostat on the right that doesn't seem to do anything:

picc.JPG

Some pics of the 'thing' on the left:

picd.JPG

pice.JPG

pica.JPG

Is this also a thermostat??

One complication is that there's a solar water heating system in there as well (which works fine in decent weather) ... all the cables run into conduits though so it's not that easy to see what's connected where!

picf.JPG

I have an engineer coming in a couple of weeks to do a service on the Rayburn (they were booked solid for over a month), but as far as I can tell that's not the issue. When you turn the central heating on the radiators initially get hot but then cool off - I assume because the hot water takes priority and the system thinks it's not up to temp. There are radiators with no TRVs in the bathroom and downstairs toilet (presumably connected into the water heating circuit), and these get very hot whenever the boiler is running. As an aside (not relevant here), the central heating is controlled only by TRVs on the radiators ... there's no room thermostat anywhere. So I would expect that to run continuously when working.

Many thanks in advance for any thoughts / advice!
 
Domestic hot water should be set to 60-65 ‘C which is enough to kill off legionella and other bacteria. Hotter than that presents a scalding hazard.

The smaller ‘Honeywell’ unit in your photo is the same thermostat as on our hot water cylinder and in our case is used by the heating system to control the domestic hot water temperature via a three-port valve.

The larger unit in your photo, looks like another thermostat.

Just quite what is controlling what is hard to say without seeing the system or a circuit diagram of how it’s all plumbed up.
 
Thanks folks. There is an immersion heater, but it seems to be directly switched from the mains supply:

picg.JPG

There's also a fairly normal looking valve:

pici.JPG

As an aside there's another thermostat towards the bottom of the tank ... I can try tweaking that one down to see if it does control the water temp.

pich.JPG

I assume one of the thermostats could be for the solar water heating, although the chances of that getting the tank much above 40C are probably quite slim in the UK!
 
Immersion heaters often have an integral thermostat. To find out, you need to switch it off at the wall an remove the cover on the tank unit. If one is fitted you’ll see the adjustment dial.
 
My guess would be its linked to the KINGSPAN* SOLAR WATER SYSTEM circulation whereby your hotwater tank functions as the energy store for the water heated in the roof panels----pointless effectively cooling the tank contents by reverse radiation if the roof panels are not up to a higher temp than the tank contents? as such it might be a temperature sensor switch for solar circulation via the controller????

API-Energy-active_solar_heating_system-600x600.jpg


* why not contact Kingspan for a functional diagram of their typical SOLAR water system- nothing online I could see on their website but the tech will be old now
 
Last edited:
Should perhaps have said the diagram on left is for south facing roof panels on right for east/west panels- not much difference to be honest.
If it were my system I would be keen to find out when the system/heat exchange fluid was changed last??
 
My system is run by TRVs only. Each radiator, except bathrooms, has an app controlled trv which talks to a hub. Any individual TRV can call for heat. No need for a room stat because each room can be set to any heat you desire and any time program you desire. In effect the house is totally zoned.
 
My system is run by TRVs only. Each radiator, except bathrooms, has an app controlled trv which talks to a hub. Any individual TRV can call for heat. No need for a room stat because each room can be set to any heat you desire and any time program you desire. In effect the house is totally zoned.

Does the pump stop when all rooms are at their desired temps.?
 
Does the pump stop when all rooms are at their desired temps.?
The boiler and pump only come on when any trv calls for heat. At all other times they are off.
 
The thermostat towards the bottom of the tank appears to have been wired in Twin & Earth cable - this is NOT correct for flexible connections or high temperatures. In addition you appear to have a broken Fused Connection Unit (often referred to as "Fused Spur")

Prepare youself for (a) sucking in of air between teeth, and (b) a larger than expected bill.
 
Should perhaps have said the diagram on left is for south facing roof panels on right for east/west panels- not much difference to be honest.
If it were my system I would be keen to find out when the system/heat exchange fluid was changed last??

Quick update on this. I booked an inspection of the solar water heating with Kingspan (£226!), and the engineer gave it a clean bill of health apart from the glycol level in the fluid. This does need changing ... he said it normally lasts about 10 years, and he suspected it had never been replaced before. Needs a return visit for this, which he's booking.

He also confirmed that the top two thermostats on the hot water cylinder were for the Kingspan system (and are fine), and that the bottom one (for the gas water heating) had failed and needed replacing. The Kingspan logs showed that the water temperature had hit 104C at some point :eek: (but this is well above the maximum output temp. for the boiler? :dk: ).

He also recommended that the immersion heater (which we never use) was replaced as it doesn't meet current safety standards.
 
Quick update on this. I booked an inspection of the solar water heating with Kingspan (£226!), and the engineer gave it a clean bill of health apart from the glycol level in the fluid. This does need changing ... he said it normally lasts about 10 years, and he suspected it had never been replaced before. Needs a return visit for this, which he's booking.

And the charge for this is another £328 :eek: although £148 of that is for 20 litres of glycol delivered to the house first (which arrived yesterday). Saving money on hot water is an expensive business :D

Per the videos that grober kindly posted above it's a pressurised system, so fluid replacement isn't really a DIY job. But fingers crossed it shouldn't need any more attention for a good few years.
 
Always a problem taking over a place and getting to understand the heating system,the first photo show the immersion heater at the top,if you take the silver cover off it will have the pre set temp just like the thermostat on the side of your hot water tank.if you switch it on for hot water it will cut off at around 68 degrees ,of course you do not use that very often,I see you have a solar water heating and so one of those thermostats maybe wired into that,you mention that the rad in the bathroom and toilet have no tvr's well thats ok although many like to have them my bathroom dos not have any
 
The two thermostats at the top are for the solar heating - one is a normal thermostat and the other is some kind of safety cutoff (I think that's what the engineer said). We don't ever use the electric immersion heater but potentially handy to have it there in case the boiler (which is actually a Rayburn/Aga range in the kitchen) packs up! The house has solar power too (the account for which still isn't fully transferred to us after 3 months!) so there has definitely been a bit of a learning curve :)

The radiators with no TRVs get very hot (boiler temp) ... the one in the bathroom has a towel rail over it and is OK but the downstairs toilet is like a sauna when the heating is on :D It does have a manual valve but I can't budge that ... reluctant to use more than manual force on the knob as I don't want to shear anything off ...
 
The radiators with no TRVs get very hot (boiler temp) ... the one in the bathroom has a towel rail over it and is OK but the downstairs toilet is like a sauna when the heating is on :D It does have a manual valve but I can't budge that ... reluctant to use more than manual force on the knob as I don't want to shear anything off ...
Take the knob/valve cover off and use pliers on the valve shank? If it’s that stiff I would also use a Stilson wrench to support the valve body while trying to turn it
 
NOTE: at least one radiator in any installation should not be under control of a TRV but should run at boiler temp . Depending on the house design this is usually the 'big one ' in the hallway or the same area that the room stat is in . The bathroom is also a popular place for the non TRV radiator.
 
NOTE: at least one radiator in any installation should not be under control of a TRV but should run at boiler temp . Depending on the house design this is usually the 'big one ' in the hallway or the same area that the room stat is in . The bathroom is also a popular place for the non TRV radiator.
We normally use a bathroom rad as a bypass.
 
NOTE: at least one radiator in any installation should not be under control of a TRV but should run at boiler temp . Depending on the house design this is usually the 'big one ' in the hallway or the same area that the room stat is in . The bathroom is also a popular place for the non TRV radiator.

Yup I'm used to it being a radiator (or heated towel rail) in the bathroom. We don't have a room stat in this house!
 

Users who are viewing this thread

Back
Top Bottom