Analogue to digital hot water tank thermostat?

18 Dec 2011
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United Kingdom
I have a Drayton HTS3 strap-on hot water tank thermostat which can be adjusted using a small screwdriver. The maximum hot water temperature can be set to my liking but, every now and again, the temperature at the taps becomes much too hot for me. I can usually correct this by readjusting the thermostat but the problem invariably resurfaces in due course. Is it feasible/sensible to think about having the analogue thermostat replaced by a digital one and, if so, what might be involved in terms of modifying the wires connecting it to my Potterton Prima B boiler, please?
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Why do you need to change it. You set the temprature to the how hot you need it at the tap, over 60 degrees c and you cause greater scale build up. No tank stat is that accurate and in all cylinders stratification takes place (hotter at the top than bottom). My advice would be set it and leave it alone, if you want that level of temprature control then install a thermostatic blending unit to the hot water.
Drayton do a wireless digital cylinder stat.

To get this to work, you would need to mount and wire the receiver. It requires a permanent 240 volt feed to it, and this would have to be taken from the same supply which feeds the rest of the CH system.
The wiring from then on is simple. The current wires from the clinder stat would then go on terminals 1 Common, and terminal 3 call for heat(probably - not 100% cos i dont know exactly how yours is wired without seeing it). You may have to include a heating satisfied cable if your current one is 3 wire and not 2.

Then all you would do is mount the sensor head where the current cylinder stat is, and mount the transmitter unit nearby.

Its potentially alot of work for nothing - have you thought about buying a new standard cylinder stat? yours may have come to the end of its useable life. Cylinder stats are pretty good out of the box and a new one will probably solve all your issues without having to pull in cables etc
It sounds like you are asking the cylinder 'stat to do something it isn't meant to do.

The purpose of a cylinder 'stat is to identify when the hot cylinder cannot absorb more of the boiler's output, not to control the water temperature at the outlet.

Cylinder 'stats are not exactly high precision devices in the first place.
They are made with a fairly wide hysteresis loop to stop them switching the boiler on and off every time you glance at them, and in their usual position, towards the bottom of the cylinder, they only reflect the temperature of that stratum of water in the cylinder - the water near the outlet may be much hotter, or it may be nearly the same as the lower strata. That depends on other factors, including how well insulated the cylinder and pipework is, how long the system is allowed to stabilise after hot water is drawn off, the amount of water drawn off etc. etc.

The hysteresis loop of a 'stat often seems to grow wider, and the accuracy of calibration drifts as they age, so a 'like for like' replacement may help, but do bear in mind that controlling the temperature at the point of delivery is not the primary purpose of the 'stat.

If you really need that level of control, then, as ratonnastick said, a TMV or similar would be the answer.
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Ratonastick pretty much summed it up. Honeywell do the rf2 pack 4 which us wireless. I have the business end of that linked to an Evohome controller to run my thermal store.

Still having fun with the various settings....
Thanks, everyone. I guess a digital thermostat isn't the answer so I'll rethink my requirements in the light of the advice you've provided.

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