Programmable digital cylinder stat

siricosm · Oct 24, 2024

I bought one of these:

and I am using it to control the temperature of my hot tub, and it works great. Does anyone know of a device like this that does not use plugs but rather can be wired in? I am looking for something to control my cylinder, that can be programmed down at night with the ability to set the differential. This model works great for the hot tub and you can set the on/off temperature as you like in increments of 1/10th of a degree, and it only costs £22.

ShaunCorbs · Oct 25, 2024

https://amzn.eu/d/eL5pTvK

Basher · Oct 28, 2024

The above controller has 10A rated relay contacts. If you have a 3kW immersion heater that's about 12.5A, so there might be a lifetime/reliability issue?

UFH programmer/ thermostats usually have 16A contacts, so that might be a better way to go - also more DIY friendly as built to fit on the wall etc.
Don't know about the resolution or hysteresis of this, but a possible solution here:
Amazon productView: https://www.amazon.co.uk/KETOTEK-Underfloor-Thermostats-Programmable-Temperature/dp/B082Y36YTC/ref=sxin_15_pa_sp_search_thematic_sspa?

siricosm · Oct 28, 2024

ShaunCorbs said:
https://amzn.eu/d/eL5pTvK

This one looks like the right idea, but from the description is does not seem to be able to program different temperatures for day/night.

Basher said:
The above controller has 10A rated relay contacts. If you have a 3kW immersion heater that's about 12.5A, so there might be a lifetime/reliability issue?

UFH programmer/ thermostats usually have 16A contacts, so that might be a better way to go - also more DIY friendly as built to fit on the wall etc.
Don't know about the resolution or hysteresis of this, but a possible solution here:
Amazon productView: https://www.amazon.co.uk/KETOTEK-Underfloor-Thermostats-Programmable-Temperature/dp/B082Y36YTC/ref=sxin_15_pa_sp_search_thematic_sspa?

I only need an amp or two, just enough for the switched live to the boiler and the pump. The UFH stats are programmable, but you can't set the differential from what I can see. I use one of these (similar) for my room stat already, and it appears to have a differential of 1C, so I am thinking that would result in cycling.

The plug-in one in the first post has the ability to set the differential, and the ability to reduce the temperature at night. The differential for a hot tub (1200l) is usually around .5C, not sure what would be ideal for a cylinder (thermal store) but the mechanical one I have is 8C or more, and that is way too wide. It effectively means I have to run the tank 5C hotter than required.

Basher · Oct 28, 2024

siricosm said:
This one looks like the right idea, but from the description is does not seem to be able to program different temperatures for day/night

siricosm said:
I only need an amp or two, just enough for the switched live to the boiler and the pump. The UFH stats are programmable, but you can't set the differential from what I can see. I use one of these (similar) for my room stat already, and it appears to have a differential of 1C, so I am thinking that would result in cycling.

Apologies, for some reason I just assumed the cylinder you mention was electrically heated, perhaps because I had the hot tub in mind!

If you have a boiler heating the cylinder, do you not already have a programmer and the ability to set temp and time for heating the cylinder? It sounds as if you already have a cylinder stat, though its performance is not as you would wish. It would help to understand what system you have!

siricosm said:
The plug-in one in the first post has the ability to set the differential, and the ability to reduce the temperature at night. The differential for a hot tub (1200l) is usually around .5C, not sure what would be ideal for a cylinder (thermal store) but the mechanical one I have is 8C or more, and that is way too wide. It effectively means I have to run the tank 5C hotter than required.

To answer your original query directly, there are programmable cylinder stats available, settable for different temps day and night, and which have switchable hysteresis, here's a poor example (looks an old version!) CP4-HW-OT - OpenTherm® Programmable Cylinder Thermostat - EPH Controls - https://www.ephcontrols.com/section/cp4-hw-ot/
With a boiler heating the cylinder I suspect the thermal inertia of the system would mean short cycling would not be an issue, even with little differential on the cylinder stat.

siricosm · Oct 28, 2024

The boiler is a heat only boiler with external pump that heats a thermal store, controlled by a mechanical cylinder stat. I had a thread on it a while back when I was designing it.

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I am too cheap to heat the hot tub with electricity. I added another exchanger (like the DHW) and am heating it from the boiler.

I am not familiar with OpenTherm, but maybe that is the only way to get programmable temperatures for day/night, and a reduced differential.

Basher · Oct 29, 2024

siricosm said:
The boiler is a heat only boiler with external pump that heats a thermal store, controlled by a mechanical cylinder stat. I had a thread on it a while back when I was designing it.

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I am too cheap to heat the hot tub with electricity. I added another exchanger (like the DHW) and am heating it from the boiler.

I am not familiar with OpenTherm, but maybe that is the only way to get programmable temperatures for day/night, and a reduced differential.

Thanks for the background. I should have looked!
I wasn't specifically suggesting OpenTherm, and not sure it will add anything.
There's no indication of the control system in the original post, did I miss it?
Do you have any sort of programmer external to the boiler?

siricosm · Oct 29, 2024

That thread is pretty old, but it is pretty much exactly what I have now, except that the boiler has been updated to Ideal's modern equivalent of the FF360. The controls are all on the diagram, it is just very simple. A mechanical thermostat controls the cylinder temperature, which the boiler heats directly (the cylinder has no coil). The rads are controlled by TRVs with a smart pump that runs all the time.

ShaunCorbs · Oct 29, 2024

siricosm said:
This one looks like the right idea, but from the description is does not seem to be able to program different temperatures for day/night.

I only need an amp or two, just enough for the switched live to the boiler and the pump. The UFH stats are programmable, but you can't set the differential from what I can see. I use one of these (similar) for my room stat already, and it appears to have a differential of 1C, so I am thinking that would result in cycling.

The plug-in one in the first post has the ability to set the differential, and the ability to reduce the temperature at night. The differential for a hot tub (1200l) is usually around .5C, not sure what would be ideal for a cylinder (thermal store) but the mechanical one I have is 8C or more, and that is way too wide. It effectively means I have to run the tank 5C hotter than required.

Couldn’t you run two ?

siricosm · Oct 29, 2024

That could work. Some timers are SPDT, so one could put two controllers like the one you linked downstream of that, one set to day temperature, the other to night. The probes are way smaller than the pocket in the cylinder, so two of them would certainly fit. It is just a shame one can't get that functionality in one device.

To be honest, I doubt setting the tank down at night would save much, I would probably get more from the ability to lower the tank temperature by using a smaller differential. I could probably go 5C lower all the time, so that might actually make a difference.

Programmable digital cylinder stat

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