dual thermostat on thermal store, latching relay...

[Gareth J]

I recently put in a thermal store to hoover up some excess solar PV in summer and wind when it's available. With a boiler for backup.

I wired it with a latching relay so that a top thermostat calls for heat, which runs the boiler, while a bottom stat turns off the boiler once up to temp. Wiring followed some suggestions somewhere or another and works as anticipated...

However, as the top stat is above the take off for rads/ufh heating water, I can forsee a scenario where I'd like some heating but for whatever reason (some wind/solar has happened for example) where the top stat is satisfied but the bottom stat is stone cold and there is no warm water availability for ufh/rads.

I feel like I need to rewire so the bottom stat alone fires the boiler when heating is required. Leaving the top stat redundant for now. (I have a second top stat for summertime DHW top ups)

Feels like I might be missing something though, I'd still like some hysteresis in the system to stop short cycling of the boiler. Maybe I just need to lower the top stat to inline/just below the take off for the c/h.

Thoughts/comments/experiences appreciated!

[Countrypaul]

I have a Thermal store and have a thermostat to turn the heat pump on when the temperature just below the UFH take off gets down to 25C and turns off when it gets to 35C. I use a W1209 which are remarkedly inexpensive but do need a 12v supply to operatate.

If your thermostat is only there to turn on/off the boiler to make sure there us enough HW for heating then I think you need to just move the top thermostat as you suggest.

[Gareth J]

What a useful gadget! Just looked it up. I could find a way to fit one and get it some 12vDC but think, yes, moving the top stat is less invasive, though that isn't totally straightforward as it'll mean digging a new pocket through the insulation and some rewiring too.

[ecogeorge]

How about using a contactor/relay with normally closed and normally open contacts.......and a thermostat with Normally closed (option) contacts.
Wire top element through normally closed contacts ........ tank heats top up to temp and thermostat closes contacts when satisfied..
THis then changes the state of the contactor onto the other contacts and relay then switches to btm element.........
As soon as water drawn off and top of tank requires heat the thermostat will switch back.
This ensure no power lost and top always at set temp. Relay is only consuming current (coil only-miniscule) when btm is heating (surplus )
If you are worried about bottom overheating fit a NC thermostat to the btm element.
Used this successfully on a 2 tank combo for many yrs .... run hot tap -hear a clunk as contactor switches back to top )
George

[Gareth J]

Thanks George, yes I use that combo on this tank to flip between top and bottom immersions for the solar PV.

However, I can't see that it'll help me control a boiler. The boiler has one take off (bottom of tank) and one input (top of tank). To get any heat into the bottom of the tank, the boiler first has to heat the top of tank up before the hot works it's way incrementally down.

The scenario I'm trying to avoid is running the bottom stat (calling for heat for heating) alone to fire the boiler and, if the bottom area is stratified a bit, the boiler pumping in a small amount of hot into the top only for neighboring hot water to then satisfy the stat and cut out the boiler. Short cycling.

[Countrypaul]

I did not realise that your boiler fed into the top of the TS.

In my case the ASHP input is about 1/2 way up the TS with the UFH at about the same level. If the top half of the TS is below 35C then that will be warmed up to 35C even though the ASHP feed is below that level. The top immersion will only heat the top portion of the TS and will provide no heat to the lower part of the TS used for the UFH. The immersion is run off an Eddi so will divert excess PV power but also will power due to a timer in the Eddi so even during winter there should be enough heat in the upper part of the TS.

The thermotats that controll the ASHP for the UFH part of the TS are located slightly below the UFH upper take off and slightly above the UFH return.

I don't use the ASHP to provide the heat for the DHW higher temperature.

[Gareth J]

Thanks, yea I have a similar heat pump coil though no heat pump installed yet.

Test situation now is to simply turn the top stat up to max, so it's never satisfied, that should let the heating circuit of the boiler be controlled only be the bottom stat. Just need the right mix of weather conditions and residual store heat to test it out. Over the weekend hopefully.

Thanks

[Gareth J]

Update (I accidentally refound this thread after searching for something slightly different);

Found that, with one stat, it's fine. The boiler doesn't short cycle too badly.

It's helped by a pump overrun timer I put onto the boiler pump. It continues circulating water round the boiler for a preset time, the idea being to take some of the residual heat out of the boiler and put it into the store. Thermostatic control based on the boiler water jacket temperature would've been better but pump and boiler are unfortunately separate and no spare wires to/fro. I digress; upshot is quite a bit of lag between boiler cut in/out.

[Martin W]

I have a thermal store with upper and lower thermostats, connected to a pellet boiler. At the moment the thermostats are redundant because the boiler stat has taken over. But this means that the boiler cycles up a lot more often then necessary. So I'm keen to install a relay to allow the tank thermostats to take back control, and I wondering if you would not mind directing me to the wiring scheme you found for this, or let me know how you did it? Many thanks...