Weather compensation on a thermal store

[solveweb]

I am just starting out designing my own system - oil condensing boiler, wood stove, solar thermal, connected to a thermal store. I am a complete novice, but I am trying to get my head around it all! Please tell me if I am off my trolley!?!?

I have started looking at control systems... and I understand that general opinion seems to be that weather compensators don't make any sense because the boiler starts when the tank is cool and carry on until the whole of the tank is at full temperature, and for that in between period you can use a thermostatic blending valve, taking water from top and bottom of the tank, to get the optimum return temperature for the boiler.

However... I think I might have found a valid use for a weather compensator - But NOT by connecting it to the boiler, but rather by connecting it to the radiator feed FROM the thermal store. I don't know whether its possible... BUT... If you could control the blend temperature flowing from the thermal store to the radiators (again by combining two feeds one from top of tank one from bottom) and blend down to whatever temperature the compensator thinks is necessary, then surely this would be a good way of using a weather compensator to avoid unnecessary depletion of the heat in the store...?

Please tell me if you think I am off my trolley, and hopefully add why you think that?

 

[Onetap]

"I have started looking at control systems... and I understand that general opinion seems to be that weather compensators don't make any sense because the boiler starts when the tank is cool and carry on until the whole of the tank is at full temperature, and for that in between period you can use a thermostatic blending valve, taking water from top and bottom of the tank, to get the optimum return temperature for the boiler."

Weather compensator would be typically a 3-port mixing valve on the flow & return to/from the radiators. That works, done that.

You can also get 3-port (or 4-port) valves which are intended to maintain the boiler return temperature above 55 degC, to prevent flue condensation & corrosion. You may be confusing these two valve functions. This isn't needed by a condensing boiler, it's useful with a solid fuel boiler. You could also restrict the opening of the mixer to prevent the return getting too cold.

If the oil boiler has a modulating burner, the thermal store is more of a liability. If it's an on/off burner, or the heat demand is below the burner turn-down output, then the store acts as a buffer vessel, giving the boiler a sensible run time.

Where would you get a controller for this stuff?

Forget the stuff about two feeds from the top, one from the bottom; that's what mixers ( valves?) do.

 

[solveweb]

Thanks Onetap. I've read quite a few of your posts in other threads already and I am starting to get there. Please excuse me if I am still making far too many mistakes though.

You may be confusing these two valve functions

Yes I think I was. I'm a (keen) novice, sorry! Okay so making sure my boiler condenses all the time is simply a case of taking the coolest possible feed from my thermal store (i.e. the bottom). Is that right?

Weather compensator would be typically a 3-port mixing valve on the flow & return to/from the radiators. That works, done that.

Are you saying that its normal to be able to use a weather compensator to control output from a thermal store? And this is a valid approach? Which weather compensator would you recommend for this?

Where would you get a controller for this stuff?

I don't know. I was... kind of hoping you could tell me that.

Thanks[/quote]

 

[ALEC1]

google ESBE valves the have what you want...

 

[Dan Robinson]

Shame I couldn't finish my experiment on weather comp'd thermal stores using a 4 pipe boiler and stratification.


Damn sticky flow switch on the hot water control screwed up the test and wasn't found until I converted it all back to a standardish set up.

Maybe if I get a quiet Autumn I will revisit it.



Interested to see what Onetap has to offer on that idea:


Two stage heating of the store. Effectively turning the bottom of the store in to LLH. controlled by the WC circuit of the boiler; with the boiler's hot water sensor circuit monitoring the top half that is heated to the set point for producing hot water.

No physical separation - but the idea was to enable solar energy and any wasted hot water energy from smaller drawoffs to be used for some space heating.


In my case I was flummoxed by a bit of debris holding the store's hot water shunt pump on constant. Without any visual indicator I didn't know until I was working close to the store one day and noticed the very faint hum.

So obviously that was mixing the temperature layers in the store. Of course the heating system pump and the boiler's pump could have the same effect and the idea would still be a barmy one.

Still. All in a good cause of education I suppose.

 

[solveweb]

google ESBE valves the have what you want...

Thank you. The series 90C controller looks like its designed for exactly this purpose.

 

[Onetap]

google ESBE valves the have what you want...

Second that.

 

[Onetap]

Okay so making sure my boiler condenses all the time is simply a case of taking the coolest possible feed from my thermal store (i.e. the bottom).

Yes, why would you take the boiler return from anywhere else?

Are you saying that its normal to be able to use a weather compensator to control output from a thermal store? And this is a valid approach?

It's just a variable/modulating flow temperature circuit, a standard detail, used for decades for space heating. It's only recently that the availability of cheaper electronics has made suitable controllers available for domestic users.

The other options are;
a constant temperature circuit (in which the flow temperature is whatever the store temperature happens to be, possibly dangerously hot) with control of the room temperature by on/off control of the pump by a thermostat;

or a constant temperature circuit in which the flow temperature is regulated by a thermostatic blending valve and thermostat room temperature control as above.

Weather compensation has many advantages over both the above, but costs more.

Which weather compensator would you recommend for this?

I don't know. I was... kind of hoping you could tell me that.

I don't know either. I had the luxury of controls contractors and BMS controllers. I've used obsolete BMS controllers for my own systems, but it's not something you could sell to a customer. You need to be able to program them. The manual is the size of a few telephone books and I read it. I have work in progress on a new controller, but just for my own interest.

As DanR said, the other function of the store is a neutraliser/hydraulic disconnector/Low Loss Header so that running the oil boiler does not induce flow in the wood burner.

I've never been convinced that you could get a useful temperature difference from stratification in a thermal store but will await the outcome of DanR's experiment with interest.