Pellet stove to buffer store circulation problems

[ChasF]

I have connected a 14kw pellet boiler stove to a 200 litre buffer tank. The original aim was to produce some lower temperature water for the underfloor heating circuit whilst feeding some rads upstairs. The boiler stove is fitted with a circulation pump on the return side. There is a 55 degree thermovar connected between the flow and return. I have not been able to get any flow through the tank and the stove just goes into the overtemperature alarm state and turns itself off.

There is an automatic bleed valve fitted to the top of the tank so I'm fairly confident it's not an airlock.

Any suggestions as to what can be done to encourage circulation?

 

[oz-plumber]

It could depend greatly on how you have set up the pump and the Thermovar valve.

If the stove is going into overheat, something is dangerously wrong.

Turn it off and don't use it until the existing problem is fixed.

 

[ChasF]

The stove closes down when the water temperature reaches 85 degrees although the pump continues to run until the temperature drops back to 60. The Thermovar is fitted as an anti condensation device in the return to the boiler. There is also a flow control (balance) valve fitted between the flow and Termovar. The pump is internal to the stove and comes on whenthe boiler temp. reaches 60 degrees. I don't know if the speed is adjustable manually but not in the software.

 

[oz-plumber]

Are you a Plumber?

There is an obvious design flaw in the setup you have tried.
Could be something to do with pump location and the thermovar valve.

If it is the way I read it - it will never work and hence your problem.

Like to help more.

Just don't use the stove until the system has been setup correctly.

 

[ChasF]

Yes I'm a heating engineer but have no experience of buffer stores. The system desgn is based on the information provided with the Thermador tank and I have taken advice from various other people who are perhaps more familiar with these than I am. I can't see any problem with the Thermovar valve. I'll post up a diagram of the system in a few minutes.

Edit: The system doesn't work so I can't use it!

 

[snowhead]

As above leave it turned off till the design is sorted.

Was the boiler existing, did it work OK before the Buffer was fitted?
Or is it newly installed, is it a new or previously used boiler?

Is the Thermovar part of the boiler, or just added into the external pipework?

Where did the idea for this come from?

Any pics of the install?

 

[ChasF]

The stove is new originally only connected to the rads upstairs. The boiler to buffer store connection is as per the Position 2 diagramon this page:

https://www.oilstoves.co.uk/webdocs/technical/Valves/LK825_TERMOVAR_922913.pdf

 

[snowhead]

Are you getting circulation around the by pass loop?

What's the balancing valve set at?

 

[ChasF]

Yes circulates around the bypass no problem. The balance valve is open but half closing it makes little difference.

 

[oz-plumber]

Are you sure you have installed the mixing valve correctly?

You have to have the pump pulling water through the mixed port.
The valve won't operate correctly if you are pushing water through the hot or cold side to the mixed side.

Stupid question..have you installed the valve correctly?

Hot to hot, cold to cold, and mixed to suction side of pump

 

[snowhead]

What controls are there for the boiler, it should be switching off before the high limit?
Is the pump still running when it's off on high limit?

What model stove , boiler is it.

 

[ChasF]

If I had the valve the wrong way round it wouldn't circulate through the bypass??

 

[ChasF]

The stove is an Extraflame Melinda Idro.

 

[ChasF]

This is a diagram of the system installed

 

[MC_Bob]

I'd be joining directly to the tank with the stove and a deperate pump setup for the radiators from deperate tapping. That looks dangerous.

 

[ChasF]

What looks dangerous?

 

[oz-plumber]

The fact that there could be a situation where the rads and underfloor system are not circulating, and the mixing valve from the buffer tank may close due to high tank temperature, and the stove will only circulate water though itself, at a reduced rate through the flow control valves.

 

[ChasF]

Thanks for the replies so far.

These are the diagrams supplied by Thermador with the buffer tank, as you will see my sytem diagram is very similar the only notable differences bein the location of the pump and the addition of the loading valve (themovar) which opens at 55 degC.

 

[MC_Bob]

Their diagrams show an unrestricted path back to the buffer. So they're very different .

 

[ChasF]

The problem turned out to be a blocked pump. I've not come across this before on a new install and the pump was a right pain to get access to.

It's all working fine now.

How would you set the two balance valves?

 

[oz-plumber]

I would set the balance valves fully open.

I wouldn't want much, if any, restriction to flow through the stove.

 

[ChasF]

The factory setting is speed 3. Once the pump was unblocked there was clearly too much around the thermovar bypass. The boiler tapping is about an inch from the top and the air bleed is fitted to the tapping rather than the top of the boiler so the pump suction was causing a lot of noise in the top of the boiler. I turned the balance valve down until all went quiet. The other vlave balances the flow through the tank with the flow through the rads. . At the moment it is fully open but I'm concerned that all this flow through the tank will reduce the temperature differential across the tank. As yet I've not been able to run the system long enough for the tank to reach its full temperature. I'm inclined to back it down a bit before I start but before I do that I'll make sure that at least one of the rads remains open.

 

[missplumb]

if you run the return directly from the buffer to the mixer valve then the buffer should heat up fully. your heating circuit is taking the heat out of the buffer and it cant reheat as it has to go through the radiators to create a circuit. Also i wouldnt have the expansion vessel on the flow or shunt circuit, the diaphragms are not designed to take a constant heat over 70 degrees and will fail.