Termovar Diverter Valve Question

[pine4ever]

First post here although I've been reading threads for a while. I'm putting together a system with unpressurized storage based on the Tarm layout below:

I'll skip the fossil fuel boiler for now but it will be nice to have the option to add it later if required. Other than that the only major difference is that the loads will be radiant slabs which will require a mixing valve ahead of P-1.

My understanding from the notes that came with the diagram is that ZV-1 open and C-1 runs whenever a zone calls for heat creating a flow through TV-2 that is regulated so that the feed into port 1 remains at the set point (around 140F). If it falls below that flow is redirected out port 3. If the boiler is running this gets made up by hot water from the boiler through TV-1. If not TV-1 will be closed and flow goes through storage.

I'm curious how well this last case will work with low temperature loads. I think C-1 and TV-2 are going try and keep the water in the loop supplying the loads at above 140F (which is higher than needed) and prevent the flow from port 3 to storage from being a cool as it could be (100F assuming a 20F delta across the radiant zones).

Does anyone see any problems with this? Would it make sense to use a diverter with a lower set point temperature (I don't know about the Termovar but the ESBE valves have replaceable elements down to 120F)? If anyone else has dome something similar it would be great to catch any gotchas before its all full of water

 

[velvetfoot]

tv-1 is boiler protection, I think. Water recirculates around the boiler without going anywhere else until it gets up to temperature.
When boiler is running, and the zone pump and valve(s) are open, water circulates through the zones, controlled by tv2 so that 1=x. Excess circulates through storage.
Tv-1 controls minimum boiler temperature for protection and Tv-2 controls the temperature of the water going through the zones.
That's the way I look at it.

 

[pine4ever]

tv-1 is boiler protection, I think. Water recirculates around the boiler without going anywhere else until it gets up to temperature.
When boiler is running, and the zone pump and valve(s) are open, water circulates through the zones, controlled by tv2 so that 1=x. Excess circulates through storage.
Tv-1 controls minimum boiler temperature for protection and Tv-2 controls the temperature of the water going through the zones.
That's the way I look at it.

Makes total sense to me. The control provided by TV-2 looks like it applies to the return flow so the supply temperature would depend on delta T across the zones. For radiant slab zones this will need reducing a bit. But either way the return flow from TV-2 port 3 will be quite warm with C-1 running on storage (boiler off); warmer than the return from the zones themselves I think. I was curious whether having that cooler water returned to the cold end of storage would be more efficient.