Sizing A Buffer Tank for ASHP?

[Myatix]

Hi All,

I was wondering if anyone can guide me with regards to scaling the correct size buffer tank for me Vaillant AroTherm Plus 7kw ASHP.

 

I have a bungalow with 2 manifolds (16 pipe runs) underfloor Heating system which contains approx. 140L of water and need to scale the correct size for my buffer tank.

 

can anyone tell me how I can calculate the size of a buffer?

 
the ASHP heating system is designed for a flow temp of 34 degrees with a DT of 5 degrees and min outside temp  -12 degrees C.

 

 

 

[HughF]

Just ufh, no rads?

 

Ditch the buffer, not needed. Total efficiency hog.

[Myatix]

Hi @HughF,

 

Yes, the house is 100% UFH and I don’t agree with you not needing a buffer.

 

You need a buffer tank on UFH to stop the ASHP from cycling on and off due to the high flow rate on ufh systems which can cause hot water to be returned too quickly and shut off the ASHP as it believes the house is up to temperature when it is not resulting in cycling and eventual damage/ a shorter life span for the ashp. See below!

 

 

So back to the original question? How do you scale a buffer tank for an UFH system?

 

Edited October 28, 2022 by Myatix

[HughF]

Schematic 30261-1011 from the vaillant schematics booklet shows ufh without a buffer, along with a lot of other interest schematics that I encourage anyone designing with vaillant to look at.

 

If vaillant say it’s ok, then it’s ok.

 

The hydraulic separation of the two pumps sorts out the different flow rates.

 

If you do insist on wanting a buffer, again the vaillant schematics show 40ltrs as standard. The volume is only there to provide a defrost capability if you’ve got your ufh loops all shut down. The separation can be as simple as an llh, a plate, or ccts.

[Myatix]

Thanks for the input! That’s interesting, but what if you have 2 ufh manifolds (9 and 7 port) each with its own circulation pump? I have 1200m of pipe and 16 loops with approx 144L capacity my plan was to put a zone valve before each ufh manifold and then remove the 2 manifold circulation pumps and replace them with a single circulation pump before the zone valves? Also plan to use Antifreeze valves instead of antifreeze in the system as this will increase the cop and reduce the cost of adding antifreeze to the entire system.

 

Are you familiar with any Vaillant schismatic’s for a setup like this with weather compensation using the SensoComfort controller and external stat. The plan is to try and balance the flow rates and based on the HEatloss Calc I should be able to run the system with a flow temperature of 35C with  DT5.

Thanks for the input!