UFH Buffer Vessel Sizing

[Dan F]

I've seen various people are on the forum using buffer vessels of around 70L, but is there any formula/method that can be used to calculate/estimate the required size?

 

The reason  I'm looking at this is because we need to decide on ASHP size.  We only really need max 6kW for heating demand, but a larger ASHP would be preffered for DHW.  The issue with a larger ASHP (say 10kW) is that it's not going to be able to module down so much and there is a risk of cycling if the buffer vessel is too small.  So I'm wondering how I would work out what buffer vessel size would be best for a 10kW ASHP (assuming it can only modulate down to 5kW output) if UFH flow temperature is 28C and total heating load is for the house 2.9kW.  

 

I assume ASHP flow temperature also has something to do with it, as a bussel vessel with need reheating less if it's at 45C than if it's at 35C, meaning less cyclig?

 

 

 

 

[joe90]

I have a 4KW ASHP fir both heating (UFH) and DHW and it copes fine. I have a buffer tank fir the UFH but some here do not, not sure I really need it but it means I have  spare capacity to heat the floor Rather than wait fir the ASHP to cycle up to temp and it has an immersion for backup. Mine is 80litre. I heat both tanks to 48’ and manifold blends down to 25’.

Edited May 22, 2020 by joe90

[ProDave]

Mine is a 5kW LG ASHP and I run it with no buffer vessel. I don't see any issues at all.  My max heating demand when -10 outside is just over 2kW

[joth]

@ProDave do you have a separate pump for ufh circulation and a blending valve?

This is the setup our ASHP installer has recommended (8.5kW ecodan, no buffer or LL header) and I'm wondering what happens when the ufh thermostat drops below setpoint so is "calling for heat" but the ASHP not on or is charging the cylinder? The ufh pump will be pushing against a closed valve??

 

So do you need to lock out the ufh circulation pump to only run when the ASHP is active for that zone?

 

Put another way if we wanted to run the ufh pump all day to balance temperature of rooms (like @Jeremy Harris) we have to have a low loss header, right?

 

[dpmiller]

the UFH pump only circulates in the loops,  when the TMV is closed the UFH is pretty much isolated from the ASHP circuit.

[ProDave]

42 minutes ago, joth said:

@ProDave do you have a separate pump for ufh circulation and a blending valve?

This is the setup our ASHP installer has recommended (8.5kW ecodan, no buffer or LL header) and I'm wondering what happens when the ufh thermostat drops below setpoint so is "calling for heat" but the ASHP not on or is charging the cylinder? The ufh pump will be pushing against a closed valve??

 

So do you need to lock out the ufh circulation pump to only run when the ASHP is active for that zone?

 

Put another way if we wanted to run the ufh pump all day to balance temperature of rooms (like @Jeremy Harris) we have to have a low loss header, right?

 

The UFH circulating pump only turns on when one or more zones is calling for heat.

 

There will be a short period when it turns on before the actuators have opened (hot wax actuators take a minute or 2 to open) when it will be pumping into a dead end but it does not seem to bother the pump.

[PeterW]

1 hour ago, ProDave said:

The UFH circulating pump only turns on when one or more zones is calling for heat.

 

There will be a short period when it turns on before the actuators have opened (hot wax actuators take a minute or 2 to open) when it will be pumping into a dead end but it does not seem to bother the pump.

This is why the motorised auto balancing actuators are good as they respond in 3-5 seconds. 

[joth]

Thanks all!

 

3 hours ago, dpmiller said:

the UFH pump only circulates in the loops,  when the TMV is closed the UFH is pretty much isolated from the ASHP circuit.

 

Yeah think I got that bit fine --  it's the opposite case, TMV is open (drawing in from the hot flow source rather than bypassing) but the ASHP isn't feeding the UFH zone (e.g. because it's being used to heat the UVC, or it's forced off during peak-rate price surge) that was my concern.

 

3 hours ago, ProDave said:

The UFH circulating pump only turns on when one or more zones is calling for heat.

 

There will be a short period when it turns on before the actuators have opened (hot wax actuators take a minute or 2 to open) when it will be pumping into a dead end but it does not seem to bother the pump.

 

Got it. So in summary, using a bufferless design and allowing 24/7 UFH recirculation are mutually incompatible. If we want the option of running the UFH pump when the heating is not on (e.g. to balance temperature in the very sunny south facing rooms with the cooler open plan north side of house, per JHarris's setup) we need to have a buffer tank of some sort. (And sizing it is the follow on question per OP)

 

Final question: am I right in using "Buffer tank" and "Low loss header" interchangeably? Or is there a technical difference between them?

 

 

 

Edited September 16, 2020 by joth

[joth]

4 hours ago, joth said:

 

Got it. So in summary, using a bufferless design and allowing 24/7 UFH recirculation are mutually incompatible.

 

Just found this was already discussed in depth last month over in this other thread, although I'm drawing different conclusion to there!