Mitsubishi Ecodan split buffer location

[Strand]

Hi,

I'm looking some advice on our ASHP installation. I think that the buffer is in wrong location to work in my setup.

Have a look at the plan and tell me waht you think.

Its Mitsubishi Ecodan Zubadan 12 kW EHST20D-YM9D/PUD-SHWM120YAA with 2 zone box PAC-TZQ1-E.

Underfloor  heating is about 35m² and radiators 73m² (second floor will be added this summer and is 50m²). Zone 1 has wireless temperature controller/remote from Ecodan. Buffer tank is 120L.

Edited April 16, 2022 by Strand

[JohnMo]

A buffer would normally sit between and be connected to both the supply and return.

 

But can also be done the way you have draw it, the buffer should be vertical and the bottom is connected to the heat pump as drawn.  Your way adds system volume to the whole system and/or to the zones that heating.

[Strand]

But this buffer location will not help against short cycling. That was the reason I wanted them to install it.

[joth]

13 minutes ago, Strand said:

But this buffer location will not help against short cycling. That was the reason I wanted them to install it.

It adds volume to the system, so will reduce short cycling. The benefit of inline is it requires one less pump. The downside is you can't charge up the buffer during off-peak/high PV generation without also circulating water through the emitters. (The ecodan controller has a "smart grid" mode specifically designed for precharging the buffer like this)

(I guess you could have manifold actuators shut off the UFH loops and an automatic bypass valve to allow for this. This is what I'm vaguely considering myself, but it's a bit off piste)

 

[JohnMo]

3 hours ago, Strand said:

But this buffer location will not help against short cycling. That was the reason I wanted them to install it.

As mentioned above, for a boiler or heat pump to run for a decent period of time i.e. not short cycle, the heating system requires a given volume.  The boiler or heat pump will switch it's self off, when the return temp is too close to the supply temperature - delta T is no longer within the prescribed limit.  This happens quite quickly with a system volume that is too small.

 

The boiler or heat pump doesn't care how the downstream system is configured, as long it's working volume is adequate or in the case of a hot water cylinder there is enough volume in that to move the heat away and the return temperature is within a defined range.

 

Your system as drawn is really a hybrid of a heating system and cylinder.  There should be no reason why it would not work.  The other option is to instal the conventional way across the supply and return and then split the zones after the buffer, but this would make your two zone box a single zone box.

 

One thing I thought about while writing this, was if the buffer (as drawn) had a coil in it you could run cold water through it, to pre heat the water before it enters your hot water cylinder.

[Strand]

At nights I have set up a zone1 room temp drop from 22 deg C to 19 deg C. When that happens the pump starts to short  cycle. I think it's  because both returns come from the tank. And as zone 1 water return  temperature was higher than zone 2 it will still get the higher return temp.

 

Shouldn't  at least the pipes on buffer be different? Return  from the zone's  should be at the bottom and from the buffer to the 2-zone box be at the top? 

 

 

2-zone box has it's own two pumps  and a 3-way valve  for zone 2. It's built like a pass through even if both zone pumps should be off there is still flow in the main lines between the pump and 2-zone box. 

 

No coil in the buffer. Cold water straight to the ecodan 200L DHW tank.

 

[JohnMo]

So is zone 1 radiators? What flow temp

Zone 2 is that UFH? what flow temp?

 

When you say short cycle, how long does it run for?

 

A more detailed drawing may help that shows what you have and the flow temperatures

[JohnMo]

Do you have lots of tcv/thermostat valves in the system?

 

Zone 1 temp reduced, flow stops. If there are tcv in zone 2 closed also, you then stop the flow back to the boiler and the boiler has nothing to work with, so switches on off.

 

Thoughts?