Air to air question.

[Beelbeebub]

The traditional A2A split unit has a AI vke outdoor unit (with compressor) and a singke inside unit with a flow and return line between them and also (usually) a condensate and power/data cable. 

 

Each inside u it has a corresponding external unit. In effect each one is stand alone. 

 

There are multi splits. 

 

These have several indoor units and one outdoor. 

 

Crucially they all appear to have one flow and return (plus date etc) lineset to each indoor unit. The number of indoor units is fixed by the number of "branches" on your outdoor unit and you have to pipe a radial system with lots of pipes going to and from your main unit. 

 

Is there a resin why there are no systems where the I door units can all be connected to a common flow and return, much like a wet central heating system?

 

Although the flow would be live to each indoor unit, a valve could prevent any gas flow into any "off" units so the heat output would be minimal. 

 

It would seem this method would make the piping of such whole house systems with potentially 8 or more indoor units, much easier. Just pipe to unit 1, then on to unit 2 and so on. Obviously the units would need to be designed for this. 

 

Is there a reason why this isn't a thing? 

 

What am I missing? 

[PhilT]

3 hours ago, Beelbeebub said:

Is there a resin why there are no systems where the I door units can all be connected to a common flow and return, much like a wet central heating system?

http://www.cool-info.co.uk/multi_dx_systems/Multi-split Systems.html

This appears to show five different setup variations based on a common central heating style flow/return rather than individual dedicated flow/return lines

[akjos]

What you're describing is exactly what is being used in commercial settings for many years: VRF Systems, i.e variable refrigerant systems. You'd have experience them most probably in hotels and similar.

Essentially there are big condenser units and then there is a main trunk with both hot and cold refrigeration lines branching to each unit. The cool thing is they can even balance themselves out, i.e if one unit needs cooling, and another heating, they can work between each other as condenser and evaporator and not use the big outside unit. Really cool stuff, but overkill for residential for sure.

Edited August 19 by akjos

[Beelbeebub]

I'm aware of VRF systems and thry are overkill for domestic situations. 

 

Which is why I was wondering if there was a simplified version for domestic systems. 

 

The need for simultaneous heat and cool is small in domestic situations (the possible exception being DHW production in summer) 

 

What would be useful in domestic situations (heating particularly) would be the lower pipework requirement of a "series" system vs the current "radial" systems (aka multi split)

 

For example a typical upstairs would have 4 or maybe 5 rooms to heat. That is towards the maximum of most domestic multi splits and would require 4 or 5 pairs of pipe to be run from the outside unit to the top floor. 

 

If a single pair of pipe could be run to the first unit. Then on to the second and so on it woukd reduce installation cost and disruption. 

 

I would envisage the indoor units would come with 4 ports as standard, an in pair and an out pair. The out pair would be blanked off with plugs by default and the pipework (basically a post of tee connections) would already be in the unit, avoiding the need for onsite brazing. 

 

This would reduce the skills and material for install. It woukd also make adding an extra unit very simple. 

 

As adding extra units would be simple there would be less need for a high power density in the units. They could be simple 1kw units with basically a finned tube radiator, simple fan and a valve making them cheaper. If your room needs more heat input, just add another. The lower power density would also mean lower air flow requirements thus quieter. 

 

Is there a practical reason why this system doesn't exist? I don't think the system would need variable refrigerant volume or any fancy valves beyond the normal reversing valve in the outdoor unit and a open/close valve on each individual unit to shut off any hot gas flow into the coil if that room doesn't require heat. 

[Archer]

Just to add that we have a 14kw residential scale VRF system (Mitsubishi Electric). It's a City Multi ducted A2A system so probably designed for small retail or office units primarily but  works fine - we have 2 indoor units running off a single outdoor unit with the intention to add a 3rd in the future which this gives us the flexibility to do easily. 

 

There is an indoor junction box which controls refrigerant flow between the indoor units and the outdoor. It was about £500 more expensive than the equivalent non-VRF multi-split but has a lower operating noise (not sure why). 

[Beelbeebub]

My understanding is that VRF system have branch boxes that contain some valving - which is fine but extra expense 

 

My "proposal" was for the pipework to be dead simple. Effectively the same layout as a 2 pipe wet central heating system. 

 

The major difference would be the indoor units would have a valve roughly analogous to a TRV but electrically controlled that woukd either allow gas to flow through the finned coil (and turn fan on) or not. 

 

If you want to add a second indoor unit to a room all you need too is tap into the existing flow return and wire up the electrics. More or less the same as adding a radiator to a room or even extending the system on to a new room. Rather than having to run a new flow/return set from the outdoor unit (assuming you have enough spare ports) 

 

Was really just an idle thought on how we could make installing A2A systems easier and quicker (thus cheaper). 

 

[Archer]

I think in principle that should work, the larger VRF systems are hideously complex (any large scale MEP), but we had the same need - to separately control each zone/ indoor unit and to have a single outdoor unit (mainly because of space and noise issues). It was surprisingly hard to find a unit that ticked all of the boxes. Many of the multi-split's seem to work off a single control (ie. you can't adjust each flow separately). The VRF branch box is a neat solution - hopefully it won't turn out to be hideously unreliable or hard to maintain. It looks quite straightforward and the piping for ours at least was very simple. 

 

Just seems that there isn't really a market for smaller VRF / individually controlled systems at the moment, hence the lack of choice.