What is the 3rd pipe for on a VRV

[viamphie]

Hi guys first post ( and probably the last) so if this sounds silly be kind.

I'm quite new to the industry only have hands on experience. Ive installed a few VRV and VRF systems and I was wondering why Mitsi use 2 pipes but Daikin use 3. Anyone point me in the direction of where I can find more info. What is the 3rd pipe for? and is it better than a 2 pipe system. Clearly both systems work as required through a switch box ( BS BOX ) so I'm a little confused.

[Brian_UK]

Hi, with a two pipe system it can only operate like a basic heat-pump; it can either cool or heat, it can't do both at the same time.

With a three pipe system you can provide cooling, heating and heat reclaim.

I'll dig up a link for a tech book, be back soon...

[edit-1]Link to Daikin US site, some downloads of tech manuals for VRV and BS boxes.
[edit-2] Better put the links in this time -DOH!

http://www.daikinac.com/commercial/d...c=docs&page=28

and

http://www.daikinac.com/commercial/d...c=docs&page=58

[The Viking]

Viamphie,

As Brian said, for most manufacturers the difference is if it's a heatpump or a heat recovery(simultanious heating and cooling) system.

BUT...
You mentioned Mitsubishi (Electric), they are the exception. They are doing with 2 pipes what (to the best of my knowledge) everybody else needs 3 pipes to do, provide heat recovery.

They do this by getting the outdoor units to feed a high pressure vapour/liquid mixture down the "liquid" line. This goes to an accumulator in their BC boxes. The indoor units that requires cooling then takes liquid from the bottom of this accumulator whilst the units in heating gets vapour from the top of it.

Cleaver kit...

[viamphie]

wow thats a lot of reading.......... thanks I'll get back to you in November lol.

Its the heat claim bit I'm not sure of..... I know that unit 1 can cool while unit 2 heats due to the bs box changing the flow

lets hope I can get through tose manuals quickly

[viamphie]

vapour/liquid mixture down the "liquid" line. This goes to an accumulator in their BC boxes. The indoor units that requires cooling then takes liquid from the bottom of this accumulator whilst the units in heating gets vapour from the top of it.

Cleaver kit...

thats just changed my perpsective on the system. so would i be right in saying that if the unit takes the liquid the system returns down a different path than it would have if it had taken vapour....... hence 3 pipes

[The Viking]

My previous post should now make more sense...

[Thermatech]

For Mitsubishi City Multi VRF heat recovery.
Think of a very simple system.
1 outdoor unit, 1 B/C distribution box & two large indoor units of equal size.

If one indoor unit is flat out cooling & the other indoor unit is flat out heating the system is in full heat recovery mode & the cooling unit becomes the system evaporator & the heating indoor unit becomes the system condenser.
The outdoor unit shuts the outdoor coil with solenoid valves & only the compressor operates to pumps hot gas to the B/C box which in turn sends the hot gas on to the heating indoor unit(condenser).
The refrigerant exits from the heating indoor unit as a warm liquid & returns to the B/C box.
At the B/C box the warm liquid is diverted through a subcooler circuit to eliminate any bubbles in the liquid.
Now the subcooled liquid is sent from the B/C box to the cooling indoor unit LEV valve & through the indoor coil (evaporator).
At the exit of the cooling indoor unit the SH vapor goes back to the B/C box & gets dumped into the suction pipe & back to the outdoor unit compressor to repeat the cycle.

Full heat recovery from the unit cooling to the unit heating & only 2 pipes needed with only the compressor running at the outdoor unit to pump the refrigerant.

Ofcourse the system has to cope with any % indoor units cooling & any % indoor unit heating. When more units are cooling than are heating the system runs in 'cool main mode' & the outdoor unit sends a hot gas / liquid mix to the B/C box. In the box is a gas / liquid seperator which is just a large horizontal tube. The liquid falls to the bottom of the tube by gravity & the hot gas remains at the top. The hot gas then gets sent to the heating units & the liquid is sent to the cooling units.
The clever bit is how the outdoor unit controls multiple outdoor coils to produce the hot gas / liquid mix.

[viamphie]

brilliant explanation.... makes it easier to visualize in my head thanks

[sinewave]

Only trouble with these increasingly 'Clever' systems is there's even more to go wrong and invariably it's harder to diagnose thus leading to ever increasing repair bills for the hapless customer!

[The Viking]

Only trouble with these increasingly 'Clever' systems is there's even more to go wrong and invariably it's harder to diagnose thus leading to ever increasing repair bills for the hapless customer!

Ahemm,

I do agree with you, KISS is my motto.

But the 2 pipe concept in the "City Multis" is one of the oldest (still operational) VRF solutions out there. So nothing new I'm afraid.

[Thermatech]

We started using City Multi In the UK in 1992 & it was already a fully tried tested & established product in Japan.

Essentialy it is still the same but has had 7 major upgrades progressing from R22 through R407c & now R410a.

Thats always a good thing with the Japs ,,, they develop new ground braking technology & then constantly develop & improve.

Daikin VRV , Hitachi Set Free are basicaly the same original concept but gradually improved over the years.

So lots of what we first learned about VRV / VRF in the early 90's is still good for todays product.
I still use an early 90's ME remote contoller as a service tool because it still can communicate with any City Multi system made from 1992 untill 2008 because they still uses the Mnet control system.
Last week I was using my Daikin remote controller form the mid 90's to set addresses at the indoor units for an I touch central controller because it still works on the latest VRV111 systems.

Try out that concept with the Koreans or the Chinees manufacturers.

Some of that oridginal R22 VRV / VRF equipment is now being replaced with the more efficent R410a kit.
The endusers realise that equipment has saved vast amounts of energy over the years & any slight increased cost for service & repare is worth while in order to capitalise on the energy savings of heat recovery.

[Peter_1]

This is a good link for AC selection and working principle in general http://www.3dair.co.uk/download-files/r407c-guide.pdf

[viamphie]

Cool !!!!!!!!! ( no pun intended )

loads of more info cheers guys.

[marc5180]

Great explanation Thermatech, thanks.

[Obi Wan]

What is the 3rd pipe for?

I always thought it was the drain pipe

Obi Wan

Hello everyone, its been a while.

[Peter_1]

Hey, welcome back Chewy..euhh..Obiwan.
If you say it's the drainpipe, then you're probably right, you can know it.

[Obi Wan]

But seriously, on the Daikin heat recovery you of course have the liquid pipe and suction gas pipe, the third pipe can be either suction or discharge gas. If the system is operating in heating or heat recovery mode than the 3rd pipe will be discharge gas, however if all the indoor units are in cooling mode than it will be suction gas (double suction).

Obi Wan

Hi Peter, Trust you are keeping well. Did you get to the bottom of your little problem yast year?

[Peter_1]

Yeah, thanks with your help and the girl we both know.
Some wondered how I knew all this. It became the largest dossier ever I made for the court, 277 pages included many copies of course but my dossier was +/- 88 pages.

The judge followed me almost for 100% and the damage I calculated for the end-user was approximately 70% to 75% of the initial installers price.

[Obi Wan]

That’s quite a result, glad I could help

Obi Wan