Too Diverse or not?

[scales]

Hi Ladies and Gentlemen

A simple question i hope for you skilled air conditioning people.

I have 2 quotes on my desk for a 3 pipe VRF/VRV system, the total cooling load is 56Kw spread over numerous individual offices.

The question is one unit is rated @ 18Hp with 130% diversity and one unit rated @ 24Hp with no diversity. Which one is correct and which one would you purchase?

Regards
Scales

[multisync]

Hi Ladies and Gentlemen

A simple question i hope for you skilled air conditioning people.

I have 2 quotes on my desk for a 3 pipe VRF/VRV system, the total cooling load is 56Kw spread over numerous individual offices.

The question is one unit is rated @ 18Hp with 130% diversity and one unit rated @ 24Hp with no diversity. Which one is correct and which one would you purchase?

Regards
Scales

Depends upon the layout of the building and the chances of the whole place being on full duty. a 130% diversity isn't too bad and not beyond reasonable. The main problem is during very cold weather rather than hot. Although that again depends upon layout and building fabrication. A newer build will need less heat that an old one so you may find it has plenty of spare even at 130%

[bigor_2]

24HP
This summer has shown that 130% diversity not work.

[lasket]

Personally I think 130% is cutting it far too tight, especially if the system is going to be your primary mode of heating in the winter. ie. no rads etc.
I generally keep my diversity below 110% unless I know very accurate info about the building layout, fabric and how the system will be used, ie will 15kw of the sytem always be on cooling in a switch room etc.

[james10]

Who is the manufacturer of these units? are they by the same of different manufacturers? some claim to be able to go to 200%

[scales]

Who is the manufacturer of these units? are they by the same of different manufacturers? some claim to be able to go to 200%
130% diversity is Mitsi heavy and 100% diversity is Hitachi bothe systems are the same price.

[scales]

130% diversity is Mitsi heavy and 100% diversity is Hitachi bothe systems are the same price.
Sorry the Mitsi heavy is no diversity and the Hitachi is 130% sorry for the error

[dannycool]

Hi

130% diversity seems a little close to the edge to me.
I think your system would probably struggle during high summer or during a bad cold snap.
But it may depend on the occupancy of the rooms your serving, ie:are they all in use at the same time?

Im guessing the potential installer has quoted it based on making indoor model sizes fit into an outdoor size.
I would bet that he hasnt allowed for any drop off in capacity caused by long pipe runs or defrost cycles.
My advise would be to shop around for quotes and report back with your findings.

Hope this helps

Regards

Danny

[al]

compare like with like,ask Hitachi to quote with no diversity and mitsu heavy with 130 diversity, i would suspect Hitachi have quoted this way to keep down the cost, happens a lot here esp with mitsu electric. I personally would not go with 130%, you have absolutely no redundancy if the temperature falls or rises excessively.


alec

[desA]

It would seem that the 24kW option, with no diversity, would give you the best 'bang-for-the-buck', if both options are the same price.

[Thermatech]

A common problem which I trouble shoot on VRF systems during winter months is perceived poor heating performance.
This almost always is due to over index / high diversity with the design being only based on cooling capacity.

It turns out that many sales engineers assume that if the system can do the cooiling duty then heating will also be ok.

A major problem any VRF system with 130% over index has is with maximum heating performance during very cold weather.
130% capacity of indoor units all requiring maximum heating but only 100% capacity outdoor unit to pump the hot gas to all those indoor units.

1/ The outdoor unit can only ramp up to 100% capacity so performance at all indoor units is reduced.
2/ The indoor units are the condenser coil & if the room temperature is cold then the system will be overcondensing & system performance will be further reduced.
3/ When the outside air temp gets lower than about 6 deg C then the outdoor unit needs to defrost about once every hour & heating performance is further reduced as the system is not heating during the defrost period.

So alot depends on your building & requirement for heating.
If the VRF system is the only form of heating in the building & the heat load calculations show that at +5 down to -5 deg C outside temp the heating requirement is over 50KW then the ac company need to show evidence that the system can provide the required capacity incuding all the correction factors for pipe lengths / defrost & -5 low ambient.

If you have an existing heating system in the building which you can use as backup during the colder winter weather then the problem is solved.

This last winter in the uk we had weeks of below zero weather conditions & this becomes a problem for any heat pump system which is slightly undersized.

[techguy]

The above are all correct it depends on the building design etc.

BUt if you go for the 100% option with no diversity. Your gaurenteed the system will operate when necessary even in extreamly cold weather such as was experienced last winter.

What most manufacturers will not tell you is that all electric VRF/VRV lose capacity in cold weather and because of defrost. Sometimes as much as 25-30%.

So if you go with the diverse option your already down 30% capacity on every indoor unit at full load. If this winter is similar to last winter you could be down 50-60% on each indoor unit.

so a 3kw indoor unit may only supply 1.2 KW of heat per hour. This may not be enough to start the indoor fans as most VRF/VRV hold off the fans until 35 degrees C is achieved accross the indoor units coil sensors.

Personally I would go for the 100% option Diversity should only be used in hotel situations never in office environments unless there is mixed use.
IE some areas such as meeting rooms which may not all require heat at the same time. As the main occupied areas.

[techguy]

Apologies to Thermatec.
I basicially repeated what he said. Should have read his post first.

He is quite correct Low ambient and diversity are a NO NO.

Stay with 100%

[scales]

This is getting tense with the suppliers all wanting the purchase order, heres one explanation the Hitachi (130%) guys are stating.

All rooms will never require maximum cooling or heating as the rooms will all be at different stages in temperature i.e. some will be close to required set point and require little cooling/heating while others may require full capacity, so the idea is the full capacity will never be required.

This is all getting very confusing???????????

[r.bartlett]

We did a MHI VRF 4 month ago, it self addressed and ran up a treat first time : never missed a beat since.

Stuck with your choice I see it as a no brainer.

[Makeit go Right]

Heres one explanation the Hitachi (130%) guys are stating.

All rooms will never require maximum cooling or heating as the rooms will all be at different stages in temperature i.e. some will be close to required set point and require little cooling/heating while others may require full capacity, so the idea is the full capacity will never be required.

I guess the kit has been bought and is on site by now. I wonder which way you went on this.

The supplier and installer normally confine themselves to the system in normal running operation and ignor startup. But if, say in an office, they switch off over the weekend (or even over night). Come the morning, a cold and frosty one, the system starts up and, naturally the condenser will frost a bit and then need a defrost cycle.....but the building has no heat in it to defrost the outdoor coil.

So, it can be a slow curve, warming up the building on a cold morning. Diversity would make things a little harder and longer. Nice for the girls, but not for vrv/f. :D