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Why not use a "ductless heat pump farm" for commercial buildings?

Created 11/29/2016 by Jack Zeiger
Updated 1/12/2017 by Jack Zeiger
130 views • 4 comments

OK, I'm going to show my naivete here and ask a stupid question. I recently moved from a fairly academic setting to a utility and am beginning to have to deal with the real world. One thing I’ve noticed, for instance, that I hadn't really before, is that, pretty consistently, smaller equipment tends to be more efficient than larger equipment. It’s even built into our codes. HVAC equipment is allowed to be less efficient in the larger size categories. This is somewhat counterintuitive. I would have assumed there was an economy of scale and that larger equipment, with bigger motors and more ways to control things, etc., you would be able to get some savings with larger systems. Apparently not. So maybe it’s cost? Wrong again. I am looking at a bid that is over $900,000 for a 40-ton VRF system. This is over $20,000/ton! I can buy small ductless heat pumps for $4,000/ton all day long. How can it make sense to spend 5 times as much for a system whose components are less efficient, and arguably little if any more efficient as a system.

I’m wondering, in this case, a 26,000 sf office bldg., if it would make sense to install, rather than a large VRF system, a “farm” of 8-12 5-ton multi-zone ductless heat pumps. You wouldn’t have building-level control, and there may well be times where one unit is in cooling and another is in heating, but if they are hooked to “connected” T-stats (BPA’s current term for web-enabled T-stats), they would be controllable remotely. If you zone it correctly, you should seldom need simultaneous heating and cooling from a single outdoor unit, and you should be able to do this for under $200K, right? What am I missing? Why does no one do this, or do they? I understand they do do this in Japan. Why don't we? Wouldn't this be cheaper and much more efficient than a bunch of standard rooftop units? We have pretty much eliminated ducts from homes. Is it time to start doing that with commercial, too? What am I missing? I'd love to hear thoughts from you smart people out there who have been dealing with HVAC longer than I have.

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Comments (4)
Travis Hardy on 11/29/16 on 02:44 PM (Pacific Time)

Stab in the dark here, Jack... I'm thinking it may have something to do with the power consumption of 8 separate multi-zone systems operating 5 compressors and 40 air handlers simultaneously versus a single, large VRF system.  I don't think the efficiencies on the DHP farm could be considered cumulative or even valid when compared to a single system (e.g. the bank of DHPs as a whole would not have a higher efficiency rating than a single multi-zone could). Considering the power consumption of 40 different units and 8 large compressors, I don't think the consumption of a 40 ton VRF HP system would even come close... like I previously warned, stab in the dark. 

There are also many disadvantages of the DHP system for this application, depending on end-use of the facility.  Is it indoor ag or industrial production?  If so, you have virtually no environmental controls (inadequate particulate filtration, zero fresh air intake, etc.) with the DHP system.

My guess on the $900,000 price tag is that it includes duct installation, environmental controls, and guaranteed efficiency and performance ratings, none of which would be guaranteed or offered as part of the DHP setup.

I'd love to hear other opinions and hear knowledge from others, as well... I will do some more research, as well, to support or contradict my assumptions...

David Bopp on 11/30/16 on 07:49 AM (Pacific Time)

Jack,

VRF is an amazing technology for many commercial applications because of its ability to shift heat around from one zone to another without ever engaging the compressor.  This is wonderful in applications where you have need to stay very closely around a set point and the way the building is built leaves some areas needing heat while others need cooling.  Schools are a great example of this where a class on the west side late in the day on a cold day could be overheated by solar gain while the east side rooms are freezing.  With a central system neither may be comfortable.  With separate dhp's they may both be comfortable but not as efficient as a vrf that would literally move the heat from the west side room to the east side room. 

If there is no appetite for heating and cooling at the same time the appetite and payback for vrf would be drastically reduced.  In which case a dhp farm may work well but it is going to come down to controls and zoning otherwise one person who wants to be 2 degrees warmer turns up there dhp which overflows into the next zone where they find it too hot and potentially turn their unit to ac to cool it down... battling thermostats with separate systems that have no intelligence to know what is going on or help find a solution. 

I am wondering if your cost figure for the VRF includes DOAS (dedicated outdoor air supply).  Any dhp/vrf solution will need doas to take care of indoor air quality and ventilation requirements for code.  Splitting the heat and ventilation into separate functions allows both to be better at what they do rather than compromising the efficiency of both to get one unit to do it all.  However, you would have the extra cost of the ducting, fans, hrv, etc in the vrf bid if it is a complete HVAC system bid. 

I would evaluate if doas is included in the vrf bid and then evaluate the reality of being able to control dhp's and isolate their zones well enough to prevent thermostat battles (whether across a floor or between upper and lower floors).  Make sure you are comparing apples to apples for final comfort and ventilation along with energy savings.  The first principle of low energy design is to make the occupants comfortable.  If don't achieve this they will make themselves comfortable at the expense of the energy savings that was designed in thereby potentially negating all designed savings and invalidating any payback. 

Dave

Fred Gordon on 12/01/16 on 08:21 AM (Pacific Time)

I am probably as naïve as you, but I think that large objects get stuck on roofs because they don't require as much design, even if it maximizes the length of duct work.  Just grossly oversize and you're good to go (except for cost to the owner and efficiency, which lose).   Design of a central ductwork system is "standard", and you can bid for design and lots of firms can pull a design off the shelf, tweak it, and bid low. 

Decentralized HVAC is getting more efficient and also reduces ductwork costs.   But owners have a pocket with money for equipment because that's what they've always pay for.  Owners hate paying for somebody to do something that's not standard- the designers get scared and ramp up the cost, it's tough to figure out who is expert enough to do specialized work.  Design is money with and no hard object to point to for it; owners and developers don't know how to know if they got their money's worth.   I know there are expert answers to all these questions, but owners, and developers, aren't experts, and their hired experts mostly know how to replicate and minimize design risk and cost.   It's tough to break something more complex or even just different into the system.   BTW the sheet metal guys don't mind running miles of ducts, the HVAC firms don't mind selling large oversized systems, everybody does okay.  With change, they're not so sure.  

Jack Zeiger on 01/12/17 on 03:11 PM (Pacific Time)

David, you make some good points. Yes, there's some possibility of dueling thermostats, and it's true that all zones will be rejecting heat or cold to the outside whether they are heating or cooling. I love the elegance of the VRF concept. I'm just not convinced that it works that way in the real world as much as the sales reps would like you to believe -- not enough to overcome 10-20% more efficient equipment and a cost of the system several-fold larger. Yes, the VRF system bid here includes DOAS, and you would have to figure that into the DHP solution. But again, we have a lot of money to play with here. Could you design a DOAS system for a 26,000 sf building for $200K? I hope so. If so, you're still half a million $$ ahead.

Thanks for chiming in, Fred. As usual, I suspect you are right on the money. Getting someone to try this is not going to be easy, and I'm sure there will be unforeseen difficulties, but I suspect they will be happy that they did and maybe we'll learn something in the process.

Any engineers out there willing to give this a try?

Does anyone know of a good controls system that would tie all of these single units together to be able to control them as a system?

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