Has anyone done a VRF system serving various zones using say heat pump VRF units and then providing outside ventilation air from a separate dedicated unit serving the same various zones? Is there any merit to this concept? I know that VRF units can be very efficient and quiet as well as some newer units can provide 100% ventilation air with ducted distribution to the individual zones of the VRF system.
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Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5868 thumbs up
March 5, 2013 - 9:51 am
We are working on a project now doing just that. The benefit is the same as any system with a dedicated outside air system. You can control the ventilation air separately and utilize energy recovery and/or economizer. The DOAS can be set up to handle the latent loads so the VRF only has to deal with the sensible loads. Typically there is a significant energy savings with a DOAS as well.
Robert Sutton
P.E., C.E.M, President/OwnerSutton Engineered Systems, Inc.
March 5, 2013 - 9:59 am
Yes thanks. I am actually thinking of VRF outside air units that have reheat from refrigerant and can control leaving air in all entering conditions without and energy recovery. A dedicated source of conditioned outside air with a unit that delivers that air to each zone served by their own VRF cooling/heating heat pump unit. These units come in sizes of 1200 cfm each. That can serve quite a few rooms. Exhaust air heat or energy recovery is so expensive to install and purchase. This is a lot less costly.
If the energy use from these variable speed units that use compressor technology that has unloading features then across the board they could use less energy during occupied hours.
Don't get the DCV benefit but then that is not free either.
I would model against a base builiding system of splits with OA to each AH unit.
Scott Bowman
LEED FellowIntegrated Design + Energy Advisors, LLC
LEEDuser Expert
519 thumbs up
March 5, 2013 - 10:00 am
We also have several systems that are either based on VRF, or use a hybrid of GSHPs and VRF, both with DOASs. Most of these include ground coupled systems. VRF is especially useful in historic renovation as routing the refrigerant piping is much easier than water piping, and sometimes ductwork is not easy to run, so console type units are needed.
I agree that delivering the outdoor air right to each zone has a lot of benefits, including some reduction in ventilation volume needs, but does require additional ductwork in some applications, so has to be built into the budget early.
Robert Sutton
P.E., C.E.M, President/OwnerSutton Engineered Systems, Inc.
March 5, 2013 - 10:02 am
Also - clarification:
I am in Florda, we don't use economizers. Too humid.
Robert Sutton
P.E., C.E.M, President/OwnerSutton Engineered Systems, Inc.
March 5, 2013 - 10:05 am
Scott- I agree but I am also saving a lot of ductwork in the base systems at each zone. All I would need is duct for the OA delivery. I get individual zone temp control, OA to each zone, minimal duct, low noise levels, etc.
Robert Sutton
P.E., C.E.M, President/OwnerSutton Engineered Systems, Inc.
March 5, 2013 - 10:10 am
Marcus
I was concerned about the model of DOAS with a constant air volume at all time during occupied hours to all zones? Is that a problem? Suggestions?
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5868 thumbs up
March 5, 2013 - 10:25 am
A problem for modeling, for operations, for system design, etc. - could you be more specific?
Scott Bowman
LEED FellowIntegrated Design + Energy Advisors, LLC
LEEDuser Expert
519 thumbs up
March 5, 2013 - 10:29 am
I think you are worried about energy use. Since you are only providing required ventilation, and you have a ventilation effectiveness of 1.0, you are dramatically reducing your volume of air being delivered. Using heat recovery and other tools you mention to reduce energy going into this airstream, and if you can size ductwork a little generous, you should be able to keep the energy use down on this system. We have not had issues when able to use this kind of system meeting very high performance goals.
Robert Sutton
P.E., C.E.M, President/OwnerSutton Engineered Systems, Inc.
March 5, 2013 - 10:31 am
I suppose I am getting ahead of myself. What I meant is to model a DOAS system typically would involve some assumptions. If we deliver full OA continously to all zones are we overventilating and could maybe get a credit for that? And, if we shut off OA during occupied hours and use VRF to maintain low setpoints during unoccupied - we could get a credit for that.
Most DOAS are not able to vary flow to zones, just not a great or successful thing to do VAV for an OA unit. So I am just planning as stated above to do the full OA at all occupie hours.
I also get the benefit of low HP fans, no duct losses for the zones, low noise. Do all those things have a place in an analysis and a credit/
Robert Sutton
P.E., C.E.M, President/OwnerSutton Engineered Systems, Inc.
March 5, 2013 - 10:38 am
Scott - I am not even considering ER in the incoming OA air stream. Costly and requires extensive ductwork to make that happen. Plus larger fans and HP associated with it. KISS principal really. Have you done this on any buildings? Like I said to Marcus, the overventilating could be good, but not maybe worthy of a credit, and the low noise I am not sure. The rest of it for energy use would definintely be lower than split systems with OA direct.
Scott Bowman
LEED FellowIntegrated Design + Energy Advisors, LLC
LEEDuser Expert
519 thumbs up
March 6, 2013 - 12:44 pm
We almost always use a wheel type energy recovery unit, with the ability to temper the air being delivered directly to the spaces. There is not significantly more ductwork as it all goes to one unit, and exhaust is more generalized than supply. We feel the energy savings are more than the fan penalty.
Robert Sutton
P.E., C.E.M, President/OwnerSutton Engineered Systems, Inc.
March 6, 2013 - 12:54 pm
At $3.50/cfm these unit are expensive to purchase and maintain. I have used them too but I am trying not to add unnecessary costs that will likely get scrapped during pricing by the contractor/owner. So, to keep it easy and cheap, I am tryng something a little different, that's all. If I have a building with no real exhaust other than toilets then the rest of my exhaust has to be relief air. At 85% of OA supply maybe. So, I have ducts and grilles for the exhaust side and for the supply side too. Otherwise, I can let the building relieve by gravity with one outlet and a damper.
Trace 700 has VRF systems in it now and I was just thinking of modeling that for the zones/AHU and the OA too. That was my question. Has anyone modeled this and had a CIR or sucess doing it to show significant energy savings?
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5868 thumbs up
March 6, 2013 - 1:28 pm
I am not sure why you have to over-ventilate and you certainly can vary the flow while recovering energy (DCV systems come to mind).
We have modeled VRF and shown significant savings. We have modeled DOAS and also shown significant savings. We have modeled energy recovery with mixed results. Sometimes the extra fan power out-weighs the energy savings from the energy recovery. The relative amount of savings will depend on the baseline system and the specifics of your project. Hard for me to say whether you will get significant energy savings as I am not familiar with your project. The questions you are asking is why we do energy modeling - to find the answers during design. While there are generalities, each project is a somewhat unique situation.
Model the system you describe and then add the alternatives you wish to evaluate. Everything you bring up has a place in the analysis/evaluation of the optimal system choice.
Robert Sutton
P.E., C.E.M, President/OwnerSutton Engineered Systems, Inc.
March 6, 2013 - 1:44 pm
I suspect that providing full time OA from a 100% dedicated unit will tend to overventilate. The reason I say that is that you can't do VAV or DCV with a 100% OA unit. Shutting dampers and riding fan curves is not good for most systems unless you have chilled water coils that can react to that. I am using a DX VRF unit for 100% OA that is ducted. I can't shut off the air to spaces so it may be that I overventilate during low occupancies since one OA system may serve several rooms from that one unit. Each room will not have DCV capability in that setting. So, I may overventilate at times.
My building will have a rooftop DX HP for baseline and either ducted DX split system heat pumps or this VRF system with non-ducted zones and ducted OA system to send OA to the zones.
I agree that I have to do it to see and am suggesting that since I have not done it and need to sell the concept to others on my team, that I don't want to waste time and money doing it if it doesn't or won't fly. Only getting paid so much for the analysis and modeling anyway. If we can get certified by less complicated means like super high efficiency units and GBCI wants to rake me over the coals to get this VRF study done then it could be senseless to attempt and delay the certification. If others have done it with success that is good. I just wanted to see if there were any CIR responses or negative feedback on this VRF system concept since that is what I need to know. It will still comes down to me selling it I suppose if it will produce significant savings and not delay approval.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5868 thumbs up
March 6, 2013 - 2:44 pm
Sorry if I misinterpreted your original question and took it to DOAS land. At least you are getting paid something to do your analysis unlike my volunteer time here! :-)
GBCI won't have a problem with your modeling a VRF system. We have done so in eQUEST which requires a work around so modeling it directly in Trace won't be an issue. I have no negative feedback on a VRF system. When they first came out I was highly skeptical but they model out very well and the one project we have followed up so far has very low energy use, in part due to the VRF system. We are in Pennsylvania so not sure how climate will affect the results.
Robert Sutton
P.E., C.E.M, President/OwnerSutton Engineered Systems, Inc.
March 6, 2013 - 10:53 pm
Volunteering is always unrewarded. Thanks for what you do.
And thanks for the information and confirmation on the VRF systems. I think after checking that some manufacturers don't have all of the data files ready for this in Trace. I am checking still on what they have and it appears even with the data available the setup in Trace is not perfect. I have some doubts about the OA piece too. Modeling the oa unit in all weather conditions could be tricky compared to loads of the zones since the units have to maintain neutral discharge air to the spaces. That come with some effort, refrigerant circuit reheat, variable speed, etc. The curves for that may not even be done.
Did you do 100% oa on any of your models? Any suggestions?
Robert Sutton
P.E., C.E.M, President/OwnerSutton Engineered Systems, Inc.
March 7, 2013 - 7:56 am
Marcus
I am going to email you a copy of one manufacturer's instructions for TRACE setup to show you what I mean.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5868 thumbs up
March 7, 2013 - 9:37 am
We are not experts on Trace. How we do it in eQUEST would not be too helpful. I would suggest you contact Trace customer support or post your issue on the Trace user's group at onebuilding.org
Robert Sutton
P.E., C.E.M, President/OwnerSutton Engineered Systems, Inc.
March 7, 2013 - 10:02 am
Marcus
Thanks again - I will check that out.