Seasonal bi-directional HVAC ductwork?
I’ve been doing a lot of searching trying to find a super efficient form of ducting that changes its path based on heating vs cooling and have had no luck. I’m in central NY state (Albany) where we have a fairly even split of cooling vs heating. I’ve always heard supply vents up high and return vents down low for optimal cooling and the opposite for optimal heating. I have a geothermal heat pump and am currently in the middle of putting on an addition and completely redoing the existing structure so I have a decent amount of flexibility in redesigning the hvac (I’m doing the work myself too). To me it seems the best thing to do would be to size ducting of the same size going to both a high position and a low position in each room and then have a set of diverters to redirect which set of ductwork is supply vs return based on heating vs cooling mode. Thoughts?
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I think you might be overthinking this. I would read through this:
https://www.priceindustries.com/content/uploads/assets/literature/engineering-guides/air-distribution-engineering-guide.pdf
The challenge is more about air flow distribution and mixing than register placement. Pick the right registers, have some adjustment on them, and make sure there is both a high and low return.
If you were to try this, I’d use electrically actuated dampers so that your system would automatically do the switching for you. The easiest thing to do is probably to arrange the dampers to be powered when the cooling mode is active, and use the spring (unpowered) position for heating. A common manufacturer of these actuators is belimo, but I like the Honeywell ones better. Both are available used for reasonable prices. I’d use 24v actuators here to make it easy to do the wiring. You’ll probably need a relay and a second transformer to power the actuators, I wouldn’t trust the thermostat contacts or the furnace’s 24v transformer to be able to handle the additional load of several of these actuators.
All that said, I’m not sure you’re going to really see any noticeable difference with the fancy system. Usually just providing crossflow (supply low on one side of a room, return high on the opposite side, or vice versa) will stir the air enough to keep the room comfortable in all seasons.
Bill
Air velocity is higher for supply ducts than it is for returns, so for the same air volume the return duct needs to be bigger. So in order for your ducts to be reversible both would have to be sized as returns.
I think this is a solution in search of a problem.
I think this is a solution in search of a problem.....I like that one. Yup, a lot of design consideration is needed to get the proper air flow. Reversing it, or one size fits all may not work out so well.
If you are just considering the one room you are adding. Two returns one high one low, each one appropriately sized as if it was the only return and teed together, or halve them and split the difference. Two or more supply ducts, once again appropriately sized, high and low. Then seasonally closing the appropriate full sized return vent should give you a good mix or just use the two half sized returns as is. The same approach may work in the rest of the house.
Thanks everyone. I'm still working on my final solution but I appreciate everyone's comments. Most likely I'll end up with something more traditional.
I don't recall the exact figures but the increase in heat pump efficiency between the two was worth thinking about. I expect that only a manually switched system would have a chance at a positive ROI.