HVAC selection help
My new house is under construction and I need to finalize my HVAC system soon. Some details:
Zone 4, 3800 sq feet (About 1900 first floor, 1550 second floor, 350 room over the garage), crawl space (conditioned), open web floor trusses for second floor.
I had the manual J load calc reviewed by an engineer and have sent it back for some corrections (there is better insulation than is shown and better air sealing, and a couple other things which will lower the load. He didn’t add the ventilation load though, which will raise it), but as it stands right now:
Floor one net gain: 22,272 BTU’s or 1.86 tons. Sensible loss is 22,027 BTU’s.
Floor two net gain: 21, 264 BTU’s or 1.77 tons. Sensible loss is 19,429 BTU’s.
The original quote was for:
Carrier CA14NA030 2.5 ton. One for each floor
Carrier 58STA070-1-16 53k btu 80% AFUE. One for each floor.
I requested some additional options:
92% AFUE $545 each
95% AFUE with ECM $1531 each
96% AFUE with variable speed motor $2123 each.
Metal trunk with true radius 90’s $1738 total.
Second floor insulated ductwork $869.
2 stage 3 ton condensers, $2342 total.
Fresh air intake with automatic damper and controller, 516$ each.
I am awaiting a new quote for a 2 stage 2 ton condenser, because there is no way I’m going with a three ton. I’m also awaiting a quote for the ultra-aire whole house dehumidifier. I’m aware of the building America research on it but I like it. I will await the updated load calc and see about lowering the capacity on the original quote to 2, 2 tons units rather than 2.5 ton units. I’m not sure how much cheaper that will be. Even the smallest furnace they offer will also be oversized. The low stage of the smallest multi stage unit might be close.
For the furnace at a minimum I will go with the 92%, because it has to be sealed combustion due to the units being located inside the conditioned space. I also plan to upgrade to sheet metal for the trunk line, and for the second floor possibly sheet metal for everything since it will be inaccessible after drywall is up.
The real question is how do I ventilate. I can either use the ultra-aire, which will also deal with the humidity, or do central fan integrated. If I go with central fan integrated I feel like I need to upgrade to a furnace with an ECM, which is another 2,000 dollars more expensive than the 92% with a standard motor. Humidity control wouldn’t be as good, so it would be nice to have the 2 stage AC which is another 2342. Then add another 1,000 for the actual ventilation work and controls.
This is separate from the ventilation issue, but I’d also like to find out how much more it would be for a heat pump. I’ve run through some calculators and natural gas is still a good deal cheaper, but I feel like the heat pump is a good option to have if things change. Plus, if even the smallest gas unit is oversized, I wonder if using the heat pump when it’s mild and gas when it’s colder might help alleviate any blast furnace type effect.
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Replies
What is your 99% outside design temp?
Summer design temp is 91, Winter is 24. I'm going to guess you are going to suggest sticking with heat pumps :)?
I should add, that is what they are in the closest city. The contractor used 92 and 22, which is something the reviewing engineer pointed out. The HVAC contractor said "they found those numbers work better". By that I'm guessing they mean it gives them an additional margin of error and slightly oversizes equipment.
Why two furnaces and two air conditioners? Is there some reason that zone dampers would not work?
I have to look up the capacity specs, but yeah, it's likely that the heat pumps are the best options.
A heat load ratio of ~11 BTU/hr per square foot for a 3800' house at an outside design temp of +24F seems a bit on the high side, even for a code-min house. A lot of window area, perhaps? (Which would explain the fairly high cooling loads too.)
In the past 48 hours I've reviewed a Manual-J on a similarly sized (~3300') house on Martha's Vineyard MA where half the house is 2x4/R13 w/ U0.5 windows and the larger-than-original house addition is 2x6/R20 U0.33 type construction, and it came in at about 31,000BTU/hr @ +12F for about 9-9.5 BTU per hour per square foot. But window areas were modest, and part of that is walk-out basement. But still, that's a design temp 12F colder than yours, with 1600' of that space built well below current code minimums.
Combining the ventilation for the house into the heating & cooling equipment isn't really a great idea, since it tends to over ventilate during periods of higher load, and may not provide adequate ventilation during periods of very low load. An HRV approach fixes that, and decimates the ventilation portion of your heating load.
It's pricey, but a 2-ton fully modulating Carrier Greenspeed per zone (or possibly a single 4 ton Greenspeed with zone controls) would handle handle your loads. (As it happens I also recently reviewed a proposal for a 4-ton Greenspeed/w zone control + a separate mini-split for that same house in MA, which seemed like overpriced overkill, prompting the Manual-J. That proposal minus the mini-split it's about right for your loads though, if the quote were reasonable.)
"Combining the ventilation for the house into the heating & cooling equipment isn't really a great idea, since it tends to over ventilate during periods of higher load, and may not provide adequate ventilation during periods of very low load."
This isn't a problem when you're using an appropriate controller like an air cycler.
"A lot of window area, perhaps?"
You got it, my wife loves windows. The second floor load should drop a decent amount after some updates to the load calc. The return is going to be in the unconditioned attic, but the load calc had the duct as uninsulated. That alone was a 6800 BTU load. It should be significantly less with R8. Out of curiosity, what sort of duct leakage do you usually see on your load calcs?
There was also a total gain of 9,100 BTU's for equipment, which the engineer pointed out as high and requested a breakdown. What is typical? How is it normally calculated or estimated?
Michael,
Your final decision will depend on the layout of the rooms and the bids provided by your HVAC contractor or contractors. It certainly sounds to me that the choices you are considering will be expensive. That means that you might want to consider other options.
Here are a few principles:
1. It's usually a good idea to keep your ventilation system separate from your heating and cooling system. I'm a big fan of dedicated ventilation ducts.
2. A bonus room over a garage is a great place for a ductless minisplit. Separate equipment assures that this zone will be comfortable.
3. I'm guessing that a combination of ductless minisplits and ducted minisplits would be a good way to go.
Perhaps not surprisingly, a pair of ducted mini-splits, one ductless mini-split, and a PTHP for a corner office room in the walk-out basement is what I recommended to the folks in MA.
In Michael's case the heat loads are just within the output capacity of a 1.5 ton Fujitsu mini-ducted unit per floor @ 24F, but the cooling loads are more than they can deliver. If broken into more zones, maybe.
I understand using multiple units in a mini-split setup where each unit is relatively small and even ducted units are limited to short ducts. I don't understand why one would use multiple furnaces in a central HVAC system, especially when the smallest available units are so oversized. Wouldn't a single modulating furnace with a variable speed blower and a zoned duct system work better and cost less? What am I missing here?
Reid,
Zoning a forced-air system is possible but technically inelegant. Many such systems waste energy and perform poorly. For more information, see The Achilles’ Heel of Zoned Duct Systems.
Mini-splits are both modulating and scalable downward in size, making them good solutions for zoned systems even on code-min houses. Also, with multiple mini-splits you don't have a single point of failure (except for the power lines, which would do-in a gas furnace too.)
The modulation range of heat pumps are not infinite, so when the multi-zoned single large heat pump is serving just one zone, it's often ridiculously oversized for the load of that zone.
The reasons for zoning a larger single heat pump is usually upfront cost, but it's pretty far from ideal.
The efficient or max possible duct lengths of some of the ducted mini-splits is longer than you might think (longer than I thought even a year ago), but it varies by vendor.
Martin, my take-away from that article is that zoning a single system can be done very poorly and often is done very poorly. If we eliminate options that are susceptible to poor implementation, we don't have very many choices left. Another take-away from the article and the comments is that you cannot effectively zone a single stage, single speed, dumb furnace.
I will concede that having two appropriately sized furnaces would perform better than a single furnace with zoning. The appropriately sized part appears to be missing from the default proposal here. For the cost of two dumb oversized furnaces, could one purchase one smart furnace? (I phrase this as a question because I am not educated about furnace prices.) I am defining smart as having at least two stages (preferably modulating) and variable speed blower motor. In a well designed zoned system, the ductwork in each zone would need to be designed for at least the minimum airflow of the furnace. That may be more than the heating load of that zone would require. It probably isn't more than the airflow of a dumb, oversized furnace dedicated to that zone.
Regarding some of your options, there's no way I'd pay an extra $5k in hopes of getting better humidity control out of my AC unit(s). On paper, and in manufacturer advertisements, two stage systems are much more efficient and better for dehumidification. But in real world testing, the theoretical potential is often crippled by poor implementations, and even when done right, the benefits are much smaller than claimed.