Furnace in attic or crawl space? What duct material?
I live in Northern California – 5 months of heating season – wet cold winter, no snow, rarely go below freezing. Dry summer and I don’t plan to have AC. I am planning on building a new house with around 3000 sf of living space for heating. The attic has around 500 sf under flat roof that I plan to insulate without connecting to central air since it is rarely used (plan to use space heater when needed). I looked into radient heat, ductless heat pump, and traditional furnance. Although I prefer not to use central forced air (potential duct leakage, blowing dusts through the duct), the other options just costs too much more than gas furnace. If you have other suggestions, please let me know.
Given that I am most likely to use gas furnance, which of the following 2 options is the lesser of two evils? I know it’s best to place furnace in the conditioned space but that’s not going to work for us. My priorities are in door air quality, energy efficiency, and cost.
(1) Put vents close to ceiling; furnace and most of the ducts in the insulated/not-conditioned attic space (inside the 500sf flat roof attic area). Some small percentage of ducts still have to go to the non-insulated open attic. I probably will have longer ducts comparing to option 2 since I do plan to use the flat roof attic bonus room and would keep the ducts at the perimeter of this bonus attic room.
(2) Put furnace and ducts in the crawl space and heat vents close to the floor. Clearance for the crawl space is 1.5′ (old house portion) or 2′ (addition portion). This is the preferred solution since heat rises and it is better to have low vents and I can also free up duct space in my attic.
My architect recommends (2). Since the crawl space is vented in my area, I think I can build an insulated box for the furnace and wrap all the ducts with insulation to minimize heat loss. Would you recommend this or go further and seal/insulate the whole crawl space?
For long term maintainance, how do HVAC company clean and re-seal duct leaks in 1.5′-2′ space?
For duct material, one HVAC company recommends Mylar flex ducts since most leaks are origniated from connections and mylar requires few connections. Intuitively, I think the flex tube will not last long and the metal ducts should provide better air flow. What kind of duct materials do you recommend and how to effectively seal metal ducts?
Thank you all in advance. I have learned a lot from many of you. The more I learn, the more I realize how much I don’t know. It’s not easy to figure out how to build a cost effective green home with a lot of conflicting information from different books/websites.
PS. In case you need the following data for your recommendation:
1. Our gas rate is $1.2 per therms and electrical rate is $0.12-$0.16 per KWH (lowest tier in our area).
2. I plan to use single ply white roof that comes with lifetime no-leak warantee for the flat roof and asphalt shingle for slopped roof. Per code, we will have fire sprinkler system installed throughout the living space but not attic.
3. Some experts questioned why I need 3000sf house. I hope I don’t need to share my reasons with everyone. Even if I do, we probably still will not agree on the minimum size. I spent many months fine tuning the floorplan based on my family’s needs. I am a big fan of The Not So Big House books by Sarah Susanka and have read through all the different variations of the Not so Big House, Most of the rooms are designed as multi-functional rooms. I also redesigned the floor plan many times and reduced the hallways and exterior foundation/wall by 30% from my architect’s first design. The exterior footprint of the house is practically a big square instead of multiple wings now. This is why I have a big flat roof in the middle of the house due to building height limitation that the slopped roof needs to stop at 20′.
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Replies
Amy,
If you are building a new house, there is absolutely no reason to design a crawl space that is 2 feet high. Even here in Vermont, where we often have excavation problems due to ledge (bedrock), we will hire a blasting contractor when the backhoe starts scraping hard rock. If blasting is out of the question, why not raise the house? You want a crawl space that is 3 or 4 feet high.
Since this is new construction, your foundation and crawl space should be designed to modern standards. The means it should have footing drains, a dry floor covered with a vapor barrier and crushed stone or concrete, and insulated walls without any vents. Put in plenty of good lighting for the heating contractor to work with, and you are good to go.
Martin,
Thanks for your very rationale response (as usual). But, I am limited by existing subfloor and budget -
We are reusing the existing 1.5' deep sub floor (contractor said it's cheaper to reuse) and I want the old/new floor at the same level for universal design, so raising the floor is out.
I can dig deeper for the new area and install lights to make it easier for HVAC contractor.
The big issue is cost. My contractor said that it costs around 10K to seal the crawlspace and I am already over budget (I also called http://www.bayareacrawlspace.com and its estimate is even more). My GPR confirmed that no one seals crawl space due to the cost. I am spending $250 on gas heat a year now and it will take 40+ years for the payback.
Without sealing the crawlspace, do you still recommend placing furnace and ducts in the crawl space in my situation?
Thanks again!
Martin,
Thanks for your very rationale response (as usual). But, I am limited by existing subfloor and budget -
We are reusing the existing 1.5' deep sub floor (contractor said it's cheaper to reuse) and I want the old/new floor at the same level for universal design, so raising the floor is out.
I can dig deeper for the new area and install lights to make it easier for HVAC contractor.
The big issue is cost. My contractor said that it costs around 10K to seal the crawlspace and I am already over budget (I also called http://www.bayareacrawlspace.com and its estimate is even more). My GPR confirmed that no one seals crawl space due to the cost. I am spending $250 on gas heat a year now and it will take 40+ years for the payback.
Without sealing the crawlspace, do you still recommend placing furnace and ducts in the crawl space in my situation?
Thanks again!
Amy,
It sounds like you're thinking to leave the crawl space uninsulated to cut cost, while part of the attic is insulated. I would think you'd be better off with option 1, to avoid having the furnace and ducts in a squashed and uninsulated crawl. Access would be a problem. Where ducts run through uninsulated attic, insulate the ducts well. Metal ducts are better to reduce friction, and seal the joints with foil tape.
Also even though you describe the middle area of roof as "flat", you do need a minimum slope of 1/4" per foot, so plan on those extra inches of rise.
Amy,
The best way to reduce construction cost is to build smaller. Unless you have 6 or more kids, no one needs a 3,000 SF house. If you feel you do, then there's more than enough room to put the furnace within the conditioned space and use transfer grills and hallway plenums to move the heat and return air.
Frankly, it sounds as if you need some better advice, more competent design assistance and more reasonable contractors than the ones you've been talking to.
Amy Builder,
So are you are an owner builder, professional builder? Or is this really your last name?
I agree with Robert. You need professional advise, not just general guidance. And without seeing plans and pictures of the existing structure, nobody here on GBA could give you the specific advice you need.
Amy,
While neither of your suggested options sounds great, I agree that it is better to put a furnace within the home's conditioned envelope, in the semi-conditioned attic, than outside of the envelope, in an uninsulated crawl space.
However, it's extremely important to provide good access to the furnace for regular service. So that means a stairway or a very big hatch -- and the hatch better be in a hallway, not a closet. That also means you need plenty of room all around the furnace for servicing, and a floor for the service person to sit on, and good lighting.
I agree that there are signs that the priorities of your designer are a little undependable. You've got a big house, a tight budget, a very tight crawl space, a cramped attic, and no good plan for where the HVAC equipment should go -- all signs of a disorderly design process.