UFAD - Using an Under Floor Air Distribution system in a 1980's Earth Sheltered Home?
If you look through my past posts, you'll see I always seem to bring the weird ones here. So this one is really weird.
Last week we bought a house that will hopefully be our retirement home in 20 or so years. It is effectively a 2500 sf ranch built entirely in concrete - including the roof. Dirt is mounded up on the roof and all sides except the south facing side (toward the street). In fact, from the street, you wouldn't know that it isn't just a normal ranch house.
Inside the concrete shell is the wood framed house itself. The floor is sleeper joists covered with 2 sheets of 3/4 plywood, and sleeper rafters above serve as the mounting point for the sheetrock of the 9' ceilings. From the front, and from the inside, you'd never know it wasn't a typical house.
What is different, and why I'm here, is the HVAC system. Somewhere along the line, one of the owners must have had a new system put in so it is not as originally designed.
How was it originally designed? As a UFAD - Under Floor Air Distribution. Each of the rooms has floor registers in the places you'd normally expect them - but there is no ductwork - they are just open to the pseudo crawl space and concrete slab below. The sleepers are raised so there is about 8" open below then (not to the current 18" code I know). All around the outer edges of the concrete slab at the walls, you can see the inside wall under the floor has a raised concrete footer up to the floor header - AND - it is sealed with 2" of styrofoam insulation as well. There is no insulation between the floor joists themselves, just around the outer walls to create an airtight space.
Over in the unheated mechanical room is the only place where there is a opening to the floor - right next to the forced hot air furnace.
Sadly, all this sits open, and unused. Someone, at some point later, seems to have run new ductwork in the ceiling to all the rooms and added 2 central returns in the hallway ceiling (the returns might be original, hard to tell), and then put in a standard oil-fired furnace (which is in great shape I might add). The mechanical room opening to the floor below is left open, as are the floor registers in most of the rooms.
I know that this kind of thing would have been pretty unheard of in 1980, especially residential. I've looked through all kinds of HVAC sites (including HVAC TALK), and UFAD is only ever referenced in commercial situations. And I've looked through the contractor map here at HVAC talk, and none of you are even near our place in Stroudsburg, PA.
There's no rush to tackle this - but I know the house is not operating nearly as efficiently as it could - with open, uninsulated space under the house. It's sealed everywhere but the mechanical room, but that openness obviously is a source of lost heat. Some day, when it's time to update the furnace, I will want to try to use this space again.
I don't know why they abandoned it - perhaps the contractor who replaced the original system didn't understand it. Perhaps the slab cooled the warm air too much before it got to the far side of the house. Perhaps there was a fear of moisture buildup (heat recovery ventilation systems didn't exist back then). Perhaps there it was just too far ahead of its time and the equipment of the day wasn't well suited for it.
Any thoughts from you guys?
Update after 1.5 years living with the new HVAC system
I'm resurrecting this old thread in case anyone is interested in how it turned out. I'm not looking for advice, just want to pass on experience.
This is a long post, so if that's not your style, please skip this one.
1.5 years with the underground house:
After a home energy audit in the summer of 2013, and consulting with the audit professional (has a good reputation among the builders and remodelers in the area), the leading HVAC installer in the area (good reputation), and even the original builder who I tracked down and is now an HVAC installer in Florida, we decided on a 2-stage 3 ton Carrier heat pump for HVAC (with 15KW emergency coils). We had it professional installed (a 3 day project).
We kept the overhead distribution plenum and ductwork into the rooms for supply. We used the sealed underfloor space for the return. The original owner and designer told us that while open under each room, each room space was sealed and an appropriately sized opening connected it to a central return plenum that ran under the central hallway. To all of this we added a Honeywell ERV running in and out of the return duct before it entered the air handler. An Ecobee thermostat handled the controls.
What we found over the winter of 2014/2014 is that the heat pump cycled crazily up and down, calling for aux heat as it yo-yoed beyond the 2 degree limit for first and second stage heat pump heat, even when 30 degrees outside.
A change to the ecobee thermostat settings to use the "Reverse Staging" setting helped immensely - it now ran nearly continuously in high setting, but only needed aux when the outside temp went below 0 and the compressor could not be used. I can't state enough how useful this "Reverse Staging" setting in the Ecobee was - it had an enormous impact on the need for the aux coils.
Summer 2014 brought warmer temps, but we never really needed A/C. Unfortunately, we did need humidity control, and with the house being so cool naturally, any attempt to use the A/C to control humidity required allow the temp to go well below comfort (or efficiency). We struggled through the summer with high humidity, but will invest in a dehumidify for next summer.
This fall (2014) we made a change to the ducting (yes, it cost me an extra $2000, but I think it's worth it).
We disconnected the underfloor plenum from the air handler completely, and re-connected the air handler return to a central ceiling return plenum that had been installed by one of the previous owners (after the original owner sold it). We actually had to add a couple extra return grates and 10" flex lines to get up to the 1200 CFM that we needed.
More importantly, we used the now isolated under-floor system to feed the ERV. This now pulled stale (cold) air from the floor registers out of the house completely, and as an ERV it used what heat there was in it to pre-heat the fresh air coming in. And although we don't believe we have any radon issues, exhausting the air under the house with the ERV minimizes any chance for it to become an issue.
This made another improvement to the house. One, the air is cleaner, but two, we are able to run on stage 1 nearly all the time now. I think the cement floor under the crawlspace was pulling residual heat out of the return air in the 2013 configuration, making it harder for the heat pump to raise the supply temp being sent to the rooms (too large a delta T). So this year it has not used stage 2 nearly as often as last year.
This new configuration will also make it easier to add a dehumidifier this spring, as we have a large section of accessible return ductwork to put the dehumidifier return and supply lines through.
If I were to do one thing differently about the HVAC equipment I chose, it would have been to go with a hybrid air handler using propane for backup heat. The electric coils work just fine, and probably cost about the same to run as propane would, but having propane would give me a backup heating option if power goes out. A portable generator would be enough to run the air handler and propane ignitor so we'd have heat through ice storms, etc.
About insulation in underground homes:
Going through the Ecobee logs for winter 2013/2014 and for 2014/2015 so far this year, it appears that the cement shell of the house pulls a constant 28,000 BTUs (in 2013/2014) and 22,000 BTUs (2014/2015) regardless of outside temp or wind chills. It's nice that it is consistent, but it's terrible that it is such a huge energy suck. What we figure is that the polystyrene foam insulation (that's all they really had in 1980 when the house was built) that was used all around the house's exterior before back-filling has either gotten wet, been compressed, or both. Effectively, it's not really providing any kind of thermal barrier any more, and the concrete shell walls are passing energy straight through to the soil. Granted, the soil may only be 40 degrees regardless of air temp, but that's still a big temp delta, and there is no insulation on the interior of the building (except for the sheetrock) to protect the interior air from the concrete shell. Where we have pulled awaythe sheetrock, we find that the concrete walls are a constant 62 degrees (so maybe the insulation is still somewhat functional - or maybe it's just the constant pouring of energy from the inside air into the concrete that is keeping it at 62 degrees).
In a nutshell - the idea of a thermal mass house is great, but the quality of the external insulation (and its protection from crushing) it paramount. In our case, we're planning to remove some sections of the sheetrock to either closed-cell foam the walls and ceiling, or apply low-vapor-transmissive rigid insulation and seal all the edges, and then re-sheet rock everything. I don't like this idea, but it's the only practical suggestion I've received from anyone so far.
If anyone is in the NEPA (Stroudsburg PA) area and wants to see the house, feel free to contact me.