View Full Version : Air Conditioning & Ventilation Question
09-25-2010, 08:49 PM
I have a question I'm hoping someone could answer. I'm building a new home and I'm breaking ground in 3 weeks. The house is going to be a 4" slab on grade on top of 3" of foam board with radiant heat; the walls are going to be insulated concrete forms and I'm going to have 4" of spray foam insulation in the attic.
Its going to be a very tightly insulated structure, but I'm wondering if its going to be too tight?...and I'm not sure if I need to make additional provisions for additional ventilation? My building plans were just approved by the town last Thursday and nobody mentioned anything about additional ventilation so I assume that the bathroom exhaust fans, plumbing vents, inevitable window leaks, dryer vent, range vent and vent for propane heater will provide adequate ventilation...or do I need to consider something else?
The second part of my question is that I really like the idea of the under slab radiant heat, but I don't have anything yet in terms of air conditioning. Again, I'd prefer to stay away duct work if I can, which leaves me with ductless mini systems for my air conditioning. However my understanding is I would need to buy several of these units, not to mention they really can be zoned to cool each and every room. Though I may be wrong?
The house is a 2,100 sq. ft. ranch and according to the engineer (who ran my original load calculations back when I was considering a heat pump) said I would need a 3-ton unit for heating and a 1.5 ton unit for cooling.
That being said my cooling needs shouldn't be that great. Therefore I'm wondering if I could just run a baseboard system for my air conditioning powered by a chiller and a pump? That way I could have a baseboard register in everyroom. And if I have adequate ventilation due to inevitable air leakage I wouldn't need to install ductwork, therefore this system should be relatively easy to install and would be my best option.
I'm really on a time crunch here so any advice you guys could offer would be greatly appreciated!!!
09-26-2010, 06:32 AM
1. When you use chilled water for air conditioning, you have to have fan coils...blower units. Heat works by convection but cool doesn't work that way. Each fan coil needs power, condensate drain, etc.
2. Ductless might be a good choice for the A/C. Many brands now make what the call inverter units, meaning they can be multiple zoned. One outdoor condensing unit can support up to three, possibly 4 indoor units. An air handle in every room might be overkill, and would start to get pricey. GOogle "Mr. Slim" which is the Mitsubishi line.
3. If you have any gas burning appliances, especially larger ones like heating and water heater, then tight houses must make provisions for an outside source of combustion air. Also, tight houses can have issues with "stale" air. Fresh air can be brought in through an air-to-air heat exchanger.
You need a really good HVAC company to evaluate your needs. Winging this stuff is iffy.
4. 1½ tons sounds small for 2100 sq. ft, but you are well insulated, and I don't know your climate, which is why you do the load calculation. Having said that, I was reading a study recently that showed most contractors tend to install 1/2 ton bigger, because homeowners will complain about the performance of a system built right at manual J. The system will be very efficient, operating in long "on" periods, and doing the best job of dehumidification. But if you come home on a hot day, and the a/c has been off, the homeowner wants to turn it on a feel a blast of cold air, from a bigger system!
09-26-2010, 04:17 PM
Thanks for your comment Jim. I agreee with your ventilation concerns, and although the mini-split system may be the answer I'm going to end up with a few ugly air conditioners mounted on the inside of my home.
Its not too late for me to switch to a forced air heat pump, therefore I'd like you to consider this option:
I can go with a heat pump but with PVC DUCTWORK. Ideally, I would want the PVC ductwork installed vertically inside the exterior concrete walls leading up to the vents/registers. This way the ducts would be insulated by 2.5" of foam insulation and surrounded by concrete on the sides, and that would permit thermal expansion/contraction of the PVC itself. I wouldn't be able to run the duct work vertically all the way to the top of the wall as that would create a "break" in the concrete and therefore compromise the strength of the wall, so I would need to have the registers at the base of the wall so the steel-reinforced concrete could continue above the PVC duct work to provide continuous strength to the rest of the wall.
I could also like to run ALL ductwork underneath the floor slab. The floor will be specially reinforced to ensure that it will NEVER settle. So settlement wont be an issue.
I would need to install the PVC ductwork WHILE I'm prepping the floor and constructing the walls to route the PVC ductwork so that it turns down from the walls and underneath the slab back to the air handler. In fact, this ductwork ductwork design is really no different than typical ductwork detail other than I'm replacing metal with PVC, and instead of running it under a crawl space or up into an attic I'll be running it under the slab up through both exterior and interior walls to the air handler located in the utility room, and the air compressor will be located outside the home.
I work in the concrete industry and we just pour concrete slabs for industrial and large commercial facilities, and as you know, in industrial construction all piping, electrical, plumbing, conduit etc. is ALL buried underneath the concrete slab(s), and I simply don't understand the concept behind a crawlspace?? In my opinion crawl spaces are a horrible design, as they harbor mold, moisture and mildew problems; they provide a haven for termites, insects and rodents and they create a wind cavity which is counterproductive to your cooling efforts in the summer and your heating efforts in the winter...much like the principal of running ductwork in an attic as that doesn't make sense either. For in the summer time your running 60 degree air through an attic that's 130 degrees and in the winter you're running heat that's 130 degrees through an attic that has an ambient temperature of 20 degrees. At least the ground remains a constant temperature of approx. 55 degrees, and this temperature would help cool the PVC ductwork in the summertime, but it would have to be insulated as it would work against you in the winter time. Either way you look at it underground ductwork seems to be a much better option. Again, that is providing it is properly constructed for if the slab settles and/or crushes the PVC pipes it would be disasterous!
Would you agree with this option?
09-26-2010, 05:56 PM
A really tight house begs for energy recovery ventillation system(s). Canada is ahead of us on this, as they mandate a tighter house, and without the system, you can easily get a sick building. Any burner in the house needs makeup air, any aerosol used in the house needs to eventually be vented out. In a really tight house, it just builds up. Plus many building products can outgas for months. You may not want to leave the windows open for safety and pollen, or other weather situations. Use your bathroom or stove ventilation fan, and the air it tries to push out, needs to be replaced somehow. It's much better to do this with an engineered energy recovery system. It also improves the efficiency. Otherwise, that garlic you love might just last through the night in the house! Best to allow free flow outside, and that means a viable path to get (hopefully conditioned) replacement air in; that's what the energy recovery systems do. the better ones not only temper the air and exchange humidity.
09-26-2010, 06:36 PM
There are HVAC systems which use small , high velocity duct work. Designing and balancing such a system is not a DIY job. You should get the contractor who will provide such equipment to design the duct layout and sizing.
09-26-2010, 08:58 PM
Jim, that was real helpful information. Question: Can the ductwork with the system you're describing be used under a slab? I assume so. Secondly, can you give me an idea of who makes such a system (Trane, Carrier, Goodman, Rheem, York etc)? Many of the HVAC contractors in my area are just too busy, so if I can locate the system and then hire an engineer to design the ductwork layout I'll have a jump on the game and that will save me a lot of time! If you could provide a make and model that would be even better!!
09-27-2010, 03:54 PM
Most of the high velocity installs I've seen have been on 'This Olde House'. All of those I remember had the air handler installed in the attic, but I'm sure you could put it elsewhere. There's a main duct and small round ducts to the rooms. A typical room would require more than one difuser. Since they are typically designed to be in the ceiling, it makes more sense to have the main duct up high rather than in the slab - the runs to the outlets would then be shorter with less chance of wasting energy and velocity. Don't know, but expect most companies equipment can be set up this way.
ICF walls & spray foam attic do INDEED scream for ERV/HRV active ventilation systems- don't even THINK of skipping active ventilation of some sort!
I've never seen baseboard cooling, but it would at most handle sensible cooling loads. You COULD do sensible cooling via the radiant slab, and run a dehumidifier coil on the ERV or a standalone dehumdifier to handle the latent loads. ERVs would have small diameter ducting, but not high-velocity. Condensation on the slab would only become an issue if your sensible cooling loads are very high (which it doesn't sound like), and/or you weren't separately controlling the humidity. In a very tight house that size, even a single 500-600W room-dehumidifer can keep up, as long as you keep the ventilation rates kept to a minimum when outdoor dew points are 70F+.
Radiant slabs are often impressed in to use for sensible cooling in Europe, which tends to be drier than the eastern US. Radiant cooling panels on walls & ceilings are more effective per square foot than slabs (due to the induced convection), but if your engineer was talking 1.5tons for both latent & sensible cooling I'm pretty sure you'll be just fine using the slab alone for the sensible cooling fraction. The thermal mass of the slab will be enough to keep it from short-cycling the 3ton compressor, and you may get some overshoots at times if you don't have PID control, but they won't likely be bone-chilling coolth. Just keep the relative humidity under 60% (under 50% if anyone has dust mite allergies) and all will be well.