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Thread: PTAC's in SFR's

  1. #1
    Has anyone heard of builders installing PTAC's in new homes?

    As I see it here are a few of the issues:

    1. The cost of gas has now officially made air-to-air heat pumps the cheapest way to heat.

    2. Zoned systems are highly desirable, and hydronic systems are relatively expensive. (3 PTAC's in a new 2 bdrm. home could be cheaper than a central ducted HVAC system.)

    3. Are PTAC's too noisy for bedrooms? Too cheesy?

    4. Do they leak cold air in the winter?

    5. Do they install like an electrical appliance, or is an HVAC tech needed?

    Are there any other issues to be considered before I pursue the idea further?

  2. #2
    Join Date
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    3. depends on what you consider noisy.

    4. minimal

    5. yes, they install like an appliance, but you must read the installation instructions to insure proper drainage of the condensate.



    My brother had one in his Denver high rise apt., and of course the hotels use them.
    If everything was always done "by the book"....the book would never change.

  3. #3
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    Feb 2003
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    In a small home it might make economic sense. But a PTAC is still a wall mounted AC. You can't expect it to be as quiet as a split AC system. You might want to look at vertical PTAC's. Here you "box in" the unit into a closet. It provides you with a return through the wall and the supply is a simple duct system off the ceiling. These are quiter then the standard PTAC units. Mini-splits offer you an even quiter solution but you will need an HVAC tech to install and the price of several units would probably be more expensive then a single split system with duct work.

  4. #4

    Updating

    Thanks for well-thought out information, Johnsp.

    Fujitsu now has 21 SEER mini split units, which I guess operate at a COP of over 5. Apparently they don't need strip heaters for low outdoor temps.

    I'm in the design phase of a small housing development. The homes will have a design heat loss of only 22,000 btu/hr.

    I'm afraid of being the first developer in the US to use minisplits for entire homes, but I'm almost convinced I should.

    Am I missing something?

  5. #5
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    They may still be fairly efficient at low temps, but they are not going to maintain full heat output at low temps, either. Heat pumps output numbers are stated at 70 F inside, and 47 F inside. If you look in the right place, you'll also find numbers for 17 F outside. Typically for split systems you get about 60% of the heat output at 17 that you do at 47. At 0 degrees ambient, you're down to about 40%. So an ultra-efficient heat pump takes less power to run, but its heat output is still going to drop way off as it gets really cold outside. Except in places like Florida, you will still always need an auxiliary heat source to go with a heat pump if you are to cope with severe cold.

    There are also comfort problems with using heat pumps at low ambients without any backup heat. The system still has to defrost regularly at ambients below about 40, and with no backup heat source, you are going to be blowing *cold* air on your occupants during those defrosts. That will make you very unpopular with your building occupants in the winter. So even if you had enough capacity in the heat pump to keep the house warm at design temperature (which is rare), you still need a way to temper your discharge air during defrosts. There are very few minisplits with provisions for backup heat, though. There are a few that have indoor sections that can be ducted (Mitsubishi does), which I think would be a big improvement over trying to heat all the common areas of a home, even a small home, from a single point. I would rather see an indoor section for each bedroom, and a ducted unit providing heat from the kitchen, living room, maybe a bathroom. But I don't know if there are any that combine ductability with a heat strip.

    You can get resistance backup heat in a PTHP (the heat pump version of a PTAC). But they don't defrost at all; they run heat pump until it gets cold enough that frost formation would be a problem, and then switch over entirely to resistance heat. With a heat loss of 22k BTU, that may not be a terrible thing.

    As for PTAC/PTHPs leaking heat, well, you can seal them up to keep them from being drafty. But you are still talking about cutting a big hole in the wall and filling it with a big box that's made largely of metal. So you are going to lose a lot more heat through one of those than you would through an unbroken wall.

    Some PTAC/PTHPs will allow for a very limited amount of ducting- typically this is a single takeoff that will dump some supply air a few feet through a duct into an adjoining room. But it has to be a straight shot horizontally, and everything has to be laid out to accomodate it (since you are limited to a few feet of duct).

    You may also find a hybrid of multiple solutions to be useful, such as minisplits plus electric baseboard heat. The residents can just use the minisplit for heat in mild weather, and the baseboard heat in cold weather. Yeah, electric-only heat sounds terrible, but with such a small heat loss, it wouldn't be that expensive to run, it's cheap to install, and preserves the zoning that you're looking for without needing an elaborate central system. And in practice the power consumption for cold weather heating would be similar to using PTHP's- since they would be all-resistance in cold conditions too.

    If gas is going to be accessible, there are some small direct-vent room heaters (see http://www.rinnai.us for some possible examples) that could be another good alternative for a cold-weather heat source.

  6. #6
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    Jun 2001
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    Are these going to be single family homes, town houses, or condos? 2 bedroom units?
    I'm not sure if you would save much money installing 2 mini splits and a few pieces of electric baseboard versus a split system heat pump with a simple duct system. Isn't the design temp for Denver below zero?

    Unless these are going to be public housing or super cheap I don't think people would be that happy with ptac units. especially in the bedroom

    I am seeing new homes now that have ptacs installed in the sunrooms that stick out the back of the house. That isn't a bad idea.

    brent

  7. #7

    Lots to evaluate

    Wyounger,

    Thanks for more well-thought out info. I wasn't considering a central split heat pump because I didn't think there were any that ran efficiently below 30F. I'll research that some more.

    Gas fireplaces that are usually considered aesthetic could be integrated into the cold weather strategy also.

    Brent,

    Right, I think I've abandoned the PTHP idea, but I'm still considering the minisplits, mainly because of the amazing SEER numbers, and quietness. These are 20' wide townhouse units.

    You can get away with Zero for design temp. But even at zero, those minisplits look like they still have a COP of 1.5 - 2.0
    Some sort of second stage heating would be required unless you really oversized the unit. (Oversizing usually sounds like a bad idea)

  8. #8
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    It's important to not confuse the difference between efficiency and capacity.

    Heat pumps are still efficient even at 0 degrees ambient- relative to electric resistance heat... in other words, the COP at zero degrees is still way above 1. That is true even while their output falls way off because their power consumption drops off amost as fast as their output does as ambient temperatures get cold.

    But since their capacity isn't sufficient to be the sole heat source in most climates, you will need an auxiliary heat source. If that's going to be electric backup, the heat pump keeps running, and the backup only has to cover the shortfall between heat pump output and the amount of heat you need. That way you are still getting everything you can out of you efficient heat source- the pump- and minimize how much heat you need from your backup heat source, which isn't as economical to run.

    Ideally, though, you size the backup heat so that if need be it can do the whole job by itself; that's a cheap way to prevent anyone from going without heat if the heat pump should malfunction in cold weather. With a heat loss of 22k BTU, though, 6500 watts could do it, and 8-10 kw would have a comfortable margin of reserve.

    Most of the efficiency advantage of the inverter type minisplits is under less than full load- just like two-stage central systems. Under more extreme conditions, though, there's not much difference. I'm looking at the product data for a nominal 13-SEER split heat pump at -3 F and the various matches are still showing COPs of 1.7-1.8. So there is plenty of *efficiency* there even at temperatures somewhat below zero; it's just that the output under those conditions isn't going to be sufficient by itself.

    Incidentally, you told us your heat loss. What will the heat gain be?

  9. #9
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    Jun 2001
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    How many bedrooms do these units have? I am assuming the first floor is one open area. You might be able to get away with 1 mini split on the wall but probably not. Upstairs how many units would you have? In Denver would it be acceptable to not have air conditioning in the bathrooms? The smallest mini splits that I am familar with are 9,000 btu cooling. Most mini split heat pumps don't have any electric heat or maybe 1 kw.

    Did you figure out your cooling load?

    Brent

  10. #10

    Mini-splits for townhomes

    No A/C in the baths is fine. End units have 3 bds., interior units have 2 bds. At 9,000 btu/hr, these are a little big for a bedroom, the above suggestion of a vertical PTHP servicing two bedrooms could work.

    The East and West facing window sizes aren't final yet, but we can assume a total cooling load of 15,000 btu/hr.

    Electric baseboard is fine for backup. Again, perfect zoning.

    Since building costs are $100/sq.ft., saving the furnace floor space is also significant.

    [Edited by kevin_in_Denver on 06-26-2006 at 05:30 AM]

  11. #11
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    Feb 2003
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    Remember, hotels use PTAC's casue they're cheap, provide individual room zoning and can quickly be swaped out when broken by having a few spares on hand. It is not efficent to have hundreds of these running over a big single central system.

    Now if your townhome has any interior rooms, they must be heated and cooled by leaving outer room doors open.

    Are you putting in hot water heaters for each townhome?
    I'd think about hydro-air. Get a small air handler with a hot water heating coil in it. Run a heat exchanger off the HW heater or get one that has a HX already built in.

    Take a look at:
    http://www.enviromaster.com/index.asp

    Several mini-split makers offer ceiling mounted units that allow small branch take offs that could feed aditional rooms. They have built in electric strip heat and hot water coil options.

  12. #12

    DHW

    Thanks for reminding me about the domestic hot water. Solar would be the primary source, so electric resistance may make sense.

    It almost seems like change in this industry is happening fast enough that the perfect solution will crop up soon.

    This design won't have interior rooms (just hallways and closets) but I would say that in general the interior rooms of a well sealed and insulated home don't need heating or A/C, just ventilation if necessary.

  13. #13
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    Jun 2001
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    160
    It seems like these homes are going to be marketed as "upscale". Correct me if I am wrong.
    I do not think most home buyers would find it acceptable to have mini splits hanging on the wall. Around here (SE Pa) heat pumps are perceived as being not as desirable. If nat gas if available you definitely use a furnace. If it is not available most developments choose between heat pumps and propane furnaces based on the price. Most people would see a heat pump system in a upscale house as a negative. That view might be changing slightly because the price of nat gas is going up.

    Brent

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