Trying to make an informed decision about what type of air source heat pump to install in my house (new construction).

I'm building a steel SIP house - don't want to oversize it, but I also don't want to be hot in the summer time. My options seem to be a 3.5 ton compressor matched with a 4 ton air handler at 13 SEER, or a 5 ton 16 SEER with a two stage compressor.

Is the 16 SEER two stage compressor worth the very significant cost increase compared to the single stage compressor? Repair costs will undoubtedly be significantly higher as well. Lots more stuff to go wrong, lots more stuff to challenge the service technician.

Any experience or knowledge to share?

Come on Dana1, where are you when I need you in my thread?

I would use one of the simulation programs to estimate annual expenses with the different units - if you don't use much, then expensive equipment isn't justified.

I'm in Louisiana, it's like a sauna, wrapped in a jungle - cooling and dehumidification needs are significant. On the other hand we rarely get below the 40s in the winter.

If your area is like Houston, it seems that almost everyday from mid May through September is close to peak design design day parameters. On a really bad day it might be 98-100 instead of the normal 93-95.

Where is the split between 1st and 2nd stages? My GUESS is that if manual J calcs say 3.5, you may very seldom ever use the second stage of a 5 ton. Is the 16 SEER throughout the range (i.e., what is the efficiency in 1st stage where you will probably spend most of your time)?

I intuitively think single stage.

Bruce

If prices warrant it, consider a smaller 20 seer unit that will do almost all of the work and then a second cheaper, low seer unit (perhaps a 2 ton mini-split) to help it out on the hottest days.

Posted By Jelly on 29 Sep 2010 07:52 PM
Come on Dana1, where are you when I need you in my thread?

Asleep at the wheel, of course... ;-)

The amount of solar gain a particular house picks up isn't a simple thing to model- Manual-J estimates are generously oversized for most homes.  When in doubt, go smaller, especially if you've optimized the glazing & overhangs for low summertime solar gain, have a "cool roof", and higher- R walls.

Steel SIPs are easy to make air tight, but you know for sure what your actual infiltration rates are? It makes a huge difference on how much latent-load you're dealing with, and with a low-gain very-well insulated home your latent loads of a leaky house can often outpace the sensible loads even on hot days.  Very high SEER units typically dehumidify less than their less-efficient cousins, and you're likely to need either a "dehumidify mode" or  a separate dehumidifier to stay comfortable under all conditions with a high SEER unit.

I'm not an AC pro, and there are so many relevant details to factor in, methinks that "design by web-forum" won't work very well here.

Many thanks to all who chimed in. The funny thing is that many of the local AC pros don't know where to start. I guess my house isn't typical, so I'm not getting much help going that route. I agree with going smaller - I would much rather undersize than oversize. But the big challenge is the distribution of the air. A smaller air handler may not be able to deliver enough air to the various far ends of a pretty large and spread out space.

A third option has presented itself. A 4 ton air source heat pump matched with a 4 ton air handler puts me up to 14 SEER but still using a single stage compressor. This allows me to squeak under the wire and still qualify as an Energy Star home if I pursue that, yet not spend an exorbitant upcharge on the 2 stage unit.

I do worry about oversizing, but there is a lot of square footage. And there's an additional 600 sq ft of conditioned attic space that I don't believe the Manual J calcs really take into account.

Dana, no I don't know what my actual infiltration rates are. I have casement windows, and the doors have weather stripping and foam where they meet the panel. What I think you're saying about the latent versus sensible load is just that while the AC may very well cool down the house enough, it would really need to keep running to dehumidify the air properly?

Bruce, yes the climate is similar to Houston - two seasons, summer and Christmas!

With casement windows in a metal SIP house it's probably way tighter than the typical LA home, which would mean LESS latent load than typical in summer, but possibly MORE latent load in the low sensible load November-March seasons when outdoor dew points are lower, but the AC isn't running, and thus not removing ANY moisture from the air. To dehumidify it's not a matter of how long the AC is running (although that counts), but the temp of the coil and volume of air passing through. High SEER units are higher temp higher volume since having to chill the coil further than the minimum necessary for sensible cooling reduces the COP. Lowering the air volume makes for cooler coil temps and more condensing of moisture out of the air. When there's no or low sensible load, there's no dehumidification to speak of going on. (In my New England home the latent load is pretty much the WHOLE load during most of the summer season, and the central AC only runs a few 10s of hours per season, even during a much warmer than average summer like we had this year. The dehumidifier runs a much higher duty cycle, rising & falling with outdoor dew points.)

Don't sweat the additional 600 square feet of conditioned attic- the additional cooling load that represents wouldn't likely come CLOSE to breaking over the generous overestimation of a Manual-J calc on the rest of the place.

I know you've told me you're not an AC guy, AND HVAC design-by-web-forum is not wise, BUT...

I think I hear you saying 4 tons may be over doing it in a 3300 sq ft tight steel SIP house? I won't hold you to anything

I know rule-of-thumb methods are not very scientific, but one AC guy told me his "rule-of-thumb" method is 400 sq ft per ton. That would put me over 8 tons! Outrageous! But another rule-of-thumb method I've heard for a SIP house is to half the typical rule-of-thumb method for a traditional house. That gives me 4 tons, which is half a ton over what the Manual J calc said. But if Manual J calcs are typically oversized, well then who knows. I think my head may explode!

I literally don' t know if 4 tons is a reasonable number, too high, or too low without doing the Manual-J and making adjustments based on educated guesses about just how tight the place is, etc. If Manual-J says 3.5 tons, 4 is probably oversized for what's really a sub-3 ton "real world" load, but probably not by more than 2x unless you have essentially no windows, and your SIPs are R30 or something.

I suspect 8 tons would be overkill for even standard construction in LA that size, but probably not by more than 50%. :-)

And 2x oversizing isn't nearly the efficiency & maintenance hit that 4x would be (had you gone with the 8 ton rule.)

You're probably overthinking this. No estimates are perfect, and if it turns out your true load is 2.5 tons you'll be glad you didn't back off all the way to 2, and the efficiency hit by running 4 tons is pretty minor.

Well yes my SIPs are about R30 walls and roof. They're 4 inch panels with polyurethane cores.

DO not use "rule of thumbs"     ....3-1/2 tons seems reasonable for your house

With R30 SIP walls & roof the predominant solar gains will be the doors & windows + infiltration factors. If Manual-J came up with 3.5 tons, you may be under 3, but probably not under 2. Let's call it 2.5, just for the sake of argument, in which case a 4 ton would be a 1.6x oversizing factor- which is enough to matter, but not a holy disaster. A 3.5 ton would be 1.4x oversized and matter less.

Going with a 3 ton unit would be even better from an oversizing P.O.V., and if the peak loads were ACTUALLY 3.5 tons per the calc, you'd still be only 14% undersized for the peak load, which is also not a disaster- you likely have enough thermal mass that in practice you'd still be comfortable even if it coudn't maintain the temperature setpoint on the hottest hours of the hottest days. (I'm not enough of an AC guy to know how well a 3 ton compressor works with a 4 ton air handler & coil.) When it's not keeping up you have a 100% duty cycle, and drier air.

I LIKE dry air, a lot. Probably should move to California. You guys are great and I really appreciate you sticking with me.

The big nut to crack is regarding distribution, and that seems to be a significant challenge with a layout like mine. The inside air handler and coil can only be sized up half a ton from the compressor. So let's say I have a 3 ton compressor and a 3.5 ton air handler. That may very well be oversized for the load, or even just perfect. But will the 3.5 ton air handler deliver enough cool air to a total of 19 registers, including the farthest, which is 45 feet or more from the blower, plus a couple of turns.

The bigger I size the air handler up to handle distribution, the more oversized the compressor will be for the load, and the closer I get to a sick or humid house. That's what I keep waffling about.

I share your concern. What do your HVAC guys say about the adequacy of the fan size and air distribution? It can be calculated (and should be a part of Man J, I believe).  How many total CFM are required?  How many CFM are the units rated for?

I think the issue is likely duct size and static pressure rather than the CFM capacity of the fan. Getting the right proportional balance so that rooms get the right amount of air will be a challenge

What is the HP and static presssure difference between the fans in the various units?

Can you upsize the ducts to minimize static pressure? Are you using hard duct or flex? (when I had some AC units replaced in Houston earlier this year their eyes glazed when I mentioned hard duct....very frustrating)

IF the system is limited by static pressure, a fan with more CFM may not generate a higher static pressure and would not move any more air.

Can the contractor put a higher HP fan in the furnace without voiding the warranty?

Bruce

Bruce, good questions and I will pose them to the HVAC guys. So far none of them have mentioned any of those issues in terms of problem solving the distribution. Mostly they just want to upsize the whole thing by several tons.

Jelly;

upsizing moe than a 1/2 ton will put you at risk for "sick building syndrome" 

I have 14 supply ducts on a 2-1/2 ton airhandler , the longest run being 40 feet and is the coolest room in the home.

The duct sizing is in Manual D rather than Manual J. Manual J calculates the heating and cooling load by room. That is the first thing to do. Once you have that information, Manual D is used to calculate the sizes of ducts to provide the right amount of heating and cooling to each room. If your HVAC guy doesn't have these calculations, you have the wrong HVAC guy.

Ok, I have a new HVAC guy who actually uses Manual J (and Manual D). He ran the numbers and comes up with 3.5 tons, too.

Here we go... he'll install a 3.5 ton single stage unit, BUT his recommendation is a two stage 4 ton unit. The reasoning is that most of the time it will run at only 3 ton capacity (until the dog days of summer when we can bump it up to the second stage at 4 tons if need be). This means the two stage unit will actually better dehumidify than the single stage, or so he says.

That sounds opposite from the info I'm getting here. Does it sound like he's just trying to sell the more expensive equipment?

And does anyone have an opinion on the UV light? Do they work, is it worth it?