We are building a new house in Austin and we just got the quote back for the HVAC system. I have a couple of questions that I was hoping maybe some of you could answer. Our total footage is right about 3,600. We have a small second level of about 900sqft that consists of two bedrooms/bathrooms and a small game room for our kids. The HVAC company is quoting two units with electric heat pumps. It looks like they are Carrier Performance Series 16 seer. Part of our downstairs will share a unit with the upstairs space. I wasn't sure how that works, but the HVAC guy tells me the 5 ton unit he is purposing for the shared unit will be more than enough. He gave me an option for a 3rd unit for just the upstairs, but he said it would be overkill and not worth the extra $3,800. How effective are shared units with zoning like he is purposing? Should I be concerned about electric heat pumps? I do have natural gas available. I want to do this right the first time. Thanks in advance for the help.

I don't think it is standard practice to share a unit with the upstairs, but I'll let the pros weigh in. However, I have a single 5 ton unit that serves 2400 sq ft down, and a room about 1,000 square feet upstairs. Three zones...two downstairs, and the one room upstairs. The other part of my upstairs is served with my 2 ton unit and it is about 1300 square feet.

Mine was set up like it was due to remodelling stages and timing. My system works as it should, but my 5 ton is a multi-stage 16SEER VS heat pump with and ECW Control board. It is fully communicating and will make adjustments based on loads and which zones are calling and at what capacity they are requesting.

If my home had been built traditionally, I probably would have still done the same units, but I would have made a 4 ton for the downstairs and a 4 ton for the upstairs (or whatever Man J called for) and zoned two up and two down. I would have stayed with a heatpump everywhere, multistage and VS system in order to properly handle the large variation in room sizes and heat loads.

The more than enough comment would make me suspect. You don't want more than enough with AC you will never get the humidity down. Also if split across floors it would need to be zoned and multispeed for me.
You need the ManJ calcs, sizing based on Sq Ft and tons is useless.
I would consider doing split system for the 900'ft area. Wish I did, my upstairs is much bigger (2500ft) but since kids moved out it would be nice to have flexibility to ac/heat smaller areas.

Thanks guys. The quote says three zone and the units are multi speed. Not sure if it makes a difference but the builder is foaming the exterior walls, some interior and attic.

What is the total tonnage he is putting in? With an all-foam house, you need to make sure he is not putting in too much. You will have moisture issues in no time. With all foam, I would not do anything less than a two-stage condenser with dehumidifier tied to the indoor unit.

It looks like 7 tons total. He also mentioned not doing the 3rd unit because of the foam. He said the exact same thing about the humidity.

but did he size it with the manual j?

I am not sure. I can ask how he sized it. I know he looked at the plans and has come out while we are framing to look over everything. They are a group called West Austin Air.

a dual zone system with two stories is done all the time. you'll be fine

Don't do electric aux heat if you have natural gas available.

The heat pumps are only efficient down into the mid 40s or so. When we have cold snaps below 40, the electric aux heat strip is running all the time. If the heat pump tries to run during this time, you are basically just wasting electricity. My HVAC licensed Uncle says you might as well put it on emergency heat during really cold snaps.

My worst electric bills are in January and February, not August like you might expect.

If they have a gas furnace option for the heat pump then maybe do that, but I would question whether natural gas is cheaper than running the heat pump in the mild cold periods.

Just to reiterate what other people are sharing here. An A/C unit does two jobs dehumidifies the air and drops its temperature. By oversizing the A/C unit you get short cycles that blast cold air in and drop the temp rapidly but don't pull enough air across the coils to dehumidify it. Not fun, makes it clammy in your house and also reduces compressor life.

The manual people keep referring to us ASHRAE Manual J. You can download the pdf and read up on it yourself. Here is a online calculator to help you do it yourself.

http://www.loadcalc.net/structure.php

I haven't used this one, but a quick glance looks like it can get you close.

Not sure why you'd want an electric heat pump, that sounds expensive with little chance of it paying for itself.

Heat pump is not that much more than gas heat installed. Also, the problem with putting in gas heat in a foam house is that you will have to punch holes in the foam for exhaust and combustion air.

Sounds like this guy is probably recommending the best solution based on the build of my house. Thanks again for all the help.

I'd say a single 5 ton on a fully foamed and air sealed 3600sq ft home is probably right on the money if that's what they arrived at via calculation. Make sure they put enough air returns in.

Just curious... Why do you say nothing less than a 2 stage condenser? I don't even really know what that means, but we're building a similar sized 2 story house with foam (3400ft2) and the initial quote I got was 16 seer - 1 stage - 7 ton unit with zoning. We're still in the design phase, but I want to be sure I'm on top of this.

MAS444 said:

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Just curious... Why do you say nothing less than a 2 stage condenser? I don't even really know what that means, but we're building a similar sized 2 story house with foam (3400ft2) and the initial quote I got was 16 seer - 1 stage - 7 ton unit with zoning. We're still in the design phase, but I want to be sure I'm on top of this.

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A two-stage condensing unit has a low speed and high speed. In low speed, the compressor runs at around 70%. The idea is to have the compressor running on low speed the majority of the time (except when it is 95+ outside). This will help to lower utility bills and pull more moisture out of the house (a necessity in foam houses). The reason for a dehumidifier in an all foam home is to pull out moisture in the winter time. A/C units only dehumidify when in cooling mode.

They are putting in two units correct?

From the way I read the GCs estimate, it's only 1 unit. Thanks for the response. Very helpful.

MAS444 said:

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From the way I read the GCs estimate, it's only 1 unit. Thanks for the response. Very helpful.

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5-ton is the largest residential split system. I am fairly certain they are quoting two systems.

Probably so then. Thanks again.