I'm trying to get a properly sized central air conditioner install in my home. I'm doing radiant heat and don't need a furnace. I want the central air to provide cooling in the summer and some air exchange when needed since SIPs are really air tight. Here in Los Alamos, NM it rarely gets above 90 F but while in the high 80s it is to uncomfortable not to have an AC. So I don't need a large system to begin with, but I set the outside temperature to 95 F on the code. SIP aren't very common around here and I haven't found an HVAC contractor familiar with them. Right now I have one giving me bid and due to ducting restraints between floors we thought it would be a good idea to split the floors into to different zones. I told him it was a SIP house and very air tight and well insulated. He said OK and left. I had the feeling he wasn't going to do a heat gain/loss calc so I found some HVAC software online that will do residential for $45 and had some good reviews. http://www.hvaccomputer.com I did a heat gain/loss on my home and came up with the following figures. First Floor 1250 sqft Sensible Heat Gain: 7,772 BTUH Latent Heat Gain: 984 BTUH Total: 8,756 BTUH Second Floor 1550 sqft Sensible Heat Gain: 7,443 BTUH Latent Heat Gain: 501 BTUH Total: 7,944 BTUH Total House Heat Gain: 16,700 BTUH Which is about 1.5 Tons. So I called up the HVAC contractor and told him about 1 Ton per floor should be fine. He baulked at the number and said it is hard to even find 1 Ton units. We then had a conversation and basically he only uses sq-ft and CFM. He basically said you need 1 CFM per sq-ft and 1 Ton = 400 CFM. So I need a 4 Ton unit and a 3 Ton unit and maybe I could get away with a 3.5 and 2.5 Ton units. He said the blowers are matched with the condensers so he can't get a 1 Ton condenser with a 4 Ton blower. He said if he found a 1 Ton unit and installed it we would barely feel the air comming out and it wouldn't work. Does anyone have some thoughts on this. How do I get the CFM up but keep the BTUH heat removal down. Is this guy even realistic. Any help would be greatly appreciated. Mark

Mark,

Dump-The-Dude! An oversized unit may not run long enough to remove the humidity. You should also consider an ERV. A good place to ask specific questions would be HeatingHelp - The Wall(at the bottom click on 'Questions').

These clowns that base Heating & Cooling by the sqft just kill me. They seem to think that all houses are built the same crummy way.

Mark,
 Actually it's not the bottom, but at the top(Admin - Where is the %$*&!!# EDIT Button?).

Thanks for the link, I found the wall just fine. I called up Trane (Manufacturer) and asked them a few questions. The guy was somewhat helpful. He basically said they don't get into air flow and ducting issues, so he can't say much about that. He did say you don't want to put an extra large blower on a condenser which my sub stated also. He basically said I will have a hard time properly cooling my house because a large unit will blow enough CFM but will draw too much BTUH and shut off quickly, a small unit won't work because it can't blow enough to circulate the air. I will check out that link to see if I can't find a solution. I'm SIP and ICFs have had to deal with this before. What good cooling solutions are there? Mark

Not being a cooling professional, I would say to size the A/C properly to cool the house. And, use an ERV to handle the fresh air. I believe that some ERV's(and HRV's) can also be used to manage humidity.

Thanks for the link to that forum. After posting there one of the members suggested a ductless split AC system. I believe it will work for me and it has the added benefit of being easily zoned. I may even attempt to install it myself. I will have to install a seperate fresh air intake system but that shouldn't cost to much.

Thanks again,
Mark

Check out this website. They have a good tutorial for duct and Central Air design. It all comes down to cfm and duct size and the air exchange rate you need. Whoops, this time I'll actually post the web address. http://www.descoenergy.com/

Mark,
I'm running into the same questions you had back in January.  I'm building SIP walls, ICF foundation, and want radiant heat, but need A/C to keep comfortable when the temps and humidity both reach 96 (like they've been this last week).  I'll also need an HRV/ERV and want to know what you ended up doing on your project. 
you can reach me direct at [email protected]

Thanks,
Ben

Ben

I decided to go with the VENMAR HEPA 3000 HRV. Venmar also makes a ERV but since I live in a dry climate I didn't need it. I then put in a 2.5 Ton Spacepak miniduct system by from ACDirect's website. Since the Spacepak is a high velocity system I couldn't couple the HRV with it. Which is actually better so the HRV can cycle on and off in the winter to and I won't have to turn on the AC fan on to get it to run. The Spacepak system doesn't require any tin duct work so that made it easier to install. I also didn't add a heater to it since I have radiant heat. I spoke with the AC Direct crew and they helped me put together a plan and designed the system for me and his HVAC guy used the same code I used. We agreed to up the size of the AC to 2.5 for the whole house which should keep it cool and get a reasonable air flow. I just put up the sheetrock in my house so I haven't even tested the systems. So I don't know how well it will perform. Hopefully I'll find out in a couple of months unfortunately that will be right after the summer ends. Hope that helps.

Mark

Mark,

I really like the idea of miniduct systems, but have always been a bit concerned that they might be too noisy. Are you at the point where you can determine whether the noise level of the faster moving air is significant?

Dick Mills

Thanks!
Did you install the systems yourself?  If so, what experience level do you have going into it?  Are the AC direct people able to size a system for me based on my floor plan and location?

Keep me updated if you wouldn't mind.

Ben

In my post above I meant to say I can't couple the HRV and Miniduct system. Hence they were installed independently of each other.

Yes I installed the miniduct system myself and it wasn't difficult. Now having said that I still haven't tested it yet so maybe I completely screwed it up. Who knows? As far as noise goes from what I read it really depends upon if there are any bends near the exit. The sheetrock just went up so I haven't finished the electrical and hence haven't had power to the blower or compressor yet. The hardest part was installing the copper between the compressor and blower. I was at a 60 foot seperation so I had to install 3/4 ID copper. I bought that flexible copper tubbing and it was too stiff to run through the house and when you did bend it, half the time it kinked. So I eventually went with a combination of rigid copper and flexible copper. I'm excited to see how it works, but unfortunately at the rate I'm going that probably won't be for a month or two.

The ACdirect group helped me size the system and created a installation plan based on my house drawings for free. I had no experience installing an AC before and didn't find it difficult after watching their installation videos and reading the literature. I was able to install it for $6,000 and the sub wanted to do it for $30,000+. Since the seperation between the blower and compressor was over 30 feet I'm going to have to hire someone to fill the system with refridgerant.

Mark

THanks, guys, information is helpful. I, like most people in the SIP and ICF industry, run into the problem of finding competent HVAC guys that don't just pile over the thumb and say, in my neck of the woods, 1 ton per 500sqf.
Panelcrafter, have you done your own loads of your buildings and then had to take responsibility for your decisions?
I am very uncomfortable with the thought of building air tight houses, in my case it is with ICF's, and then have to make HVAC decisions for my clients only because local HVAC contractors just go by stick-build figures.
How do you find competent people?

Yes. The key is to take accurate measurements, and understand the insulative properties of the structure.

I trusted the 'pros' and got burnt, so I took it upon myself to learn the real cause of the problem. Scenario:

• Plumbers up here do most of the HW heating work...
• Before we finished the lower level, I had a plumber rough in HW baseboards for the upper level which was forced air. The lower level was radiant powered by a WH...
• A few years later I had the money to have a boiler installed(another plumber)...
• We fired it all up and all was ok until it started to get cold...
• The system couldn't heat the house on cold days and ran nonstop...
• Everyone was pointing fingers...

So, at this point I didn't trust any of them. I started researching and learned how to perform an accurate Heat Loss calculation. I also had to relearn fluid dynamics to understand pipe sizing. Here's what I found:

• In all 3 zones the baseboard convectors were undersized(not enough BTU output)...
• They used 1/2" pex which restricted the flow and the zones were too big(length of pipe)...
• Basebaord convectors are a poor solution for an area with high ceilings and lots of windows...

So, I found a genuine hydronic pro and we came up with a solution(not easy, by now the entire house was finished and I didn't want to rip it all up). Here's what we did:

• Performed an accurate Heat Loss for every room in the house...
• In the main LR/DR/Nook/Kit are with the high ceilings, we replaced all of the baseboard convectors with baseboard radiators(higher BTU output and they were radiators instead of convectors)...
• Turned this zone in to 2 zones fed with 3/4" kitec...
• Used the baseboard convectors that we took out to replace and add to the baseboard convectors in the other 2 zones...
• We ran 3/4" kitec to the middle of the other 2 zones and tee'd onto the existing 1/2" pex...
• We ended up with 4 zones and 8 returns. Strange but we were able to utilize most of the existing 1/2" pex, feed it with 3/4" pipe and improve the flows...
• With the extra baseboards we were able to over size each room by about 60%, and the system does NOT short cycle.
• We did have to rip up the garage ceiling to install the retrofit, but the rest of the house was spared damage(a lot of snaking!)...

The system now works great and keeps every area of the house warm on the coldest days.

I am confident on any Heat Loss that I perform. All it is is math(and I wrote a calculation program) and knowing the materials. But also, I don't have to worry about A/C, it doesn't get that hot up here.

I guess that the bottom line is to have a Heat Loss & Heat Gain performed and don't allow sombody to just use their standard sqft calculation for a SIP or ICF house. They will over estimate the loads and end up costing you money.

Finding competent people? That is tricky. If they won't have a Heat Loss performed(and can show you the results) is a good indicator of who to bypass. I also don't like 'staple-up' unless they use heat diffusers. Also insist on 3/4" pipe for all baseboard convector/radiator designs. And ask to see some of their work(in progress) to see if you like what you see.