Hi folks,

My builder just got permits for new construction here in northeast PA.  It's about 3200sqft, with some energy efficiency aspects to construction:

• wet-blown cellulose in 2x6 wall cavities
• 1" Dow SIS structural foam sheathing panels over framing
  
• open-cell spray foam under entire roof cap (roof deck and gables)
  
• Superior Walls XI extra-insulated foundation panels

I've contacted three geothermal companies for quotes for a Climatemaster Tranquility 27 system with two zones.  All quotes have come back pretty close to each other with similar specs (insulated metal trunks, flexduct runs, etc.).  Two companies quoted me a single 4-ton unit in the basement, and the third company quoted me two 2-ton systems - one in basement, one in attic.  When I spoke to the third company, he said that the cost for a dual-unit is about the same, but the comfort level and efficiency would be higher.  He also mentioned that because there are very few walls overlapping between the first and second floors, that the single-system ductwork reaching the upstairs would be quite inefficient in my case.

For reference, here is my floorplan:

my floorplan (PDF)

He's right, I'm not sure how a single system is going to get ductwork upstairs without many long runs through exterior walls.  You can see that the floorplan is very open, with the first and second floors being connected together by a 2-story foyer and 2-story breakfast area overlook.

I have some thoughts and concerns about two 2-ton systems:

1. Can you really just split up a system in half like that, since the upstairs system will bear more load in cooling season, and the downstairs system will bear more load in heating season - especially with an open floorplan like mine?
2. There are now two systems which have points of failure, although on the bright side I now have a backup system if one system goes out.
3. Of course, now there's no damping and no zoning control boards - woohoo!
4. The two systems still have to reach across a single floor's span to provide both supply and return ducts, but I suppose that's not as bad as a single downstairs system having to reach two stories for its returns.
5. I'd now no longer have a single 4-ton unit to operate a desuperheater - how is that going to work with two systems??
6. I wanted an electric air cleaner installed - now this means I have to pay for this twice...
   

Contractor #3 swears that the cost difference is negligible, and the efficiency and comfort benefits are very real.  I was very reluctant to go this route at first, but the more I think about it, I am starting to appreciate the potential benefits if the cost is the same.  No zoning, no dampers, each unit has variable capacity and a variable speed blower.

I plan to schedule sit-down meetings with all three contractors for a Q&A.  Before I did that, I wanted to run all this by the good folks here and get some thoughts.  Does anyone have experience with dual geothermal systems?  Is it practical to tap both of them for their desuperheater capabilities?

Thanks!

 - Chris

Posted By chrispitude on 21 Oct 2010 07:31 AM

5.  I'd now no longer have a single 4-ton unit to operate a desuperheater - how is that going to work with two systems?

We have 2 dsh's going to 1 tank.  A few extra "T"'s, 2 check valves, and perhaps a few other plumbing components (get bleeder valves properly located and installed in the lines.)  Works great for us.  Make sure your length of run doesn't exceed the manufacturer's recommendation.

An open floorplan is challenging with a lot of exchange between floors is challenging. There is usually no major downside, except usually costs, to installing 2 independent systems, but if there is someone who is willing to install 2 systems for you for the same price, go for it. Using electric dampers instead with one unit has the advantage that the full capacity can be directed either towards upstairs or downstairs, so this is a matter of correct sizing. Thermostat placing is crucial, especially upstairs. Make sure you put it in your bedroom, not in the hallway. We have built systems up to 4 different heat pumps, correctly size for each zone. We have piped the desuperheaters all in one buffer tank, unless the heatpumps were too far apart. It was expensive, but it worked like a charm.

Posted By docjenser on 21 Oct 2010 09:38 AM
Thermostat placing is crucial, especially upstairs. Make sure you put it in your bedroom, not in the hallway.

I agree.  We have 2 units, one for each floor.  Our foyer (not huge) is the only part of our floor plan that is open to the second floor, and we still experienced considerable heat exchange between floors with the upstairs thermostat in the hall and the doors to the rooms on the second floor open.  To reduce this problem, we put a remote thermostat sensor in our bedroom, and now keep the doors to the rooms on the second floor closed.

In cooling season, this helps keep the cool air in the upstairs rooms instead of allowing the cool air to more easily "escape" down the steps to the first floor.  This has reduced the load on the second floor unit in cooling mode.  In heating season, this helps keep the heat from the first floor unit from entering the bedrooms.  This has reduced the load on the first floor unit in heating mode.  This has also made it easier for us to control temperatures on each floor.

Of course, with the doors closed on the second floor, you need adequate returns.  We have one central return in the upstairs hall instead of a return in each room.  Fortunately for us, there is enough of a gap under our doors and we don't experience air flow problems.

Awesome guys, thank you for all the feedback! The upstairs desuperheater should be within the allowable 50' distance if placed and routed with care, so maybe that would work after all. I meet with one of the contractors (NOT the one who suggested dual units), and I'll see what they say when I bring it up.

So you can get a feel for what can be done, I'll explain our layout. Our packaged unit (aka "all-in one" unit) for the first floor is in the basement. Our second floor unit is a split system, with the air handler in the attic, and the compressor section in the basement right next to the packaged unit and our water heater. A line set (one insulated) and a few wires is all that connects the compressor section to the air handler. The DSH for the split unit is in the compressor section and is very close to our water heater.

Major downside to two units is generally hardware cost. If that is balanced by ductwork issues / savings, then go for it. A two unit system will be more flexible as well as providing some redundancy as written above.

Avoiding zoneboards and dampers is not a driving consideration toward two units - those components are much cheaper than a second unit.

I have a single package Envision 038 serving 4 zones on 3 floors. It works superbly, but it benefits from a generous duct chase extending from basement to attic.

Trying to force fit a single unit into a house with circuitous and constrained ductwork paths is a recipe for unhappiness.

I've spoken to two contractors so far. Both have looked at the floorplans, and indicated that it shouldn't be a problem to go with a single 4-ton unit in the basement. They both mentioned that the 2x6 exterior walls will have adequate room for chases. They also said that if I ask the framing crew to keep interior floor and wall joists lined up, it will open up options in the interior walls too. One contractor mentioned that splitting 4 ton of total requirement into two 2-ton systems is not necessarily ideal, since upstairs cooling requirements and downstairs heating requirements could each exceed 2 tons, especially with my open floorplan. Since the upstairs is mostly open and the heating/cooling requirements are not too demanding, I feel more comfortable going with a single 4-ton.  It simplifies the desuperheater plumbing too.