Choosing an HVAC system (size, that is)

[fredhargis]

Here's my dilemma: my house is (apparently) exactly the wrong size for the existing heating systems offered, I fall right into the middle of the 2 size ranges offered. More details: the house currently has an older geothermal/electric system I'm replacing with a split system. The heat load calculations (2 different ones with the same result) show that we are right between needing a 5 ton and a 4 ton system. Cost (installation or operational) is not part of the question. I'm really leaning toward the smaller one, because my wife's health conditions require really good dehumidification. One the other hand, I want as much HP use as possible in the winter to save on LP. One of the installers tells me I can't make the wrong choice, because either will work very well and I may be overthinking it. So what points should I consider, or am I overthinking it? Again, the cost considerations aren't a factor......I can deal with them.

[daddo]

A 2 stage system is best for dehumidification.

[fredhargis]

Left that part out: in both cases the HP will be 2 stage. As I understand it, the 5 ton will cool at 42000BTU (1st stage) and the 4 ton will cool at 32000 BTU (1st stage). The current system is a 4 ton single stage, and the cooling at 4 tons is not sufficient.

[Robert Adams]

Always go bigger with AC. Can't tell you how many houses here have undersized systems. A friend had bought an energystar certified house... after the first summer ad great threat of lawsuit to the builder they finally replaced both systems with larger units and now it can maintain a comfortable temp. 

     As to dehumidifing you won't see any difference with the larger system. If humidity is very high there like in coastal areas like Houston the building scientists recommend an additional dehumidifier.  Also the DOE real world testing showed no appreciable increase in operating cost if a system was oversized. The old thought that it did came from computer modeling not real world testing.

      The heat pumps are better than straight electric but personally I'm not a fan of them. They work ok but the closer you get to freezing the longer they run. They are a good solution for places with no nat gas. However I'd look at your propane cost vs Elec and see what's cheaper. With propane costs up north being low I'd bet propane. (Propane is about 60% cheaper in ohio than tx right now)

[fredhargis]

Then I would put in a 5 ton, 2-stage system.

If 48,000 BTUs is not sufficient (is this at full load?) then why are you considering it?

Well, the house was built in 1998 with a 48,0000 BTU unit and seems to have survived just fine, heatwise. In fact, the POs kept the breaker to the strips turned off and apparently got along with just the HP and whatever the 2 gas fireplaces put out. I would guess that on the coldest days they turned the strips back on, but have no facts to base that on. But 48000 is too much for cooling, the unit will cycle for about 20 minutes and poop off...it holds the temp in the house just fine but doesn't run enough to dehumidify. Yes, the 48000 is full load. Plus, with the new split systems, if the HP isn't keeping up the furnace takes over.....makes sense since your are heating against wind chill and the thermometer can't read that.

[Roly]

Is the new system also geothermal or air source ?    I would think the geothermal system would perform better in the winter than the air source type.    Check at what outside temp it loses most of its heating capacity.   Roly

[fredhargis]

Yep, replacement will be geo as well.

[fixtureman]

If you say that the unit doesn't run long enough then why would you want to go higher

[Roly]

Are you saying you want more  heating BTU's but less AC BTU's  ?    I don't know if that is possible with a heat pump.    Roly

[Roly]

As Thooks said if its not working why even consider it.    The insulation will settle more in time and it seems to be getting warmer.    Go for the 5 ton and I doubt it will be necessary but you could always add a dehumidifier if necessary.  Roly