HVAC went out 6 months before home renovation to start

[DIYKO]

In six to twelve months we'll be adding 500 square feet to our home but my 12 year old 4 ton XL14 Trane Heat Pump was just diagnosed with a leak in the evaporator and a bad variable fan controller. Odd they both went out at once but we were going to remove the 4 ton that's support 1,900 sf and put in a 5 ton unit and convert to gas during the expansion to 2,400 anyway. My question is around the interim time while I'm still at 1,900 sf. I've been told by one contractor the 5 ton XL20i Trane is too large and will cause mold issues and that I have to sign a waiver about possible mold. I have another contractor saying if I go with a Lennox XC21 that I should be fine (because it's a two stage) while we prepare for the expansion. I know both are two stage but why the differing opinion? I'm in Dallas TX if that climate makes any difference.

[BaldLoonie]

The
20i is a 2 stage with low about 55% of high. The Lennox on low is more like 75% on low so not near as much difference.

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[Shophound]

Unless the ductwork on your existing four ton system is sized (and properly installed) for five tons, you may have problems with the five ton system even after the addition is completed and tied into your existing ductwork.

Consider a separate heating and cooling system for the addition, which would enable your existing system to be replaced right now without upsizing (provided it was sized properly in the first place). It will add somewhat to the addition's cost, but would likely better heat and cool the addition than if it were tacked onto the existing ductwork.

[DIYKO]

Unless the ductwork on your existing four ton system is sized (and properly installed) for five tons, you may have problems with the five ton system even after the addition is completed and tied into your existing ductwork.

Consider a separate heating and cooling system for the addition, which would enable your existing system to be replaced right now without upsizing (provided it was sized properly in the first place). It will add somewhat to the addition's cost, but would likely better heat and cool the addition than if it were tacked onto the existing ductwork.

Absolutely on the duct work, both plenums are being replaced with an addition to the return to match the 5 tons needs, the supply is being expanded to handle the new runs to the addition. We actually are adding two additions and the second 1600 sf will be running on a separate unit. The 500 addition or should I say expansion is on the main house where the 4 ton is, it's more of an expansion to a great room and a master bedroom so the contractor felt it was easier to incorporate than separate on the main house.

[DIYKO]

The
20i is a 2 stage with low about 55% of high. The Lennox on low is more like 75% on low so not near as much difference.

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Excellent information. Thanks!!!!!

[beenthere]

Adding 500 sq ft isn't going to increase the cooling load by 1 ton. May even decrease it a bit. The existing wall/walls where the addition is added will no longer have a heat gain. the addition will be built to todays codes so it will probably have much better insulation and less air infiltration. And may decrease the rest of the homes infiltration rate.


Should have a load calc done to see what size you really need.

[dbailey]

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[beenthere]

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[DIYKO]

I'll get the calc done before we make any decisions. Thanks

[motoguy128]

I'll get the calc done before we make any decisions. Thanks

Abosultely, and if in doubt... undersize. A slightly undersized system will always use less energy than an ideally sized or a slightly oversized system, it just might fall behind for a few hours in the hottest and coldest weather. Load calcs already have built-in "fudge factors". Furnaces are slightly more effcient on hte highest stage, heat pumps are usually more efficeint in high stage at the same temperature. Only AC is a little less efficient in high stage, but any losses on the hottest days will be made up with longer run times the other 99% of the time. Smaller equipment also means smaller ductwork, and if doign a zoned system, it reduces the size of the bypass damper, reduces noise and overall smaller systems just makes everything work better and the home more comfortable. You jsut can't expect the systme to recover 5, 7 or 10 F in 30 minutes like the massively oversized furnace people are used to.

With 80% furnaces going away in some parts of the country in May, there will be a whole new wave of oversizing, as already oversized 80% furnaces get replaced with the same input rated 90%+ furnaces. Sometimes I think mfg's could help their customers out by using the max output rating in their model numbers.

[GREGHVACGUY]

If the controller for the fan went bad. could they put in regular motor in and if the leak is not that bad you could limp the unit till a new one is put in. Have you asked them about that.

[DIYKO]

They never mentioned the regular motor option but he said the leak wasn't fixable and I'd have to run on auxiliary heat and I'm Texas and warmer days are less than 60 days away, so I that lead to want to just swap it out now. Fortunately i have a wood burning stove and have been able to keep the house warm while I make this decision but momma's getting tired of the fires.

[Cooked]

Adding 500 sq ft isn't going to increase the cooling load by 1 ton..
......................

It certainly could, remember this is in Dallas, TX. Some years ago in a house I owned in northern Texas I added a 500 sq ft TV/computer room with three exposed walls and lot of windows, one door and a huge vaulted ceiling. It added right at 1.25 tons to the load.

........................Should have a load calc done to see what size you really need.

That's simply the best advice to be had!