Heat Pump Replacement Help

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SAS

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We have a 12 year old 1.5 ton heat pump that has been nothing but trouble since we moved into our house last January. The condenser fan has once again died and I just can't see putting more money into this unit. It is used to heat and cool an upstairs bedroom suite of roughly 750 square feet. The air handler is in the attic adjoining the bedroom. When the unit was operating, it could not handle the late afternoon heat in northern Maryland.

I have had several contractors take a look and provide a quote. Only one of them did a heat load calculation; his results pointed to a 1.5 ton unit. I'm a bit concerned about the size, given that the current unit (when it worked) did not adequately cool the space. Taking that into account he has recommended a 2 ton Bryant unit - either a 13 SEER Legacy or a 15 SEER Preferred unit. He also believes that the existing 5/16 line needs to be replaced with a 3/8 line.

A second contractor has recommended a choice of 3 Trane units, 2 of them are 2.5 tons and the third is a 2 speed compressor rated at 3 tons.

The third contractor went with the Bryant 2 tons, but also suggested "oversizing" with a 3 ton 2 stage unit.

My questions for anyone who can help me are:
What size unit should I go for? 2 ton, 2.5 tons or a 2 stage unit which would normally run at about 2 tons but have the ability to go up to 3 if/when needed?
Does the brand matter? We had great performance from our Trane units in our previous home, but there's quite a premium to pay over Bryant. Is it worth it?
Does the line set need to be replaced?
 

Jacobsond

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If your contractor did not do a heat load calculation he is just guessing. Do you want to have something installed on a guess. I wouldn't. Ask the other contractors to do the heat load and prove their recommendation. If they wont find another contractor. With AC to big is not good. You want it just right.
 

SAS

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If your contractor did not do a heat load calculation he is just guessing. Do you want to have something installed on a guess. I wouldn't. Ask the other contractors to do the heat load and prove their recommendation. If they wont find another contractor. With AC to big is not good. You want it just right.
Thanks, I decided to go with the contractor who performed the heat load calculations. I had him price out some Trane units since we had very good luck with them in our previous home. And after doing some homework, I also told him to go ahead and replace the lines as he had suggested.
 

Dana

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It's highly unlikely that a 750 square foot master suite in MD would have a heat load larger than what could be delivered by a 1.5 ton compressor. In fact the 1.5 tons is probably oversized for the heat load, unless you literally sleep with the windows open, or the ducts leak great gusts of air and are located in an unconditioned attic, changing the pressure dynamics between the attic and conditioned space raising the load.

But if you have a lot of west-facing window &/or limited attic insulation, or the ducts & air handler are in the attic above the insulation you could conceivably have cooling load that high.

The solution isn't a matter of picking the right sized compressor- the whole system and it's configuration relative to the thermal & pressure envelope of the house have to be considered. Using dumb rules of thumb for cooling like " a ton per 500 feet" is probably how the original installation came up with at 1.5 tonner. But it's likely that the cooling load of the space itself is less than 1-ton, but putting the air handler & ducts up in the attic to pick up heat directly and drive air-infiltration raising the latent load drives the load to over 1.5 tons while it's operating.

But a 2.5 or 3 ton unit to condition 750 square feet of space is completely nuts, unless it's a west-facing greenhouse or something, with huge afternoon solar gains.

It's usually better to hire an energy nerd, not an HVAC contractor to run the heating & cooling load numbers, with an eye toward optimizing what you have. There may even be ductless or mini-duct mini-split solutions that could handle both heating & cooling with a 1-ton compressor, that won't drive air infiltration.

Hopefully your contractor with the 1.5 ton solution did the due-diligence and had no thumb on the load calculations, and does the duct & register boot sealing well.
 

SAS

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Well, we do have most of the factors that would add to the cooling load - except for the greenhouse. One contractor refused to upgrade the existing unit without ripping up the floor to re-route the line set and returns, and completely redesigning the duct work. He didn't feel that he could supply a system that functioned properly working with the constraints of the current environment. While it's nice to know that he has high standards, that's just not a practical approach.

So our big problem, as you suggested, is late afternoon solar gain. If I'm not going to reconfigure the ducts, etc. then my biggest question regarding sizing is whether the old unit before it died was delivering 1.5 tons of cooling. If it was, then 1.5 is not enough, despite a very careful analysis by the contractor I've chosen. Due to our concerns and experience with the 1.5 ton unit, he suggested we go with 2 tons. How much risk do you see if 1.5 is really the right answer (since he will improve the current configuration by sealing any leaks and adding some insulation)? Will putting a 2 ton unit where 1.5 is really called for create a problem with humidity? And is that a bigger risk than undersizing it at 1.5 tons?
 

Dana

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If you went with a 2 ton solution and the ducts were marginally undersized you're at higher risk of icing up the air handler coil. Tightening up the ducts (and house) and insulating does a lot for system efficiency, as long as the duct sizing is adequate for the air flow. Typical air leakage on duct systems that haven't been meticulously sealed is ~15%, and not only does the air go where you don't want it (like the attic), it imparts a pressure difference between the attic & conditioned space to drive air infiltration. If it's as bad as "typical" those total losses & increase in load could easily be 20-25% of the total cooling capacity, and if you tighten it all up you'll get most of that back.

Undersizing the compressor but having very tight ducts & house means that when it's running a 100% duty cycle it dries out the air considerably, which makes it comfortable even if the room temp is higher than the setpoint. If the ducts and house are leaky it won't dry out nearly as much. Without running the numbers on just how much load you'd be shedding with more insulation and air sealing, it's hard to say how close you are to the real margin. But 1.5 tons is a lot of cooling for 750' of code-min construction- it's about 2x what it would typically take to get there, with reasonably tight ducts and a Manual-D duct layout.

Re-using a poorly designed duct system limits the efficiency & comfort potential somewhat, but it's understandable that you don't want to live in a construction zone. For 750' of space a mini-duct type mini-split might have been a solution, depending on the room layout of the master suite (probably a luxury bath, a walk in closet or two and a big bedroom?), but it would take a bit of design work to figure that out. With a mini-duct cassette (essentially a tiny low-profile modulating air handler) you can install the thing somewhat centrally to the rooms it serves, often completely inside the thermal & pressure boundary of the house, where it can't drive air infiltration or pick up parasitic load in a hot attic.
 
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