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Shopping HVAC - Upstairs
Old 12-30-2015, 04:30 PM   #1
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Shopping HVAC - Upstairs

DW and I are shopping for a new HVAC system (proactively) for the upstairs, which is about 1200sqft. Current HVAC is a 80%, 10-SEER TRANE split system. Furnace and air handler in the attic, A/C unit outside. We live in Cary, NC, home was built in 1998, and has the original HVAC system

My DW wants to go with a single stage, single speed 'everything' either TRANE or Carrier unit around $6300-$6400. A couple of the HVAC companies have told us that the variable speed or multi stage
systems are not worth the cost, and have more repair issues than the single/simpler HVAC systems.

We have talked with, and received quotes from 4-5 different HVAC companies. I personally have been sold on the value of having a variable speed, and dual-stage A/C and dual-stage gas valve both for cost savings, and humidity control. However, the price is about $1500-$2000 more.

The past few summers have been extremely hot (several weeks in the mid 90's). We do have a small leak/s in the coils, and the freon has been refilled a couple times in the past 5 years. We're thinking its time to bite the bullet and replace system. The old TRANE system just can't keep up with the heat, and would run all the time when temps got into the mid 90s.

Question - is the additional cost of a variable speed/multi-stage furnace and AC worth the cost and the potential headaches ?
Are we making a mistake with going with a single speed/stage 14SEER TRANE or Carrier HVAC ?

BTW - I have read through some of the threads on the HVAC-talk forum website, and even though those folks are supposedly the experts, kind of think, and get the impression they are biased toward the higher end systems.

Thank you !

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Shopping HVAC - Upstairs
Old 12-30-2015, 10:52 PM   #2
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Shopping HVAC - Upstairs

Never ever ever will I buy another home with hvac installed in the attic. We replaced a heat pump with a 2 stage gas unit. The blower is 2 speed (not variable). We live in MD. The cost was about the same as the single stage. I think it is 48k and 80k btu and maybe 30k AC. The contractor had a special deal : buy the furnace and get the AC free. Too good to be true I thought but it appeared to be accurate based on 2 other quotes. Keep in mind they had to extend the gas line to the attic and add the air inlet/exhaust through the roof. Some contractors were hesitant to quote the conversion. The unit is a Carrier and about 92% efficient/14 SEER. It seems to work very well. What is needed is attic exhaust fans to take some load off when it's really hot. I am thinking of getting a solar attic ventilator.

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Old 12-31-2015, 09:42 AM   #3
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How much of your HVAC bill is a attributed to heating and how much to cooling? I'd assume in Cary, NC that AC would be the big $ amount. So why not concentrate on the AC side? I would think a single stage gas furnace would be fine (it's been the standard just about forever), I doubt you'd see much incremental % efficiency increase, and that would be pretty low $ for heating.

Going from 10 SEER to 14 SEER should be a big deal for you, efficiency-wise. Does your current set-up handle the humidity OK? The comment 'running all the time when temps got into 90's' is a good sign - running a long time is what takes the humidity out. So don't go too much larger in AC capacity, or it will shut down quickly and you will be left with the humidity. But maybe a 2 stage AC, versus single stage or fully variable makes sense?

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Old 12-31-2015, 10:06 AM   #4
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Have you looked into mini-split heat pumps? They're supposed to be highly efficient and easy to install. The power plant sits outside, connected to the indoor cooling/heating coil by refrigerant lines.

I was talking to an HVAC guy the other day, and he recommended Mitsubishi.
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Old 12-31-2015, 12:16 PM   #5
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@ERD50 - I agree - A/C is more of the issue for us. We have a dual zone. Can't really tell about the cost of cooling the upstairs vs. downstairs between the 2 systems. (Gas heat for both)

With regards to my comment 'running all the time', I was implying that the system could not drive the temperature down to the set point. When it was 95 degrees outside, I could only get the 2nd floor cooled down to maybe 83 degrees.

Humidity upstairs was ok, and really not bad.. wouldn't say it was unbearable during the hot days/weeks. It was more about the overall temperature.

All the duct work and vent pipe is already in place and sized correctly. Contractor agreed to testing and pressure checking the existing coolant lines, before cleaning and replacing the R22 freon with R-410A.

Thank you
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Old 12-31-2015, 12:25 PM   #6
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Degree-days for us locations are available to compare systems capability, eg:

Heating & Cooling Degree Days - Free Worldwide Data Calculation

The mini splits are "ductless" which eliminates a huge cost for cases of retrofit.
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Old 12-31-2015, 01:54 PM   #7
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I am a licensed mechanical engineer, and a little biased to doing everything myself. With that said, I would likely want to do some calcs on the heating and cooling requirements before suggesting a system. Has someone suggesting the High priced system, done a heat loss calc and cooling load analysis?

I can tell you I installed an 18 SEER two stage heat pump with a variable speed fan gas furnace, 2 stage. The 2.5 ton HP and gas furnace cost me about $3500 to install myself, replacing an older 80% eff furnace. For me, the cost of the equipment is small compared to the improved efficiency, but then if you are the mercy of the HVAC cartel to install, you will pay a lot more for the best system. I could have spent about $1000 less in equipment and got a SEER 14 rating, and a lower efficiency furnace.

In the Seattle area, I set my cross over point to 35 deg F, where the gas furnace takes on the heating. The high efficiency HP runs great (Goodman higher end (same as Amana etc) is just fine don't let anyone tell you its junk). I look at the system performance, including the COP (coefficient of Performance) at the design temp range. At 50 deg, my system heats over 3X as efficient as an electrical based furnace. With a lower SEER rating, you would likely see something from 2 to 2.5 COP for that temp.

The main thing I looked for in the HP was protection for the scroll compressor. This involves controls and a suction accumulator (before the compressor). The HP is really a very simple system (to me), and the primary consideration is the brand of components, and controls for protection of the equipment. Typically you won't find the better controls and accumulator in lower SEER rated systems. The furnace should have a GE variable speed blower. This makes a big difference in setting your duct velocities for HP versus gas furnace flow. The motor drives the air to a constant flow, so you have consistent proper heat transfer for both the indoor coil in HP mode, and the gas furnace when firing gas. Without this you ducts might run too cool for HP flow, or too hot under gas firing. You set the blower speed on the controls for each stage of heat and cooling and mode of heating.

My last home I installed a single stage HP and it performed well, but not nearly as well as our 2 stage HP, with the higher COP rating.

As someone noted, for your small area, you might consider a DYI install of a split system. They are used all over the world for small area heat/cooling. They are lower in cost, but typically you will not find them to be as efficient.
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Old 12-31-2015, 02:20 PM   #8
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"Without this you ducts might run too cool for HP flow, or too hot under gas firing."

A common suggestion in the house remodelling biz in the case of an existing gas furnace is to "just speed up the fan" on the old terminal block selectable speed models. That is supposed save the budget-beleagured client a barrel of money for their new addition by increasing the air to reach the new second story or whatever. Big risk if the equipment and ductwork is not sized correctly.

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