Need advice on replacing our HVAC system

PA12August 15, 2012

I live near Philadelphia and our 2 story home is ~2400 sq ft and I am considering replacing our HVAC system within the next week or so because our AC is no longer cooling the house (so we have been without AC for the past 2 days), so any advice would be appreciated. I am just starting the research so I may not be familiar with all the terms or have all the necessary information. I am hoping you will help me to know what questions to ask.

Our current system is a Carrier and it's 19 years old. We had to repair our AC last summer and again earlier this summer but it has failed us again. So maybe it is time to replace it.

I have had a few contractors come out but their differing advice is confusing me. Our current system is 80% efficiency furnace with a 10 SEER AC.

Some contractors are recommending a 2 stage system while others are saying a single stage is sufficient. Some say it is not worth getting a higher SEER than 13 because it'll take too long to recoup the cost in energy savings as compared to 16 SEER, for example. One person suggested a 4 ton capacity (current system is 3.5 ton) while another said stay with the 3.5 ton and get a stronger blower.

Can you help this newbie? Thanks!

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tigerdunes

80 KBTU furnace var speed 2-3 stage 95%+ efficiency with a 15 SEER rated AC condenser.

New thermostat, lineset, filter box

What size is existing AC condenser, probably 3 1/2 ton?

Carrier Infinity, Trane, or Rheem.

IMO

    Bookmark   August 15, 2012 at 7:15AM
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tigerdunes

On the AC condenser if existing was satisfactory, go high efficient, either 3 1/2 ton sgl stage or 4 ton two stage. 15+ SEER.

IMO

    Bookmark   August 15, 2012 at 8:59AM
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mike_home

A 2-stage AC helps reduce humdity. It can run on the low stage for longer periods of time. This dries out the air without making the house too cold. You will find you can set the thermostat 2-3 degrees higher and still feel comfortable. The 2-stage equipment is only available in whole ton sizes.

A SEER 15-16 rating AC is a good compromise on cost and efficiency. Higher ratings than this begin to get expensive. The pay back is very long for the climate you have in the Philadelphia area.

    Bookmark   August 15, 2012 at 9:27AM
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tigerdunes

Two stage is not absolutely necessary for best dehumidification.

A high eff sgl stage AC or HP will do just as good of a job if sized correctly and especially if stat has DOD feature for AC cooling.

IMO

    Bookmark   August 15, 2012 at 9:42AM
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PA12

Current AC is 3 1/2 ton. Furnace is 80% but because our basement is finished and new pipe vent will be difficult to install with the structure of our house, I plan to stay at 80%. Someone suggested a 4 ton blower? Will that help?

To clarify, there is sgl stage and 2 stage for AC and furnace separately? Or if I get a 2 stage for one is applies to the other?

    Bookmark   August 15, 2012 at 10:08AM
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PA12

Tigerdunes -- when you say "...especially if stat has DOD feature for AC cooling" - what is DOD?

And when you say "A high eff sgl stage AC ...", is that anything higher than 16 SEER?

    Bookmark   August 15, 2012 at 10:14AM
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PA12

Mike -- if you're recommending a 2-stage AC, then I should move up to a 4 Ton then? When contractors are quoting me for a 2 stage AC, are they automatically including a 4 ton?

    Bookmark   August 15, 2012 at 10:17AM
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tigerdunes

"To clarify, there is sgl stage and 2 stage for AC and furnace separately? Or if I get a 2 stage for one is applies to the other?"

Today's furnaces are much more compact. An 80% eff furnace is a big mistake and will hurtnyou on resale. Review all options with dealer on installing a 95% eff furnace before deciding on the 80%.

You must have a 4 ton rated blower for a 3 1/2 or 4 ton AC condenser. This is a must.

Furnaces are available in sgl stage and two stage. The two stage high eff models have a var speed blower and there are 80% two stage models that have var speed blowers.

You must have a varvspeed blower on the furnace if you opt for a two stage condenser.

Got it?

IMO

    Bookmark   August 15, 2012 at 10:21AM
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tigerdunes

DOD stands for dehumidify on demand feature in AC cooling.

Must have both var speed blower on furnace or air handler plus have the correct thermostat that offers DOD.

IMO

    Bookmark   August 15, 2012 at 10:31AM
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mike_home

The right way to do this is to have the contractor do a heating and cooling calculation. This is known in the industry as a Manual J calculation. For a house your size it cooling requirements should fall in the 3-4 ton range, but it should be confirmed with a calculation. The amount of insulation, orientation of the house, and amount of windows will influence the result.

If you get quotes for a 2-stage AC it will be for either a 3 or 4 ton unit. Contractors tend to oversize equipment because they are too lazy to do a real calculation and play it safe.

The size of the blower needs to match the size of the AC. The blower speed will be adjusted to give the proper air flow.

A single stage AC can provide good humidity control, but correct sizing is even more important.

    Bookmark   August 15, 2012 at 10:44AM
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PA12

If I keep my furnace at 80% efficiency, will the overall efficiency still go up with a new system? Or only if I change to a 2 stage system vs a single stage? I think I currently have a 126,000 BTU furnace. Will 100,000 BTU be a sufficient replacement or do I need to go higher? Does that depend on whether it is a single stage or 2 stage? What would you recommend, if I need to stay at a 80% effic furnace?

    Bookmark   August 15, 2012 at 11:15AM
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PA12

"Today's furnaces are much more compact. An 80% eff furnace is a big mistake and will hurtnyou on resale. Review all options with dealer on installing a 95% eff furnace before deciding on the 80%."
I was told that it has more to do with where the extra vent can go out from the basement that may be a problem, or just much more work and cost. We might have to tear up our finished basement to do so.

    Bookmark   August 15, 2012 at 11:33AM
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PA12

As for a thermostat replacement, a Trane constractor says that he'll be installing a TCONT900 unit. I have read on your other posts that you recommend HW VP IAQ. Are these comparable?

    Bookmark   August 15, 2012 at 11:36AM
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tigerdunes

Before I answer, what system components is Trane dealer quoting?

IMO

    Bookmark   August 15, 2012 at 11:49AM
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mike_home

You current furnace is oversized. You choices are either a 80K BTU with 95% efficiency, or 90-100K with 80% efficiency. The size of the furnace is not affected by whether it is one, two, or three stages. The sizes quoted are the size of the highest stage.

You will problably not see much savings in fuel cost by going with the 80% furnace.

Have you figured out how much work it will be to install the PVC vent in your finished basement? You can some times get away with one 2 inch PVC pipe, but most installations will require two separate pipes for intake and exhaust.

    Bookmark   August 15, 2012 at 11:56AM
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PA12

Mike -- No one has actually given me an estimate for a new PVC vent yet. So I will ask them for an estimate. But because there is a steel beam that goes across the middle of our basement ceiling, they said they'd need to vent out to the front of the house. However, there are several windows in the front of the house, so they said it wouldn't be possible to vent out that way either.

Tigerdunes -- The Trane quote is for:

4TTX6048 condensor (17 SEER 2 stage)
4TXCD063 coil
TUD2C100A9V5 80% two stage furnace

or

single stage 16 SEER
same furnace 2 stage v blower

I am trying to compare this to a Carrier brand:
Infinity 2 stage furnace: 58CVA110-120
performance single stage 16 SEER: 24ACC642

or Performance 2 stage 16 SEER: 24ACB748

Would you say these two models are comparable? I received a quote that was about $1000 less for the Trane model...

BTW, thank you both for all of your advice so far!!! :)

    Bookmark   August 15, 2012 at 2:07PM
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mike_home

What thermostats are included in each quote? Are the linesets being changed? What type of filter does each include?

    Bookmark   August 15, 2012 at 3:19PM
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tigerdunes

Pa

I would be interested in the thermostats as well. Carrier should be Infinity control. Anything less would be an unacceptable shortcut.

On the Infinity 80 furnace, drop back from the 110 K to the 90 K size.

Either of the Carrier condensers would be fine but my preference is the Performance two stg model.

IMO

    Bookmark   August 15, 2012 at 3:50PM
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weedmeister

There is staging for the AC and staging (or modulation) for the furnace. They are separate. One does not affect the efficiency of the other.

The efficiency of a furnace is the measured BTUs you can get from the BTU rating of the furnace. So a 100kBTU 80% furnace supplies 80kBTUs. A 95% would give you 95kBTUs.

2-stage AC units usually (and someone may smack me here) come in even tonnage, 3, 4 or 5. Single stage units are available in half ton sizes.

You can (and should) have variable speed for your blower on the furnace.

DoD lets the fan run slow during AC operation to extract humidity down to your humidity setpoint. Variable speed fan profiles will also start slow and speed up over time to also extract more humidity as the temperature setpoint is reached,

IMHO, if price is an issue, a single stage AC with a variable blower would be a good choice for humidity extraction.

    Bookmark   August 15, 2012 at 4:48PM
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PA12

The Carrier thermostat that was quoted was Edge but he said he'd upgrade to an Infinity when I asked about it.

The Trane thermostat is TCONT900.

But I was mainly wondering whether you think the two models(Carrier and Trane) that I listed above are comparable systems so they should be around the same price.

    Bookmark   August 15, 2012 at 7:17PM
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mike_home

I believe the Trane and Carrier equipment are comparable when you are comparing single stage and 2-stage equipment. I am not sure if the TCONT900 thermostat has all the features of the Infinity controller.

    Bookmark   August 15, 2012 at 9:15PM
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PA12

What are the main features I should look for in the thermostat? If I go with the Trane, what model thermostat should I ask for?

    Bookmark   August 15, 2012 at 11:33PM
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mike_home

Here are some features I think are important:

- ability to set at least 4 temperature time periods per day
- 7 day programing
- temporary temperature override
- true 2-stage control (if getting a 2-stage furnace/AC)
- AC powered with battery back up
- dehumidify on demand (if getting a variable speed furnace)

I have the Infinity controller, here are some features which may not be present in other thermostats
- static pressure reading
- filter change alert
- outside temperaure display
- fault code display and logging
- fan RPM display
- maintenance reminder
- installer diagnostic settings

I believe the Trane TCONT900 is a good thermostat from what I have read. I don't see it listed on the Trane web site so I don't know if it is an older model. The XL900 seems to have the features you would want if you are getting a 2-stage furnace and AC.

If you are getting the Infinity furnace then you need to get the Infinity controller. I own Carrier equipment and have been pleased so far with the comfort and performance. The Trane equipment is a good alternative.

    Bookmark   August 16, 2012 at 8:48AM
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tigerdunes

There is not doubt that in the residential HVAC marketplace that the Infinity controller is still the gold standard that others are judged by.

Either of the systems you are looking at are top of line.

Still would want the 95%+ model.

IMO

    Bookmark   August 16, 2012 at 9:31AM
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