HVAC gurus: Variable-speed Blower is the Answer?

In My Humble Opinion
1

Our top-floor air conditioning situation in our 8-year-old Atlanta house gets worse every year, and I’m eager to implement the correct solution. Determining the “correct” solution has been difficult.

Two years ago, after learning that the wrong-sized coil was put in our top-floor unit, we switched it out with the correct size, a 2.5-ton, which matched the outside compressor. The old coil had failed anyway, so the difference in performance vs. a failed one was obvious, but it still didn’t meet my expectations.

Currently the unit will run all day and never get the temp under 75 degrees; at 7pm the temp can be 80. I haven’t measured humidity, but I assume it isn’t as low as we’d like.

Yesterday a rep from a well-reviewed company surveyed our situation and proposed this solution:

  1. replace the existing furnace with a variable-speed unit, the Tempstar VS80: http://www.chicagofurnace.com/pdf/T8MPV%2080%20GAS%20FURNACE.pdf (PDF)

  2. increasing one outlet from a 4-inch to 6, and adding an additional small return in one of the rooms (currently there is only one return, a 14-inch)

His assertion was that, even though there was nothing wrong with our existing furnace (and heating is definitely not an issue), this variable-speed blower unit in this furnace would solve our problems by reducing humidity (by moving air over the coils more slowly), increasing “throw” of the air out the vents, and consuming less electricity in the process.

Price for part 1: $2900; for 1 & 2: $3200.

The small question: can replacing a two-speed blower with a variable speed really solve my problems, even in theory? I really have my doubts; it doesn’t make sense to me (also, our existing blower/furnace seems to be in good shape, by his assessment). I’m eager to hear from HVAC professionals or others with knowledge in this area. The company has rave reviews on kudzu.com.

The bigger question: my doubts from the small question above stem from this observation: if we have an insulation company come in (which we did), the answer is: you need more insulation. If we have an AC firm come in (as we did yesterday), the answer is (surprise!): you need new HVAC components. If the answer from the experts here is that the variable-speed unit can be expected to make a drastic improvement, I’ll go for it. But what I really want is some consultant with broad, deep heating/cooling knowledge to make an unbiased assessment.

Is there such a class of professional, where for several hundred dollars they’ll survey your situation, and consider all possible solutions: attic ventilation (static or powered); ceiling and wall insulation; ductwork quality, insulation, and suitability; and AC components. Perhaps some form of home inspector (I imagine they’re not terribly busy right now)?

2

For starters- your upstairs unit is serving approximately 1200 square feet? (Big house!) If the unit is sized correctly, and it’s all in working order, you shouldn’t have a problem keeping cool.

A few things that impact sizing are:
Volume of air - the traditional one ton per 500 square feet assumes 8-foot ceilings.
Passive solar gain - if the house is a greenhouse, you need to compensate for the extra heat pouring in through the windows.
Insulation - you need to compensate for inadequate insulation, especially in the attic.

How is the insulation? Particularly the attic insulation - especially if you have a dark roof, you get a huge mass of hot air sitting on top of the bedrooms. Attic ventilation will help, eg: powered gable vents or even just improving “static” venting) and it’s cheaper than air conditioning.

Window tinting will cut passive solar gains tremendously, and if you go with a neutral gray or silver tint, you don’t even know it’s there. It’s not impossibly hard to do yourself, but having done the living room windows I can attest that it makes a noticeable difference, and I’d have been better off having pros do it - the stuff is fragile and unwieldly in picture window-size sheets.

I have no idea what the rep meant by a lower air speed will yield more “throw” - usually if you have a multi-speed blower, it’s set to run *faster * for air conditioning. OTOH, adding another return will probably help with circulation if you have “dead spots.” I don’t know if it’s going to be the magic pill for your home.

When you’re cooling - do you need all rooms or areas cooled? If not, have a look at a multi-zone system - automated dampers close off ducts leading to unoccupied areas, so the occupied areas get more cooling effort. (Same goes for heating as well.) This is more a change to the thermostat than a change to the furnace and air handler, but retrofitting the dampers into already-installed ducts can be challenging.

If money is no object, take a look at Mitsubishi’s new “City Multi” systems. Each room gets a low-profile unit, and they all run off a central compressor. Each room can be at a different temperature, and the compressor automatically adjusts itself for the actual demand, rather than just running full-power all the time. Our housemate has installed a few of these systems, and he says they’re pretty spiffy.

3

Where is the furnace located upstairs? (in a closet I presume…)
Common problems:

Problem: The ductwork is picking up heat from your attic.
**The way to check: **Take a temperature reading of your return air at the return grille. Then take it at the entrance of the furnace. If it is warmer at the furnace, then the return air duct work has picked up heat through the duct in the attic. Do the same thing for your supply air: Take the temperature right past the A/C coil (which may require drilling a small hole in the ductwork right past the coil), and take another reading at the register farthest from the furnace. Here too, the temperature readings should be very close. If either the supple register, or return grille are picking up heat, you have a problem.
The Fix: Check to see if the return flex is loose, or has come off in the attic. Seal all joints with ‘duct seal’. Insulate around the ducts.

Problem: Freon charge is low/high.
The way to check: The average A/C should produce a 16-22 degree drop, or ‘delta’ across the evaporator. A common reading would be 75 degree return air (at the register) and 55 degrees at the supply register; a 20 degree ‘drop.’ Take a reading of both your supply and return. What is your delta?
The Fix: Call a qualified technician.

Problem: The attic temp is bearing down on the second floor living space.
The way to check: Get on a 6’ ladder and place your hand on the ceiling at 3:00 p.m. Is it hot? Do you feel it in your face as you climb the ladder? (common) What is the temperature of your attic at 3:00 p.m.? Do you have ridge vents, or soffitt (sp) vents? A powered attic fan? Does it work?
The Fix: Get a working, powered, thermostatically controlled attic fan. Now. Before you make any decision on a new A/C system.

4

Tell us this:

Manufacturer of furnace- A/C
Model number of A/C unit outside.
How many square feet upstairs?
Is this unit only serving upstairs?
Where is the furnace located?
Where is the duct located?
Do you know if the duct work is made of galvanized metal, or fibrous “duct board”?
What type of thermostat, and is it programmable?
Do you program it for setback?
Are there periods of the day when it is able to keep up?
Is there a separate unit serving the downstairs?
Is the upstairs hot everywhere, or just in one room?
Is the attic finished? Walk up? Crawl space?
Do you have an attic fan? Does it work?

5

Problem number 1: At .05" w.c., a 14" return will produce 725 cfm. If your unit is indeed a 2.5 ton, at .05" w.c., it should have a return of 16". Adding the return is a good call.

Problem number 2: At .1" w.c. a 4 inch supply will produce 35 cfm; or, in other words, enough to cool a room that is 6’ x 6’. Whoever installed a 4" run in a house should be publicly beaten. How big is the room that he wants to increase to 6"? A 6" run will produce (at .1" w.c.) 125 cfm; or in other words, 1/3 of one ton; or in other, other words about 200 sf; or in other, other, other, words a room 14’x 14’. Increasing the size of the run is good call, however we don’t know if 6" is the right size without knowing the sf of the room.

Based on the little we know, I’m skeptical that the VS blower is the right choice—or at the very least the first choice. Answer the questions in the other post and we’ll give you a better answer to this question.

IMHO, I would not get the VS unit right now, although I might based on the answers to the other questions. If the house is 8 years old, then the furnace is only 8 years old. I’m highly skeptical (<----notice I used all the highlighting available to me…)that you need to scrap an 8 year old system.

My personal opinion is that you are better off with several individual masters of one trade, than a jack of all trades. From experience, I have a dim view of home inspectors, and my opinion is that you need an experienced HVAC person.

6

When it rains, it pours!

A couple of things that your posts have reminded me I should have stated in the OP:

  • the furnace/blower/coils are in the attic, above the top floor. It has the typical large square box coming up from the coil, with all the runs running off this trunk.
  • there is a single return that is situated nearly underneath the furnace, so the distance from the grill to the unit is pretty short.

This is my personal belief. The ductwork is the insulated flex-duct, but I’m thinking in the tremendous heat of the attic, it MUST be affecting the air temperature before it gets to the vent. This (IMO) is borne-out by noticing that air coming from the two longest runs is warmer than the other vents.

Manufacturer of furnace- A/C : Ruud

Model number of A/C unit outside: not available at the mo, but it’s a SEER 10 unit

How many square feet upstairs: roughly 1200 (which is larger than the 1st floor, since it covers the garage)

Is this unit only serving upstairs: yes, we have a separate unit/zone for main floor.

Where is the furnace located: in attic

Where is the duct located: the return is very close to the furnace, centrally located in a large hallway; we keep all doors open.

Do you know if the duct work is made of galvanized metal, or fibrous “duct board”: the ductwork running off the main vertical trunk is flexible, something similar to this.

What type of thermostat, and is it programmable: White-Westinghouse, I think; old mercury-switch style.

Do you program it for setback: No.

Are there periods of the day when it is able to keep up: 3am. That’s about the time where it’ll start to cycle off, if we set it to 70 degrees.

Is there a separate unit serving the downstairs: Yes

Is the upstairs hot everywhere, or just in one room: naturally, in the afternoon the rooms on the sun-side will be warmer, but none of the rooms is cool.

Is the attic finished? Walk up? Crawl space: unfinished, access via pull-down. Like most new houses, huge headroom to the peak (maybe 15’).

Do you have an attic fan? Does it work: No; certainly willing to consider ridge vents, turbines, or powered exhaust, but no one yet who has visited has suggested it.

Attic insulation: HVAC guy says insulation looks sufficient; my WAG is that there is about 12" deep of the stuff (remember, the house is only 8 years old). Most importantly, all ceilings are NOT warm to the touch.

FWIW, the HVAC guy said that my unit size and compressor are fine - and certainly he had an opportunity to go for the throat. But I’m just not convinced that a variable-speed fan would really improve things much.

Comments noted that his suggested addition of a supplemental return and upsizing 4->6 inch outlet are sane choices. The existing 4-inch is for the sitting area of the master bedroom, so it isn’t a delineated space but part of the overall master suite. But it does have two 30in-wide windows to contend with in that sitting area.

My novice, gut-feeling is that IF my units are mechanically performing as expected, and are sized properly, then a combination of attic ventilation; wrapping additional insulation around the longer ducting; and possibly adding the additional return are the most likely to produce good results…at least as first steps.

I agree with you in spirit; the problem is, all those HVAC people have hammers, and everything looks like a nail. If they don’t sell attic ventilation units, they aren’t going to recommend one. Or maybe I’m just too cynical.

7

Ok, here’s my 200 pesos, k? (in no particular order)
My observations:

  1. A 2.5 ton system is more than adequate for 1200 sf. (there should be a “30” in the model number)
  2. Ruud makes a good product
  3. A digital stat will be more accurate and do a better job. A good inexpensive stat is the Honeywell Focus 3000. A pricey [but outstanding] stat is the Honeywell VisionPro 8000.
  4. Zoning is not an issue if all rooms are hot.
  5. Attic fans are a tremendous way to cool upstairs living spaces.
  6. SEER ratings don’t speak to your comfort, as much as the energy it uses to provide your comfort. A 10 seer unit and 13 seer cool the same; it’s just that the 13 seer unit will do it cheaper.

IMO you’re spot on. Of course we can’t see, and so this diagnosis is nothing more than peanut gallery MB advice, but…I would recommend: (in this order)
1) Have the unit serviced professionally. I have more than a sneaking suspicion that the real issue is that this unit is not performing. If you’d like, we can provide a laundry list of technical things to check, and for the technician to record. (there ain’t no way that a properly running 2.5 ton unit needs 3 a.m. to satisfy the stat at 1200 sf.) No way.
2) Get an attic fan. It’s inconceivable that you don’t have ridge or soffit vents. Dropping the attic temp is cheaper than air conditioning after the fact.
3) Make sure all ducts are properly insulated, and sealed at the joints. You didn’t say what the “trunkline” was made of. If it’s ductboard, it’s possible it’s come apart.**
4) Go ahead and replace the 4" run, and add the additional return.** Check to see what the trunkline is made of, and let us know.
5) Consider blowing in insulation, particularly if you’re willing to do it yourself. (it will save money, and you can rent the machine to do it)

8

one other thing…

variable speed blowers are great and have some applications where they shine. there are pros and cons too, and if you’d like we can post them.

but…other than humidity control issues, noise issues, and a couple more, i typically wouldn’t think of a vs blower as a ‘fix’ for what you’re experiencing.

the hvac guy, imo, hasn’t addressed the root cause, so the vs option is not a wise choice until you do address the root cause.

then, and only then, should you go for a vs unit, keeping in mind you’re scrapping out a unit that’s less than half it’s expected lifespan.

9

The tech is coming today. It wouldn’t surpise me that the unit is a bit low on refrigerant, as it’s been two years since the last service visit.

It is ductboard (not sure what the base material is, but I assume fiberglas), and I think your instincts are right: based on leaking areas the HVAC rep pointed out on the downstairs unit (which I repaired last night), the attic unit may have similar leak issues. The leaks on the joints on the duct board I can pretty easily handle myself; leaks within the flex-duct runs (that seem to have been sealed using some black brush-on stuff) might require a pro; not sure what’s involved.

My uneducated guess is that a few hundred dollars invested in something like this:

couldn’t hurt, and may well help. I’ll try to take some temperature measurements in the attic over the next day or two.