HVAC efficiency - upgrade was worth it

[Brad Johnson]

This month marks the second year since I upgraded my HVAC system, installed a programmable thermostat, and added eight inches of blown-in insulation to the attic of my 25 year old home. The total energy savings, thus far, averages 37%. Some months, especially in summer, the savings approaches 50%.

That's right, the upgrade cut my total annualized utility bills by over a third even though I am keeping it slighly cooler during the summer. If the savings remains constant it will offset 100% of the upgrade cost in just under 7 years.

The system is rated at 12 SEER, but my AC-expert friend who installed it is like me - one of those guys who studies things to death (probably why we get along so well). The system is all Trane, but the air handler and inside coil are not the units usually spec'd as companions to the compressor. He did a mix/match setup using a bunch of energy tables and air movement calcs. According to his estimates my system should be functioning at just over 14 SEER. Based on my energy savings I have no reason to doubt him.

In case you were wondering, the electric bill during the 05/15-06/15 billing cycle for my 1640 sq ft home was ninety bucks ($90) vs $170 for the same time two years ago. That's with daily highs no lower than than 95 and almost two full weeks of 100 deg plus days. I was also keeping the inside temp a cool 75 during the day and cooler 72 at night.

Not bad. Not bad at all.

Brad

[Leatherneck]

COOL!





(I had to say it)

TC

[K Frame]

Every year I tell myself that I'm finally going to add more insulation to the attic.

Every year I put it off.

I just need to get the rafter chutes in place and then find someone to turn the blower on and off while I fill the attic.

CHRIS! We have a job to do!

[thebaldguy]

We replaced our old furnace (50% efficient) with a 92% model and added a high efficiency central air system a 18 months ago. We think it made a pretty big difference in gas/electric bills. Our a/c is on a saver switch, where the electric company will shut off the central air 15 minutes an hour during high use periods. They give a nice discount for agreeing to the a/c switch.

[cfabe]

What a timely topic, I was just going to post something similar.

So given the expertise here, I'd like some suggestions for my new house. The home is a 1951 1000 sqft 1.5 story with an unfinished upstairs. Current equipment is a 5 year old comfortmaker natural gas 80% furnace, and an old old Singer central air unit outside. Both seem to be operating properly. The house has about 3" of fiberglass bat insulation in the ceiling, and about half the roof also has 6" fiberglass in the roof rafters (the rest had fallen out, city violation, was removed before sale). There is no ridge vent or soffit vents, but the attic area has two windows in the gable ends. The walls probably have no insulation.

Clearly insulation is in order, but I'm planning to finish the upstairs in a few years, would I be best re-insulating the rafters that are missing, or blowing some in on the floor to fill the joists?

I'm considering the slow-rise polyurethane foam for the walls, but it's pricey, going to cost about $2500 in materials for my small house, at least from the one outfit I checked with. How does blown cellulose compare in price and efficency?

On the gable ends I have windows. A friend mentioned putting a window fan in one window to ventilate the attic on hot days, and that this would reduce a/c expenses considerably. Thougts on that?

What kind of efficency improvements have they made in a/c in the last say 30 years? I wonder how much I could save by upgrading compared to waiting until the current unit dies.

In this area (northeast ohio) we tend to have a lot of days in the summer (like today) where the temperature, espicially at night, isn't bad but the humidity is high. Right now I have my window open blowing cool, damp air onto me. I'd much rather it be dry air. Is there any solution for dehimidifiying outside air and then using it to ventilate a house? Maybe at a lower operating cost than normal a/c?

In all of this, I'm faced with the budget of a single guy making decent money, but moving into my first house and having lots of other expenses. So I'd really only want to put money into things where I'm really going to see some return.

[280plus]

Ventilating the attic will help but you don't want to be sucking humid air intothere so aware of that, blow insulation down between the ceiling joists for the best results. You can replace the missing bats in the rafters but it's probably not necessary. I'd go for a new condenser (outdoor unit) because they HAVE made significant improvements in the efficiency of those things in recent years. Go for a high efficiency (12 SEER or above) unit like Brad did. You may not achieve the 14 SEER rating he did because the indoor unit is not sized for it but every little bit helps. Around here the gas/electric companies have kits to improve your house's seal from the outside weather. Look into that in your area. Replacing the windows with double pane glass is another good step in this direction. Good luck!

[cfabe]

The windows are actually already replaced with qood quality double pane vinyl windows.

Any thoughts about how best to insulate the walls?

[Brad Johnson]

cfabe,

Blown cellulose is cheap. Real cheap. For the money it works quite well. However, the expanding foam stuff is just a wonder of thermal efficiency. It it pricey for sure, but there is probably no better insulator for a wood-framed residential application.

I would definitely install soffit and attic vents along with a thermostatically controlled attic fan. An unvented attic can easily be 30-70 deg F higher than ambient temperature, and all that heat is migrating into your home through your poorly insulated ceiling.

In most structures some 60% of the heat migration is through the ceiling, so I would concentrate there first. Your walls may be poorly insulated, but the airspace between the interior and exterior walls will give you some thermal barrier for now.

As for the windows, I have mixed emotions. Certainly older windows (or "winders" here in Texas) are horribly inefficient. However, window and door loss only accounts for around 20-25% of most of the heat migration in a home. Given the relatively high cost of replacing windows I think the money would be better spent elsewhere. For many homes the cost of replacing all the windows and upgrading the entire HVAC system ends up being roughly the same. Upgrading the system will return you more than upgrading the windows, so I feel like the mechanical upgrade is a better use of funds. That, and all the insulation you can reasonably afford, should have a significant impact on your utilities. If you are still concerned about windows you can always have storm windows installed. They are an easy, low-cost way to add efficiency to existing windows.

As for dehumidification, most dedicated dehumidifiers are just smaller versions of your central AC system. A dehimidifier will usually use just as much, if not more, electricity as your central air system. Often it is just as cost effective to simply close the windows and turn on the AC. I guess the short version is that if you intend to keep your windows open in the evening, be prepared to live with the humidity while they are. Then shut the windows and turn on the AC to dry everything out.

*EDITED TO ADD* Looks like you posted about your replaced windows while I was in the middle of typing this. That makes things simple. HVAC and better insulation will give you the greates return on investment.

Brad

[K Frame]

Plan for the future. Insulate the roof rafters now. Anything you put or blow onto the floor will only have to be removed later.

You should install soffit vents and a ridge vent before you install the insulation.

Doing so properly requires that you leave an air channel between the insulation and the roof decking, and that means you'll likely have to remove your existing insulation to do that. If you don't do that, the roof will overheat and shingle life will be shortened, possibly dramatically.

For the walls, make sure that you don't have knob & tube wiring. If you do, you can't do blow in or injected insulation.

Getting the heat out of the upstairs can very well drop your cooling expenses. On hot days without proper insulation, the ceiling in your home essentially becomes a large radiant heating panel.

"Right now I have my window open blowing cool, damp air onto me. I'd much rather it be dry air. Is there any solution for dehimidifiying outside air and then using it to ventilate a house? Maybe at a lower operating cost than normal a/c?"

A dehumidifier.

Remember, though, that dehumidifiers ADD heat to the air due to the heat from the compressor and drive motors. So even though the air in the enclosed area will be dryer, it will be hotter.

A dehumidifer won't use as much electricity as a central air conditioning unit, unless it's a huge dehumidifer. It's a simple question of size. Your central air unit has a much larger capacity because it's serving your whole house. The largest dehumidifiers are in the 60 pint range. On average, they use about as much juice as a large refrigerator.

The kicker is, though, the amount of time the system will run vs. how long a dehumidifer will run. Cumulatively, a dehumidifier may well suck down more juice because it runs longer. Conversely, if your central AC system isn't designed properly, you might very well find yourself a cold, but very clammy, home because the AC cools too quickly to effectively remove moisture. That's the problem I face, especially in my basement. I have to run a dehumidifier down there pretty constantly to protect my books because my central AC unit wasn't designed/installed properly.