In this house, we obey the laws of thermodynamics
July 29, 2006 7:46 PM   Subscribe

What can we do to get more air from the air conditioning (and the heat in the winter) to the back of the house?

100-year old house. An addition was built about 25 (?) years ago. The addition is about 40% of the house. It was built without a building permit and with some, um, interesting technique. Very little air comes out of the vents in the addition. None of the ductwork in the addition is accessible without tearing out the floors.

We have had the ducts cleaned, and an annual checkup for the furnace and air conditioner (they're both in good shape). The HVAC guy says that he could put a more powerful fan in the furnace for a few hundred bucks but he doesn't know if that will help (the furnace is newer than the addition, BTW).

Searching AskMe brought up this old question about duct sealing (with a thumbs-down answer) -- but I don't even know if the problem is leakage or blockage.

Right now running the air conditioner only on days when the outdoor temperature exceeds 30 degrees C triples our electricity bill for July and August and it doesn't get the temperature down past 26.
posted by winston to Home & Garden (17 answers total)
 
That's 26 in the coolest place, BTW (with the thermostat set at 24 or 25). The temperature in the master bedroom, upstairs at the back, is not affected at all by the AC.
posted by winston at 7:49 PM on July 29, 2006


I don't remember where now, but I remember seeing fans that would sit on vents and switch on automatically to draw air through them when they detected airflow. They would then stay on for the until your main fan stopped, plus an additional couple of mintues to draw out any remaining cold or warm air out of the duct. I don't know if they are very effective though. Maybe the mefi-mind has more insight!

Good luck.
posted by defcom1 at 8:08 PM on July 29, 2006


It sounds like your ductwork is just undersized. Blockages would likely have been detected during duct cleaning. The fans defom1 mentioned should help a lot. They are called duct or register booster fans (sample). You might also consider putting in some new ducts, they can often be added through closets etc. without tearing up floors and walls. In th meantime keep the registers in the main house partially closed to even out the air distribution. This of course hobbles them, but then the whole system will be at the same level of hobble.
posted by caddis at 11:56 PM on July 29, 2006


Things to try, in least-to-most expensive order:

1) caddis's register trick. I do this to with my own crappy HVAC system. It's a good first step, but might not be enough.

2) Check/clean/replace the filters -- they may be in the furnace/distribution fan cabinet, in the intakes or both. A dirty filter can really inhibit air flow, and you're supposed to change 'em a few times a year (although nobody ever does.)

3) Keep the air circulating in the house all the time -- a furnace tech once suggested I move the fan setting on the thermostat from "auto" to "on." This helped, but really whacked my electric bill. A couple of strategically-placed floor fans might be cheaper in the long run.

4) As others have mentioned, booster fans.

5) Get a bigger distribution fan or add ductwork.
posted by Opposite George at 1:52 AM on July 30, 2006


Oh, and hopefully the folks who recently checked out your system took care of the filter issue but don't count on it unless you it for sure. The crappy service my condo association used to hire for annual maintenance never did and it took me 5 years to figure out that I even HAD filters in my system, and that they're supposed to be changed more frequently than every 5 years.
posted by Opposite George at 1:58 AM on July 30, 2006


s/you it/you know it/
posted by Opposite George at 1:58 AM on July 30, 2006


If you think about upgrading your ductwork, don't forget to assess the adequacy of the air return. Even with adequate vents in the affected area, if that air has nowhere to go you may have problems. Depending upon the layout of your house an additional return may be either easier or more difficult to install than additional ouput ductwork.
posted by TedW at 7:23 AM on July 30, 2006


TedW brings up a good point. One thing you should check is whether you actually have returns in this part of the house! Amateurs putting in their own stuff sometimes omit the return ductwork. Without it forget any semblance of normal circulation.
posted by caddis at 7:57 AM on July 30, 2006


100-year old house. An addition was built about 25 (?) years ago. The addition is about 40% of the house. It was built without a building permit and with some, um, interesting technique. Very little air comes out of the vents in the addition. None of the ductwork in the addition is accessible without tearing out the floors.

If the ducts in the addition don't produce any substantial airflow, but other ducts do.. Well, you have a closed damper, a big leak, or the addition's ductwork is improperly coupled to the old ductwork. The HVAC guy should have addressed the inconsistency specifically, I think..

Right now running the air conditioner only on days when the outdoor temperature exceeds 30 degrees C triples our electricity bill for July and August and it doesn't get the temperature down past 26.

There are many problems to be considered: the insulation of the house as a whole, the relative insulation in specific areas, the overall heating/cooling capacity of the system, and the proper function of the ductwork.

When you say it can't cool past 26.. If this is about the addition, there must be temperature variation from room to room, but you aren't specific. Further, you need to account for occupancy, because if there is one room where everyone likes to go (with computers and TVs too, maybe?), it will naturally be hotter - there are things you can do about that, of course..



Searching AskMe brought up this old question about duct sealing (with a thumbs-down answer) -- but I don't even know if the problem is leakage or blockage.

Airflow in a house is a pretty complicated fluid dynamics problem. Properly sealing all the duct joints is important, first because it can actually seal a leak, but also because you can't effectively analyse the situation if you can't assume properly functioning ductwork.

Your ducts probably have dampers - the location isn't always predictable so search everywhere - try adjusting dampers to re-balance the amount of heating/cooling going to various locations in your house.


In the end.. If the problem is environmental control in a specific room, make sure you address drafts and leaky windows, and then consider a window air-conditioner and/or portable heater.
posted by Chuckles at 11:33 AM on July 30, 2006


Add 'missing cold air return' to that top list of ifs..

If the ducts in the addition don't produce any substantial airflow, but other ducts do.. Well, you have a closed damper, a big leak, there is no cold air return in the area or the addition's ductwork is improperly coupled to the old ductwork. The HVAC guy should have addressed the inconsistency specifically, I think..

And, you know, all the damper/leak/coupling type problems can effect the cold air return as much as they do the hot air ducts.
posted by Chuckles at 11:37 AM on July 30, 2006


First of all, if you have no explicit cold air return in the addition, but have doorways open to all the rooms, your cold air return will be your doorways or hallways in the addition leading back to the main house, but the existing cold air return on the main unit will probably still be undersized, if it wasn't upgrade for the additional air volume being sent to the addition. Some people remove swinging doors in rooms or hallways so that closing them can't cut off return air flow.

Next, I'll chime in late with the thought that replacing the central fan unit with one that can operate efficiently at a higher head pressure may be exactly what you do need to do. Normally, HVAC fans are designed for installation into plenums from which air is directed into appropriate ducting. Larger duct systems usually dictate larger plenums, and longer duct runs are best served from plenums where a higher positive pressure is maintained than that required for short runs of a only a few ducts. If your addition is a 40% increase in your original house, and the HVAC system was even only slightly oversize, I can pretty much guarantee you don't have nearly enough fan for the job.

A good HVAC tech can work this out through measurement of the static pressure in the distribution plenum, and some calculation for measured length and size of the ducts you are feeding. If the current fan can't maintain needed static pressure in the plenum, a fan which has more blades in the same pitch, or greater pitch, or greater rpm, or greater diameter, or is of an alternate design (squirrel cage blower to 2 stage blower, etc) can make a big difference. It's also possible your plenum box isn't big enough for the duct work you are driving, in which case it could be creating turbulence which would impede the flow.

If you have a small plenum, insufficient fan flow, and a generally undersized central system, you can either upgrade the central unit accordingly, or split the load using some of the new ductless perimeter units. That's usually a simple cost/benefit calculation, assuming you are OK with additional outside units, and want to establish a zoned system with 2 or more heating/cooling controls. The big advantage of a zoned system is that you can heat or cool only the rooms your are using, so that in extreme weather you can save quite a bit of money, over heating/cooling the whole house. And of course, if you have heavy heating or cooling loads in only one room, like a server room, or a home theatre, a zoned system lets you concentrate heating/cooling in the areas all those extra toys require.
posted by paulsc at 2:20 PM on July 30, 2006


You can reduce almost 40% of the heat in your house by using good curtains when the sun's bearing in. Not really relevant, but just in case you get hot in the meantime. (Trust me, I know - I lived through a week long blackout in a west-facing apt. last week. The curtain thing really makes a big diff.)
posted by DenOfSizer at 2:58 PM on July 30, 2006


My first reaction to the huge cost is that you could have lousy insulation that allows your nice cold air to seep out of your house. A contractor can test how airtight your house is, and identify where air leaks out. In an old house, this is a real possibility (even in cold climates, many houses were built with uninsulated exterior walls and zero vapor barrier).

It's possible that even after fixing your ducts and sealing up the house, your cooling will still be inadequate to get nice cold air to all the parts of your house. One thought is to install a second system that cools the parts of the house that are not getting chilled by the first system. A "duct-free" system sends chilled liquid to small fan coils chillers located in specific rooms. Each coil cools each room independently, and only as needed. This way of doing things might help you better organize your cooling: for example, the forced air AC cools main floor living areas during the day, and the other chiller system cools bedrooms in evenings.
posted by drmarcj at 9:02 PM on July 30, 2006


Thanks for all the replies. In case anybody else is going to comment yet, I'll reply to some points.

Whatever other problems might exist (e.g. insulation), there is definitely a problem with the airflow through the ducts. The registers towards the back of the house emit very little air.

This is a very small row house. Two floors, no attic (flat roof), and the basement is about 9 x 9 and 5 feet high. The footprint of the house (including the addition) is about 400 square feet. The original house is 11 feet wide and about 20-25 feet deep. The addition is 7 feet wide and again about 20-25 feet deep.

As I said, the furnace is newer than the addition (it's about 10 years old)

The main floor is all open (no walls or doors between rooms) and the thermostat is in the front room. The duct to the register in the front room is about 3 feet long straight from the furnace. There's another duct in the middle room and another duct against the back wall in the dining area of the kitchen. With the AC running non-stop on a hot day, it gets down to 26 in the front room, and is much warmer everywhere else. It's such a small area altogether that I'm sorta surprised that the one good duct in the front can't take care of the whole main floor. While the sun is a factor in the back window in the morning, the situation doesn't change all that much when the sun is in the front in the afternoon.
posted by winston at 10:08 PM on July 30, 2006


More random bits of information:

Filters are being replaced several times a year.

There is a large return in the front room on the main floor. None anywhere else.

The front bedroom on the second floor feels as comfortable as the front room on the main floor.

No room has particularly high heating/cooling needs. (Well, I guess there's the dishwasher and the clothes dryer, but the problem exists even when they're not running).
posted by winston at 10:20 PM on July 30, 2006


If air isn't flowing through the duct work, you have to take steps to discover why. That is either going to involve a physical inspection of the ductwork, by crawling under the house, looking through the basement, or gaining access through the flooring, depending on how the ducts were originally installed; perhaps, if access is really problematic, you could get an internal video camera inspection of the duct work to ensure nothing has been dropped into them, or crawled into them, impeding the air flow. After that, you have to tackle the dynamic air flow/plenum issues raised above.
posted by paulsc at 10:20 PM on July 30, 2006


There is a large return in the front room on the main floor. None anywhere else.


This is a big problem. Talk to your HVAC guy about getting returns elsewhere. Any room without one will be starved for air, especiallly when its doors are closed.
posted by caddis at 10:41 PM on July 30, 2006


« Older Did Woody Cut Mark?   |   Bass Ackwards Gas Prices? Newer »
This thread is closed to new comments.