Author Topic: 2000 square feet and 2-1/2 ton unit (Read 2827 times)

bluebayou · « **on:** June 20, 2008, 04:32:22 PM »

Does anyone know about HVAC?

I have 2000 square feet, 1400 down and 600 in converted attic space. The AC froze up yesterday. The service guy showed me how the return is too small (only 2 square feet, okay I buy that). The thermostat was set on 66 by "someone else" but if you are married then you know what I mean.

So the AC guy's explanation is that the combination of lack of air flow, the thermostat being too low (making it run 24/7 for 3 days), and not having a big enough unit all combined to ice up the unit.

Basically, he says that you need 1 ton of air for every 600 square feet. I buy his arguement, but can't afford a 3-1/2 ton unit when I have functioning 2-1/2 ton one now.

My thought is to put window unit in upstairs and either run it in peak of day to keep attic temp down or to run it at night when the attic is occupied. Either one would help to cool to get the main AC to shut down at night.

Any thoughts?
Was also thinking to put some of those turbine/vent things on the dead, unused part of the attic.
The AC guy didn't comment on using a window unit upstairs part-time. He only wanted to recommend a bigger tonnage unit. Window unit is only $100-120.

DalesCarpentry · « **Reply #1 on:** June 20, 2008, 04:53:54 PM »

Boy you got me here. I can build you a house from the ground up. Just don't ask me to work on your heating and cooling system. Oh yea you can forget about me working on your car too. Dale

Tencubed · « **Reply #2 on:** June 20, 2008, 07:20:36 PM »

We owned an HVAC business for a number of years, sold it to the guys that were working for us when we moved out of the area.

First off, it's not possible to tell anyone that you need this much cooling for this much area unless you have done considerable research on heat load, infiltration of air, space usage, insulation and a slew of other boring stuff. A tech that has been in an area for an extended period of time may be able to make a close guess but that would be all it would be.

In your case it sounds like you have a very simple solution to the problem, couple of them really. First you could put a lock on the T-stat and break the fingers of anyone that messed with it. This usually leads to divorce and lots of lawyer bills.

Easiest solution is to buy a programable thermostat that allows you to put in "set back" period during the 24 hour day. Set the temperature where you want it to be for the day and night periods then go back in and set a 10 to 30 minute period where the temperature called for is 80 degrees. Set one of these high temperature periods every four, six or eight hours as needed to allow the AC unit to defrost the coil. Set the fan to run full time then when the compressor shuts off the warm air from the house will be pulled over the coil and any frost/ice accumulation will melt off. Be sure the condensate pan under the coil is draining properly as having the coil setting in a puddle of water is a sure way to ice things up.

The length of the set back and how often they must occur you can experiment with and find what works best for you, the humidity in your area and the capacity of the machine.

I find it odd that the tech said this was caused by undersize ducting as that is seldom the case. Usually the ducting is sized to the fan on the system. If the fan is multiple speed it may have been set to a speed higher than originally specked or the entire system may have been upgraded and put on the old ductwork.

If you are consistently finding the house warmer than you want it then your ideas of venting heat from the attic, fan forced systems are better than turbines, and addition of spot cooling with a window shaker or two are viable options and way cheaper than changing the central unit.

Also consider adding outside bamboo blinds or such to stop the heat of the sun before it gets to your windows. This is one of the easiest, and least expensive, methods of lowering heat gain in a building.

You can try to explain to "someone else" that setting the stat really low will not make the unit cool any faster. You can try but it probably won't work.

Hope this helps, any questions I'll try to answer.

Mike

bluebayou · « **Reply #3 on:** June 20, 2008, 08:27:32 PM »

Hey, thanks for the thought out reply. The house is from 1944 and had the HVAC installed way back in time. The present unit is from 1998 and is a Trane High Efficiency. The ductwork appears to be a lot older than 1998. There was a lot of adding and refinishing to the house at least once. The tech that was here seemed knowledgable.........was an older guy. He talked about if it was on a slab versus the pier and beam construction (its is from 1944 after all).

One of the reasons that I picked this house was for big oak trees on West side of house and a magnolia on the northwest corner that shades the AC unit. The home inspection shows 10-12 inches of insulation in the undeveloped part of the attic, but this is in the "floor" of the attic. The attic interior is hot, as in I can feel the heat in the closets upstairs and feel the hot air leaking into the finished part of the attic through the cracks in the knee walls.

My thoughts are to close up the gaps in the closets and walls upstairs and to get some of the hot air out of the attic space.

I will look at fans instead of the turbines and see what my options are.

The thermostat is actually on of those remote unit things that hang on the wall. I could (if I figure out how to work it) take it off the wall, program it, and put it back on the plate. I just have it on a Hold Temp, fixed setting right now. I didn't bother with it because the wife is home all day with one of our kids. I will look at it more critically tomorrow, but I don't think that I can do much more than "wake, leave, return, something". Maybe a more modern unit will go on that base.

Thanks again.

Dale, if I can come up with a cabinet or framing question then I will holler at you.

beemanbeme · « **Reply #4 on:** June 21, 2008, 01:22:53 AM »

I'm not an A/C tech but I know that the volume of return air is important from talking to one in Okla when we got water in our slab ducts and went to attic ducts --yeah expensive. Simple logic sez if the return air flowing back to the Central A/C is cooler the unit ain't gonna work as hard nor as long so I'd be inclined to try the $200 - $300 window A/C upstairs before going to more expensive solutions. If it don't work, you can always sell it as you go on to other things.
From you description, it seems the downstairs is better insulated than upstairs so if you can kinda keep them seperated --doors shut and such-- to keep the cold air in the attic, it'd help.

Tencubed · « **Reply #5 on:** June 21, 2008, 07:59:22 AM »

Your second post indicates you have quite a bit of air infiltration which is a real killer for AC and heating. Since the ductwork is older, probably from an early gravity or forced air furnace, it is most likely way undersize for the AC unit. This can cause a lot of problems and if you can add more return air by putting in an additional duct or enlarging the present run it would probably help. Just doing the return air and air infiltration improvements may solve the problem but I doubt it. As you well know, the addition of insulation pays dividends big time when it come to AC costs. Those warm closets sound like a good place to add some.

You mentioned the tech was an older fellow that seemed knowledgeable about the system in general. If you have confidence in him you might consider asking him about adding a control to the AC unit that will detect a frosting problem and shut the compressor off until it self corrects. He will know the kind of control to add into the system. Not really inexpensive but it will protect the compressor from having liquid refrigerant flooding back and destroying it. The use of a set back on the stat is the easiest and least expensive fix but can let the house temperature fluctuate some. With the old, undersize ducting I would still go with your idea of a couple of window units to pick up the slack. Just set the temperature controls on the window units to come on if the main unit can not quite keep things cool enough. If you elect to eventually go with a larger central unit be prepared to replace all or most of the ductwork. Bigger units, especially the new ones with the modern refrigerants, require a lot of air movement.

The thermostat you have on the wall can be replaced with any number of different ones, I really prefer a couple of the White-Rodgers models. If you use the same stat for both heating and cooling the wiring can get a little tricky but careful attention to the instructions that come with the unit will see you thru the change. Be sure to put a piece of tape on each wire as you take it off the old stat to identify which pole it went to ON THE OLD STAT. Reason for this is there is a good chance someone may have not used the standard color code of the wires and this is especially true where the system has had add on work done in the past. Since you have a Trane (wreck) system you may have to hunt a bit to find a stat that is compatible.

When looking for an attic fan spend the few extra bucks and get one with louvers and the sensor system that turns the fan on when the attic temps get above a certain level. Best ones will not turn the fan on if the outdoor temp is higher than the attic temp. Size the fan to change the air in the attic every ten minutes or so. Be sure you have plenty of vents to allow outside air to enter the attic so you are not sucking cooled air out of the house.

Good luck with your project.

Mike

victorcharlie · « **Reply #6 on:** June 21, 2008, 10:39:44 AM »

I put in a 14 sear American Standard heat pump a few years back after having the same problems.

I figured as much as I spent on the house I need to be comfortable as I was going to spend a lot of time there!

I figure it knocked about a 100 bucks a month off the bill as it's much more efficient than my old unit.

mike243 · « **Reply #7 on:** June 21, 2008, 10:46:50 AM »

a dirty airfilter or a dirty coil or temps lower than 65 in the house can cause a unit to ice up,also low freon level.a undersized unit will not frezze up from exstended run times.your electric bill will suffer though.if the condenser coil hasnt been cleaned recently it would be hard for anybody to ck the freon reliably.if you can get to the coil ck the return side for dirt,normaly you can get by with smaller ductwork on the 10 seer units but the 13 seer & up need lots more,the window ac will work fine but if the unit has worked well in the past be cking for something dirty then low of freon,if its got a dirty condenser coil it will make things ice up worse when cleaned then you will need to find out whats wrong for sure,mike243

bluebayou · « **Reply #8 on:** June 21, 2008, 06:44:52 PM »

Tencubed,
Should I try blanketing these closets? Basically, they are set back into the kneewall kind of framing that they did on the back side of the attic roof (on the front are dormers with windows because everyone thinks that they look cool). On the back side of the room is a wall probably 4 feet high with the closets set into it. There is a crawlspace hatch thing to get into the dead space on the back of one of the closets. The dead space has plenty of blown insulation in it. I could get into it with difficulty.......will have to anyway to hide the DirecTV line and to install new outlets......could do it all in one swoop.

Would it be worth it to insulate the closets? I will fill the cracks ASAP. Going to Lowe's tomorrow for 4x4's, will look at attic fans with thermostats.

Tencubed · « **Reply #9 on:** June 22, 2008, 04:46:36 AM »

Within reason any area you can get to, top, bottom or sides, of any heated or cooled space will benefit from insulation. Better if you can use insulation that has a built in vapor barrier with the barrier facing the conditioned space.

I would definitely insulate those closet areas if at all possible. This will help keep moisture, condensation, out of these storage areas and also reduce your heating/cooling bills.

Someone mentioned heat pumps. If you do eventually change out the AC unit heat pumps are a good way to go in many areas. Depends on the cost of electricity and what other fuels are available. Some areas natural gas is less expensive to use but with todays changing energy cost weirdness it may be wise to put in a heat pump just in case you need an alternative heat source.

Lowe's, Home Depot and other home improvement stores often have stats of the type that would work for you. Not all stores carry the same products so you may have to look around a bit.

Enjoy your project, I know I always love crawling around in an attic.

Mike

mike243 · « **Reply #10 on:** June 22, 2008, 05:18:40 AM »

if you measure the temp going into the airfiler & what the supply is putting out you want a 18-21 degree diffrence,colder than that & you have a low airflow/low freon problem,if its less than 10 a dirty condenser coil is to blame most of the time,its easy to ck the basics before spending money on thermostats & pressure switches that are not meant to deal with a iceing problem,good luck,been doing this for over 30 years & the basics are normaly what cost folks money for me to correct that they could have saved

mike243

bluebayou · « **Reply #11 on:** June 22, 2008, 11:57:26 AM »

mike and tencubed (mike and mike?)- thanks for the info

Yeah, you got to love crawling in the attic. I remember when I was a kid that one of the best parts of Christmas was that I was to go up the ladder to get the Christmas decorations...........ANYTHING to get into the attic, it was awesome...dark, old relics from the 50's (previous owners) and 60's, a rifle (later discovered to be Mannlicher-Carcano), etc. Now, as an adult.....I would ALMOST rather crawl into the crawlspace with the spiders to look for the skunk than go into 140 degree space full of fiberglass and spiders.

thermodix · « **Reply #12 on:** February 25, 2009, 02:38:15 AM »

Quote from: Tencubed on June 22, 2008, 04:46:36 AM

Within reason any area you can get to, top, bottom or sides, of any heated or cooled space will benefit from insulation. Better if you can use insulation that has a built in vapor barrier with the barrier facing the conditioned space.

I would definitely insulate those closet areas if at all possible. This will help keep moisture, condensation, out of these storage areas and also reduce your heating/cooling bills.

Someone mentioned heat pumps. If you do eventually change out the AC unit heat pumps are a good way to go in many areas. Depends on the cost of electricity and what other fuels are available. Some areas natural gas is less expensive to use but with todays changing energy cost weirdness it may be wise to put in a heat pump just in case you need an alternative heat source.

Lowe's, Home Depot and other home improvement stores often have stats of the type that would work for you. Not all stores carry the same products so you may have to look around a bit.

Enjoy your project, I know I always love crawling around in an attic.

Mike

You said...Someone mentioned heat pumps. If you do eventually change out the AC unit heat pumps are a good way to go in many areas. Depends on the cost of electricity and what other fuels are available. your right for that, i think some areas electricity cost are a bit higher....

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Programmable Thermostats

squirrellluck · « **Reply #13 on:** February 25, 2009, 06:25:25 PM »

A heatpump is good but below 38 degrees it looses effeciecy and the heat strips kick on unless you are talking ground source then it does not care what the outside temp is. By the way Mike you wrote my post before I could. But my question is ,does this freezing happen often or is this the first time it has done this?