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Title: Paging IV, Gregg, etc. Re: HVAC Post by: murfvol on October 09, 2012, 05:22:26 EDT What size unit(s) is needed for a two story 1900 square foot house with no high ceilings that gets no morning sun, but a lot of it in the afternoon?
Currently it has a 3 ton/40,000 btu unit that's 12 years old. Title: Re: Paging IV, Gregg, etc. Re: HVAC Post by: Inspector Vol on October 09, 2012, 06:02:11 EDT I am by no means a hvac expert, but it depends on a lot of factors. How well are you insulated in the walls,ceilings, windows & doors? What is your climate? I assume you have a split unit now, one that services the upstairs and one the downstairs, are you replacing both?
Rule of thumb used to be 500ft per ton. I can advise against over sizing as that can be worse than a little undersized for your home. Good luck. Title: Re: Paging IV, Gregg, etc. Re: HVAC Post by: GreggO on October 09, 2012, 06:53:35 EDT IV is correct on all counts. Also, units are a lot more effecient these days than just 10-15yrs ago. Are you going "John Ward" (gas) or electric? I would talk to several contractors to see if you can find a consensus. G Title: Re: Paging IV, Gregg, etc. Re: HVAC Post by: murfvol on October 09, 2012, 07:06:11 EDT Much appreciated gentlemen. Gas is not an option (we're in the sticks and there's no line), and we have a single unit that appears to be on its last legs. Our insulation is o.k., but not awful. The ceilings, doors and windows are pretty decent.
Since we're just north of Charlotte it's good enough. Thanks much for the oversizing info too. I didn't realize that had drawbacks. Title: Re: Paging IV, Gregg, etc. Re: HVAC Post by: Inspector Vol on October 09, 2012, 07:27:15 EDT If it is oversized it will cycle on and off too much wearing out your unit faster. Also it may not run long enough to pull the humidity out of the home.
As Greggo advised seek multiple reputable hvac contractors and ask them all the same questions. I am curious as to why it is a single unit, as around here every two story home has two units. Was the upstairs added at a later date? Title: Re: Paging IV, Gregg, etc. Re: HVAC Post by: murfvol on October 09, 2012, 07:46:38 EDT Nope, it was built at the same time, but is indicative of the shoddy construction of the home. At least we had a home inspectin and knew what we were getting into so it was priced accordingly. :smile:
Title: Re: Paging IV, Gregg, etc. Re: HVAC Post by: Inspector Vol on October 09, 2012, 08:17:06 EDT Nope, it was built at the same time, but is indicative of the shoddy construction of the home. At least we had a home inspectin and knew what we were getting into so it was priced accordingly. :smile: Might consider going split unit as typically the upstairs is harder to cool but gets hotter in the winter. The split would give you flexibility and maybe more comfort. That is if it is a possibility depending on the floor plan. Title: Re: Paging IV, Gregg, etc. Re: HVAC Post by: VinnieVOL on October 09, 2012, 08:39:08 EDT On a related note, my builder installed two air returns and most other houses I've been in have just one. Anyone know why?
Title: Re: Paging IV, Gregg, etc. Re: HVAC Post by: Inspector Vol on October 09, 2012, 08:40:26 EDT Single story or 1 up and 1 down?
Title: Re: Paging IV, Gregg, etc. Re: HVAC Post by: VinnieVOL on October 09, 2012, 08:55:30 EDT Single story or 1 up and 1 down? Single story. Basement but no heating and air down there. Title: Re: Paging IV, Gregg, etc. Re: HVAC Post by: Inspector Vol on October 09, 2012, 09:09:42 EDT Single story. Basement but no heating and air down there. What is the length of the house? If it is very long they may have did that to draw air from all over the house as opposed to one space. Again, no hvac expert here but that is just what comes to mind. Title: Re: Paging IV, Gregg, etc. Re: HVAC Post by: VinnieVOL on October 09, 2012, 09:31:30 EDT What is the length of the house? If it is very long they may have did that to draw air from all over the house as opposed to one space. Again, no hvac expert here but that is just what comes to mind. I guess that's probably why. Title: Re: Paging IV, Gregg, etc. Re: HVAC Post by: RockinGrannyVol on October 11, 2012, 02:26:51 EDT Much appreciated gentlemen. Gas is not an option (we're in the sticks and there's no line), and we have a single unit that appears to be on its last legs. Our insulation is o.k., but not awful. The ceilings, doors and windows are pretty decent. Since we're just north of Charlotte it's good enough. Thanks much for the oversizing info too. I didn't realize that had drawbacks. Propane is always an option....we use propane and use it for heat/cooking/hot water....We have a wonderful dual fuel system which uses heat pump down to 35 degrees, then the gas kicks in since heat pumps become ineffecient at freezing! If you get a larger tank, you can fill it in the summer months when rates are much lower and you don't have to pay those high prices. We opted to buy and bury a 100 gallon tank -- we top it off each summer and save loads of $$$. Just a thought! Title: Re: Paging IV, Gregg, etc. Re: HVAC Post by: Inspector Vol on October 11, 2012, 03:17:56 EDT Propane is always an option....we use propane and use it for heat/cooking/hot water....We have a wonderful dual fuel system which uses heat pump down to 35 degrees, then the gas kicks in since heat pumps become ineffecient at freezing! If you get a larger tank, you can fill it in the summer months when rates are much lower and you don't have to pay those high prices. We opted to buy and bury a 100 gallon tank -- we top it off each summer and save loads of $$$. Just a thought! Not a bad idea. Of course you know I had to post this...... :dielaughing: (http://www.deviantart.com/download/177265508/hank_hill_by_xmanipulationofcoldx-d2xjexw.jpg) Title: Re: Paging IV, Gregg, etc. Re: HVAC Post by: RockinGrannyVol on October 11, 2012, 06:24:34 EDT Not a bad idea. Of course you know I had to post this...... :dielaughing: (http://www.deviantart.com/download/177265508/hank_hill_by_xmanipulationofcoldx-d2xjexw.jpg) BTW that is a 1000 gallon tank, not 100! We normally use around 200-300 gallons a year in a fairly large house (2700 sq ft). For the last year, we averaged $38 a month for propane. Title: Re: Paging IV, Gregg, etc. Re: HVAC Post by: Inspector Vol on October 11, 2012, 06:40:41 EDT $38 is good. :thumbup:
Title: Re: Paging IV, Gregg, etc. Re: HVAC Post by: murfvol on October 11, 2012, 09:14:45 EDT That's sound very reasonable. I was curious how much you used.
Title: Re: Paging IV, Gregg, etc. Re: HVAC Post by: RockinGrannyVol on October 11, 2012, 09:58:17 EDT That's sound very reasonable. I was curious how much you used. That may be 300 - 400.....but anyway, the $38/month is right because I just went and looked at what we paid to top it off this summer -- that cost was to replenish what we used last year. Title: Re: Paging IV, Gregg, etc. Re: HVAC Post by: 73Volgrad on October 13, 2012, 02:13:28 EDT I am a Mechanical Engineer by education and practice. I would recommend a 4-ton unit if you can afford it. If possible, go with a multi-speed fan and 2 speed or stage compressor operation. If slightly over-sized, the unit would run at low speed and low energy use for a longer time which will get rid of the humidity without cooling the house too much (i.e., short cycle). In a short cycle, you lower the temperature, but the humidity remains high so you may still feel uncomfortable or hot.
I bought a Lennox 5-ton 2-speed compressor and multi-speed fan unit (actually a gas furnace but you can get a fan unit). Except in hot weather, the AC runs about 3.5 tons capacity. If it runs too long or temperature does not drop, it cycles off and restarts at 5-ton and higher fan speed to bring the temperature down. When running at lower capacity and fan speed, it dehumidifies the air space well. Even when it was hot late spring and early summer, my electric bill was <$100 a month. I have nominal 2,000 sq ft cooling space with 1970s era insulation. Like warned by several above, more BTUH cooling (tons) is a hindrance to cooling the air space if the fan speed does not match. The key is to be sure the installer sets the speed on the blower fan to keep the proper air flow over the cooling coils or they could freeze over. A lot of the two-speed HVAC split systems and package units match fan speed to compressor stage. Gut feel, you could likely use a 4-ton unit and be pleased except when the temperatuer is >95° and >50% RH (typical for Charlotte). Then your house will be cool, but the humidity will be high.A single speed 5-ton unit is likely not a good selection. Title: Re: Paging IV, Gregg, etc. Re: HVAC Post by: Inspector Vol on October 13, 2012, 03:32:10 EDT I am a Mechanical Engineer by education and practice. I would recommend a 4-ton unit if you can afford it. If possible, go with a multi-speed fan and 2 speed or stage compressor operation. If slightly over-sized, the unit would run at low speed and low energy use for a longer time which will get rid of the humidity without cooling the house too much (i.e., short cycle). In a short cycle, you lower the temperature, but the humidity remains high so you may still feel uncomfortable or hot. I bought a Lennox 5-ton 2-speed compressor and multi-speed fan unit (actually a gas furnace but you can get a fan unit). Except in hot weather, the AC runs about 3.5 tons capacity. If it runs too long or temperature does not drop, it cycles off and restarts at 5-ton and higher fan speed to bring the temperature down. When running at lower capacity and fan speed, it dehumidifies the air space well. Even when it was hot late spring and early summer, my electric bill was <$100 a month. I have nominal 2,000 sq ft cooling space with 1970s era insulation. Like warned by several above, more BTUH cooling (tons) is a hindrance to cooling the air space if the fan speed does not match. The key is to be sure the installer sets the speed on the blower fan to keep the proper air flow over the cooling coils or they could freeze over. A lot of the two-speed HVAC split systems and package units match fan speed to compressor stage. Gut feel, you could likely use a 4-ton unit and be pleased except when the temperatuer is >95° and >50% RH (typical for Charlotte). Then your house will be cool, but the humidity will be high.A single speed 5-ton unit is likely not a good selection. I sent you a PM. |