[Daltex]

I am designing a replacement HVAC system for my house and stumbled again on an issue I hope you guys can help with. There are multiple units involved but I am just replacing the upstairs unit now which is the bedroom area. I have always raised the set point of the stat in the unoccupied parts of the house in the summer. At night the stat lowers the bedroom area to 70 or so.

I have always had oversized units that run no more than 30 mins. per hour even on the hottest 100+ degree days. Replacing it with the correctly sized unit and ductwork (man.j,d). Will either a HP and conv. AC (properly sized) be able to lower the temp from 80 to 70 within an hour or so when outside temp is 90? Seems like if the "perfect system" is installed it will not drop temp. very fast.

I had ruled out using a HP but don't remember why. I think it was due to complants about the cooler than body temp. air blowing on you when using the HP for heating. I now know that proper ducting and quality diffusers will minimize this effect. Any thoughts? VS of course.

Lastly I'm not using this data to go buy a system for a DIY install. I've posted in past post about my delima finding qualified HVAC contractors in my area. Main problem is they use rule of thumb (1500 sq'= 4 tons). I'm not at this house now but will post some great pics soon regarding the gas smell I posted about earlier. Unbelievable! The unit is in a raised cabnet with the return plenum under it. The cutout for the return plenum is about 4 inches deeper than the air handler. There is an open ceiling above the unit for combustion air. I had 80 square inches of unfiltered attic air mixed with combustion air etc. returning into my system. Wonder what the A coil looks like.

 

[second opinion]

I am designing a replacement HVAC system for my house and stumbled again on an issue I hope you guys can help with. There are multiple units involved but I am just replacing the upstairs unit now which is the bedroom area. I have always raised the set point of the stat in the unoccupied parts of the house in the summer. At night the stat lowers the bedroom area to 70 or so.

I have always had oversized units that run no more than 30 mins. per hour even on the hottest 100+ degree days. Replacing it with the correctly sized unit and ductwork (man.j,d). Will either a HP and conv. AC (properly sized) be able to lower the temp from 80 to 70 within an hour or so when outside temp is 90? Seems like if the "perfect system" is installed it will not drop temp. very fast.

I had ruled out using a HP but don't remember why. I think it was due to complants about the cooler than body temp. air blowing on you when using the HP for heating. I now know that proper ducting and quality diffusers will minimize this effect. Any thoughts? VS of course.

Lastly I'm not using this data to go buy a system for a DIY install. I've posted in past post about my delima finding qualified HVAC contractors in my area. Main problem is they use rule of thumb (1500 sq'= 4 tons). I'm not at this house now but will post some great pics soon regarding the gas smell I posted about earlier. Unbelievable! The unit is in a raised cabnet with the return plenum under it. The cutout for the return plenum is about 4 inches deeper than the air handler. There is an open ceiling above the unit for combustion air. I had 80 square inches of unfiltered attic air mixed with combustion air etc. returning into my system. Wonder what the A coil looks like.

There are several good contractors in this forum in your area.

Watch for Mark Beiser to post or post his name directly I am sure he is competent to handle your requests.

 

[gary_g]

I had ruled out using a HP but don't remember why. I think it was due to complants about the cooler than body temp. air blowing on you when using the HP for heating.

New heat pumps put out:

104 degree air at 55F outside
100 degree air at 47F
95 degree air at 35F
91 degree air at 25F

The above temps are at the air handler for a new 3-ton heat pump, and based on an indoor temp of 70F.

The palm of your hand is about 90F.

Best to you.

 

[Daltex]

New heat pumps put out:

104 degree air at 55F outside
100 degree air at 47F
95 degree air at 35F
91 degree air at 25F

The above temps are at the air handler for a new 3-ton heat pump, and based on an indoor temp of 70F.

The palm of your hand is about 90F.

Best to you.

Thanks Gary- What kind of temp loss is the norm for attic duct runs. I know it varies but with new ductwork, installed correctly (appropriate insulation over solid ductwork), would 5-10 degree loss or 10-20 be close in say 35F outside air?

 

[docholiday]

As you already mentioned, air patterns are the biggest concern. Try this test.

Open your hand in front of your mouth and breath out as if trying to fog a window. Warm. Now blow on your hand like you are trying to put out a candle... Cool. Does your body have the ability to alter the temperature of the air leaving your lungs? No, it's a matter of velocity and feel. Its the same temp either way.

youre right, a properly sized unit will not cool a house from 80 - 70 in one hour when it's 90 degrees outside. Mainly because it is designed to maintain even temps and humidity removal. Your home also is filled with mass such as wood, furnature etc that hold heat. It's not just a matter of cooling the air down, you have to remove heat from everyting in the home.

 

[Daltex]

Thanks Doc- So when properly sized, is it more efficient to leave it at a constant temp. even if it's like say 70 on a hot 90 degree day? Would the VS blower/compressor compensate for this on a efficiency level or not?

Never had a properly sized system so just looking before I leap.

Thanks

 

[platchford]

As you already mentioned, air patterns are the biggest concern. Try this test.

Open your hand in front of your mouth and breath out as if trying to fog a window. Warm. Now blow on your hand like you are trying to put out a candle... Cool. Does your body have the ability to alter the temperature of the air leaving your lungs? No, it's a matter of velocity and feel. Its the same temp either way.

Could someone confirm this with a good (relatively precise) temperature probe? My understanding was the reason the air is warm when you exhale normally is, well, you're lungs are warm and thus so will the air being expelled. Whereas, on the other hand, if you blow as if you are trying to cool off something that is hot (closing you lips rather tightly as you exhale) the air released from your lungs is under greater pressure and therefore expands and cools as it exits your body.

 

[platchford]

Thanks Doc- So when properly sized, is it more efficient to leave it at a constant temp. even if it's like say 70 on a hot 90 degree day? Would the VS blower/compressor compensate for this on a efficiency level or not?

Never had a properly sized system so just looking before I leap.

Thanks

With a heat pump you want to maintain a relatively constant temperature... some may say that you don't want to setback the temperature at all... it really depends on the thermostat you use... an adaptive recovery thermostat that intelligently brings the temperature back up without causing the electric strip heat to turn on is a must. Basically, the problem is this, if you have the temperature setback and the the heat pump is not able to bring the temperature up enough on its own, then your electric heat (or whatever second stage heat system you have) comes on and reduces or negates whatever savings you had by setting the temperature back. It's entirely possible that it could entirely reverse any energy savings. However, this depends very much on the design of the system installed and the thermostat used in conjunction with the system. Even with a good thermostat you should not set back your temperature too much... again this number varies depending on who you ask... personally I use 5 degrees in my own home.

 

[Daltex]

With a heat pump you want to maintain a relatively constant temperature... some may say that you don't want to setback the temperature at all... it really depends on the thermostat you use... an adaptive recovery thermostat that intelligently brings the temperature back up without causing the electric strip heat to turn on is a must. Basically, the problem is this, if you have the temperature setback and the the heat pump is not able to bring the temperature up enough on its own, then your electric heat (or whatever second stage heat system you have) comes on and reduces or negates whatever savings you had by setting the temperature back. It's entirely possible that it could entirely reverse any energy savings. However, this depends very much on the design of the system installed and the thermostat used in conjunction with the system. Even with a good thermostat you should not set back your temperature too much... again this number varies depending on who you ask... personally I use 5 degrees in my own home.

My bigger concern is with the AC side. Bringing it down to 70 from 80 on a 90 degree evening. Can a properly sized system do this in a reasonable amount of time? Also is there a difference on the cooling side of a HP vs. conv. AC systems?

By the way- What is more efficient- Set back of 10 degrees on a gas furnace or 5 on a HP?

 

[beenthere]

A properly sized system won't do it in an hour.

You may not need it to drop the temp 10*F in an hour anymore either.
With a properly sized unit, your humidity will be lower. You should be able to set your thermostat higher, and feel just as cool.

With the right equipment and thermostat, you may be able to set your stat to 74 and feel like its 70.

 

[Daltex]

A properly sized system won't do it in an hour.

You may not need it to drop the temp 10*F in an hour anymore either.
With a properly sized unit, your humidity will be lower. You should be able to set your thermostat higher, and feel just as cool.

With the right equipment and thermostat, you may be able to set your stat to 74 and feel like its 70.

Great point. I new (from you guys) that the latent heat removal would be better but I guess my brain has been trained to set the stat there so that was a concern. Not anymore. Thanks.