Hi,
I am getting quotes for a furnace and A/C change out to take advantage of the tax credits. Three contractors have done calculations to determine the A/C sizing. In each case, the indoor temp used was 76 and the outside temp was 89. We try not to use our A/C a lot, to save on electric costs, but when we use it, we want it to cool the house. Typically, we would not turn on the A/C unless the daily high were 83 to 85 or above. Therefore, the majority of the days we would want the A/C to be working are likely near or above the ASHRAE (97.5%?) design temp of 89.
The contractor's calculations seem to show somewhere between 2- and 2.5-ton, generally around 26K for latent and sensible combined. Two of the three contractors are recommending 2-ton and one 2.5-ton (his number was 27.5K).
I am spending good money and would like to get the most efficient system (see note on saving electricity, above), but I am concerned that when we most need it, it won't be able to cool our house adequately.
I have three questions for the collective experts. In your experience, does an accurate application of Manual J provide a good match (is it conservative or otherwise) or how do you select a size for all of the cases that fall between two sizes. And two, if it really is a 2-ton load and I were to install a 2.5-unit, how much efficiency would I be wasting? Finally, I have a 2.5-ton unit currently running on a simple thermostat. Can I use the next couple of days of approximately 90-degree heat to do a quick empirical study of cycle time to see if 2- or 2.5- ton would be better? (BTW, I do get the sense that our current 2.5-ton unit cycles too often when in use.)
Thanks for your response.
I am getting quotes for a furnace and A/C change out to take advantage of the tax credits. Three contractors have done calculations to determine the A/C sizing. In each case, the indoor temp used was 76 and the outside temp was 89. We try not to use our A/C a lot, to save on electric costs, but when we use it, we want it to cool the house. Typically, we would not turn on the A/C unless the daily high were 83 to 85 or above. Therefore, the majority of the days we would want the A/C to be working are likely near or above the ASHRAE (97.5%?) design temp of 89.
The contractor's calculations seem to show somewhere between 2- and 2.5-ton, generally around 26K for latent and sensible combined. Two of the three contractors are recommending 2-ton and one 2.5-ton (his number was 27.5K).
I am spending good money and would like to get the most efficient system (see note on saving electricity, above), but I am concerned that when we most need it, it won't be able to cool our house adequately.
I have three questions for the collective experts. In your experience, does an accurate application of Manual J provide a good match (is it conservative or otherwise) or how do you select a size for all of the cases that fall between two sizes. And two, if it really is a 2-ton load and I were to install a 2.5-unit, how much efficiency would I be wasting? Finally, I have a 2.5-ton unit currently running on a simple thermostat. Can I use the next couple of days of approximately 90-degree heat to do a quick empirical study of cycle time to see if 2- or 2.5- ton would be better? (BTW, I do get the sense that our current 2.5-ton unit cycles too often when in use.)
Thanks for your response.
