Best System for Phoenix Arizona
I am in a Phoenix Arizona and am updating and changing my HVAC units as part of adding to the size of the house as part of a remodel. Only thing I am keeping from the previous HVAC system is the original 1940 under slab concrete supply ducts for the bedroom area HVAC system. Those under slab concrete ducts are oversized for the new replacement unit and pushed out a lot of cold air with the 5 ton air handler previously hooked to them. The total new expanded space to cool/heat with two separate units will be approximately 2700 sq feet after subtracting closets etc. The house is roughly half adobe (with 12 inch walls) and the rest is 8 inch block walls. The only glass exposure that is not shaded by shutters is on the North side of the house. On the north side there is a new 15 foot bank of insulated glass doors with a less than 2 foot roof overhang. Those north facing glass doors get direct sun less than one hour each day. The house has always stayed very cool and comfortable.
I plan to cool/heat the one level house with two separate systems that will each roughly handle one half of the house.
One HVAC contractor had an engineering firm do a manual J calculation which he did without visiting the house but probably looked at the building plans for measurements. His square footage of area to be conditioned was off for several of the rooms some by as much as 60-70 sq ft. His brief report did not reference or account for any of the shutters that shade most of the glass in the house or the unconditioned spaces within the house (i.e. closets etc) His report used 112 degrees and 34 degrees for the Phoenix design conditions and calculated a total sensible heat gain of 71,000 btuh, total gain of 72,000 btuh and total heat requirement of 59,500 btuh with 357 square ft. per ton. The report and the HVAC contractor who hired the engineer recommended 8.5 tons to cover worst case and 70% sensible capacity, even though the report calculated only 6 tons for actual sensible + latent for the house.
After I measured the dimensions of the inside space to be cooled, windows etc. and using the 108 degree outdoor design condition for Phoenix from ACCA and plugging that information into the HVAC-Calc software, I got the following results: Total sensible gain of 36,600 btuh and total heat loss of 46,698 btuh. With each of the two areas to be cooled calling for just under 18,500 btuh sensible gain each of the areas look like each can be handled by a 2 ton unit especially after the extended data shows that each of the two units have sensible capacity of 21.5 at 78 degrees inside with Trane’s design condition of 59 OD dry bulb(much higher than actual Phoenix conditions)105 degrees outside and 20.7 at 115 degrees outside before the multiplying factors applied for higher 900 cfm fans speed and oversized variable speed air handler/furnace options which look like they would raise the sensible capacity by as much as 16%.
From reviewing the calculations from the HVAC-Calc software my thought is to get a Trane XL15i heat pump(2 ton) 4TWX5024B and the most efficient matched Trane Hyperion XL7 variable speed air handler for the bedrooms/living room area(My preference is to not have a gas furnace near the bedrooms). For the kitchen/ family room area rest of the house I am thinking of an XL15i air conditioner (2 ton) and a Trane XV80 or XV95 variable speed gas furnace with the most efficient matching evap coil. With our dry climate the above HVAC packages appear able to easily handle the sensible cooling load for our desert climate under the extended performance data that I saw. The info I saw for the XL15i heat pump was paired with the previous version of the Trane variable speed air handler (I have not been able to find the performance data for the XL15i matched with the new Hyperion 7 variable speed air handler). The 2 ton Trane heat pump/air handler combination for our bedrooms/living room area appears to be slightly undersized to cover the winter heat loss for the coldest nights that we could experience in the winter but on those rare occasions my thought is that if necessary I could simply limit or completely close the living room vent (10,000 btuh heat loss for that room alone) and there would then be enough heating capacity for 17,000 btu heat loss of the bedrooms without the living room (the bedrooms are separated from the living room and rest of the house by a door. Also there is a gas fireplace in the living room and the floors of the bathrooms heated with the bedrooms will have on demand electric radiant floor heat. Am I correct in thinking that the money saved by not oversizing the heat pump just to handle the infrequent extra heating is worth the rare times when the bedrooms/living room could be cool from the heat pump not having enough heating capacity to handle our coldest evenings? Another factor is that the living room is open to the dining room/kitchen/family room area that will be heated by a smallest two stage variable speed Trane gas furnace with lots of additional capacity. There also is a wood burning fireplace in the family room that I use most nights in the winter that puts out enough heat that the family room/kitchen area heat seldom ran when a fire is burning in that fireplace. Do I still need to get heat strips on the heat pump system as well and would that be the best way to address the heat pump undersized heating capacity?
Anyway the bedrooms/living room area tradeoff is having a heat pump that is undersized for heating the bedrooms/living room area in order to more closely size the heat pump to match the cooling load calculations. By my calculation the heat pump would have to increase by more than one size (from the 2 ton size matched to cooling load) to handle the 27,000 btu heat loss under the HVAC calc calculations. Supply registers for the bedrooms/living room area will be fed via oversized under slab concrete ducts that less than 25 feet from the variable speed air handler. With the old model Trane variable speed air handler it looks like my most efficient combination with the 2 ton XL15i heat pump was the TWE040E13 variable speed air handler (Is the TAM7A0C42H31SB the most efficient matched Hyperion 7 series air handler for this heat pump?). Can anybody provide or point me to where I can find the numbers for the newest version XL15i heat pump and Hyperion XL7 variable speed air handler? Any reason to think that the oversized under slab concrete ducts will not continue to work fine with a 2 ton Trane XL15i heat pump and best matched larger Trane Hyperion XL7 variable speed air handler package?
For the kitchen/family room area supply registers will be via new upper wall metal/flex ducts that will be no more than 25 feet from the variable speed gas furnace. For the AC/furnace combination for the dining room/family room/kitchen area it looks like my most efficient combination with the 4TTX5024A1000B 2 ton XL15i air conditioner and variable speed gas furnace (either XV80 TUD2B060A9V3VA or XV95 TUH2B060A9V3VA ) is with a 3 ton coil. I assume that has not changed with the changes and newer models of the XL15i air conditioners since the 2005 data that I have. I understand that with the XV80 I would need to have a 5 inch diameter exhaust pipe sticking through the new roof and that exhaust pipe is not necessary for the XV95 and that the XV80 is much noisier. I am told that the XV95 will cost approx $ more than the XV80. Are there any other considerations to know in choosing between the XV80 and more efficient XV95?
I also plan to get for each unit a Trane 5 inch media filter and Honeywell VisionPro IAQ thermostat that I will also use to control a Panasonic inline fan for ventilation of each area of the house. Is there any advantage to getting the XR15 versions of the above packages over the XL15i versions? I assume that the correct sizing for the totally new duct system for the AC/gas furnace combo should be matched to suit the air capacity of the oversized matching coil that is used?
Sorry about the long post but I wanted to include any info that I thought could be relevant to your consideration and advice. I appreciate and welcome any suggestions of changes or additions that you have re my plan and equipment.
Last edited by beenthere; 09-08-2012 at 06:36 PM.