Help sizing a dual fuel system
Hi. I live in Washington DC and am looking to install a dual fuel (heat pump and gas) system in our new home (a 1939 brick colonial with ~2300 sq ft). All three of the contractors I've had out have basically refused to do a Manual J so I bought a program from hvaccomputer.com and have done it myself. I'm pretty confidant I did it correctly since the program was pretty straightforward, but now I'm having some trouble deciding what to do with the data since it seems to be on the line between different sizes.
The program recommends design criteria of 91 degrees for summer and 17 for winter based on my zip code. The instructions stress that the program adds no safety margin to any calculations but suggests that in many situations people add 20% margin on furnances but little or none on A/C to maximize efficiency. However, since I'd like to do dual fuel I imagine I need to look at both the heating and colling capability of the pump.
Here's what I get:
91 34,679 (sensible gain = 31K; Latent gain = 4K)
It seems like a for the furnace, a 80% dual stage with 100K Btu's in and 80K out is probably the right size. Am I reading this right?
As for the heat pump, do you think a 3-ton or 4-ton is more appropriate? It seems 3 tons might be enough for the A/C but the switch over point would then need to be above 40 degrees which seemed high to me but maybe it isn't. Does your recommendation depend on whether I go single or dual stage (on the logic that in a dual stage, oversizing may be less problematic)? I had been thinking about either a Trane or a Bryant Evolution but neither seems to come in a 3.5 ton option. Lastly, since all the quotes I have so far are for 3 ton units, how much more could I expect to pay to go to 4 tons so I'm prepared for sticker shock if I go that way?
In DC the home of ACCA (acca.org) you should be able to find a contractor who will do a load calc for you.
Based on your info and a single stage HP, a 3 probably won't cut it. You are supposed to select a system that 100% meets the sensible gain you have calculated. I don't think you will find a 3 ton AC or HP with 31K of sensible capacity, 27 or 28K tops. So a 3.5 ton sounds right to me. The 100K 80% sounds correct too.
The 2 speed will always be a better choice IMO. But it your wallet better be fat, cause you'll need it for a cushion when the sticker shock knocks you on your butt . But can you really put a price on comfort?
BTW, I'm building a new home myself and pealing for a 2 speed HP...
You may want to give some serious consideration to putting some of your budget toward improving the thermal efficiency of your new home. Do some what if runs and see what you have to do with insulation and especially infiltration to improve on the heating and cooling load. Any money you spend in improving the envelop is a one time expense that will save you money every day.
I live west of you near the Blueridge in an old farm house that was as loose as a goose when we bough it two years ago. Air coming and going everywhere - especially through the attic. I have been working on tightening it up. The biggest improvement was sealing the attic with spray foam insulation this past summer. Summer temps in the attic were in the upper 80's to lower 90's no matter how hot it was outside. As we have moved into heating season I am shocked how little the heating systems are running. With the attic now sealed the reduced stack effect (the movement of warm air rising right out of the house) has had a huge impact on the call for heat.
The other thing that is often the case with two story colonials is poor cooling during the summer. If you have not lived there through a summer yet you may want to consider putting in two smaller systems. One up and one down. You will find that about 70% of your cooling will be done by the upper unit and about 70% of your heating by the lower.
if one system, 4 ton but have your ductwork thoroughly inspected as to size, insulation properties, overall condition, leakages, adequate supply and returns. I would ask about the feasibility of zoning controls for the first and second floor. Of course two separate systems are your best bet for great comfort.