Greetings! New to the forum, so I hope I am not re-hashing an old topic. We just bought a bit of a disaster of an old house in Seattle and are strategizing how best to make it more comfortable and efficient. The previous owners installed central air in the house fairly recently, but seemed to use an unlicensed contractor who did a terrible job. The air handler and ducting is in the uninsulated attic, and there was never a drain pan installed under it, so I'm going to have to spend at least $2000 to bring it up to code. Given that A/C is so rarely needed in Seattle, I'm actually wondering if I would be better off just ripping the system out and optimally insuluating the attic. If we were desperate to add A/C in the future, I suppose we could rebuild a system from scratch. Is my cost/benefit analysis here way off, or is proper insulation a better use of that attic? Thanks so much for your thoughts!

We live in Seattle part-time…

For cooling, you might be happy with a whole house fan that pulls hot air from the upper floor and exhausts it into the attic or directly outside. If your house is reasonably well-insulated, you can just open the windows on lower floor in the evenings when the temps fall into the 50/60F range and operate the fan to quickly cool down the house. During the hot part of the day, keep the windows closed and rely on the insulation to keep the house at an acceptable temp. You will need to work out the flow rate details of the fan and the attic ventilation area to ensure that this approach will be successful.

If you decide to do a full replacement, I would consider a heat pump that efficiently accomplishes both your cooling and heating requirements and relocate the ducting to the conditioned living space area.  Or better yet, perhaps consider efficient mini-splits that eliminate the requirement for ducting completely and can accomplish both your cooling and heating requirements.

We are fortunate not to have much humidity during the Seattle summer months and to have few days where the temp exceeds 85F.

How is the place being heated, and what is the manufacturer/model number of the AC compressor and air handler? How big is the house, and what vintage?

Ducts in attics are a bad practice, but common in slab-on-grade homes. Even putting them in vented crawlspaces would be better if you can, but that doesn't always work well for cooling the second floor of a barely insulated house.

At Seattle-style outdoor dew points it's usually OK to bury attic ducts in blown insulation (once you've sealed both the attic floor and the ducts), since there is very little risk of chronic condensation on the ducts, unlike locations well east of the Rockies. If the system is worth saving at all kind of depends on the actual cooling loads, the system sizing, and how difficult the presence of the less-than-optimally installed ducts & air handler makes the all-necessary air sealing that has to go on prior to insulating the attic. If it's a 4-5 system with a 1 ton peak load it's just a liability, but if it's reasonably matched to the load (less than 2x oversized) it might be worth re-commissioning it (even if it means installing a new/different duct system.)

If you're off the gas grid and heating with propane or resistance electricity, it may be worth considering a better class heating/cooling heat pump option rather than strictly cooling, which can work pretty efficiently at Seattle type outdoor temps, provided you make the house & ducts tight (and WA electricity pricing is downright cheap compared to much of the US.) But it takes a bit of napkin-math to figure this stuff out.

Unfortunately I don't have a ladder tall enough to get me into the attic to read the model number of the air handler, but the compressor is a Carrier 38HDR036 (3 ton unit?). The house is 100 years old, leaky as a sieve, and around 4500 square feet. Ducts are only run to some of the rooms. The house has a relatively new Buderus boiler which supplies cast iron radiators--I'd like to keep those, so I'm not sure the benefit of installing a heat pump system to use for A/C. Appreciate the help . . .

"The air handler and ducting is in the uninsulated attic, and there was never a drain pan installed under it, so I'm going to have to spend at least $2000 to bring it up to code"

Really? That seems way off mark

"The air handler and ducting is in the uninsulated attic, and there was never a drain pan installed under it, so I'm going to have to spend at least $2000 to bring it up to code"

Really? That seems way off mark

No kidding! But I've gotten several quotes and they're all in that ballpark (anyone know an honest Hvac contractor in the Seattle area?).

Is the Buderus an oil burner, propane burner, or a natural-gas burner?

At 3 tons of compressor for 4500' of conditioned space the AC probably isn't ridiculously oversized, despite the fact that latent your loads @ 55% RH are negative (no real latent load- EVER), and the 1% outside design temps are in the low 80s in King County. The air handler & ducts being in the unconditioned probably adds a half-ton or more to the load though (unless sealed perfectly, and well insulated.)

Posted By cem21 on 28 Jul 2014 11:38 AM
No kidding! But I've gotten several quotes and they're all in that ballpark (anyone know an honest Hvac contractor in the Seattle area?).

What are they doing for 2K? Insulating duct and what else? The pan? If you are in IRC regulated area, there are other options vs a secondary drain pan including a switch that disables the system once moisture is detected or a secondary drain to a "conspicuous" location. You can insulate duct yourself or hire a handyman.