SEER 13 or 17?
Hello,
I am renovating a 1918 home in Rhode Island. When finished the wall insulation will be about 50% closed foam, the rest blown in cellulose that has not settled much, the roof will be closed cell, I am using blue skin, 1.5 inches of rockwool, and thermalbuck as exterior shielding and extra insulation. All new double pane energy star compliant windows.
My question relates to how much air conditioning benefit I would gain from going to a seer 17 dual speed compressor over seer 13 single speed. I know that the 13 will cool the house easily, but will it run long enough to dehumidify, zone 5A temps and mid to high humidity, and filter the air? In the total cost of the project the upgrade is minor if the benefits are there. Any help would be appreciated.
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At the same AC sizing SEER 13 units tend to handle humidity better than SEER 17 units, but the difference isn't enormous. By far the biggest factor is sizing it correctly for the sensible load. If the SEER 17 2-speed is sized for the load at it's high speed it will probably handle humidity as well or better than the SEER 13 single speed. But without the load numbers and the model numbers there's no way to dig into it very far. If oversized by 2x or more it could be that neither of them do a great job on the humidity.
I'm curious- why 50% closed cell foam?
Thanks. The house had cellulose blown in a while ago, but I am adding dormers and moving walls around. Some of the plaster walls have not survived construction.
It’s hard to answer this question with the level of detail specified. It’s at least necessary to propose two specific models of AC and provide there submittal sheet, then at least some “pen and paper” estimates can be given. Also I will mention that comparing SEER numbers is probably not that useful. These numbers are essentially weighted averages of EER ratings, they don’t necessary take into account the fraction of time your climate zone/house spends in at a particular outdoor air temp. I would use something like weatherspark.com to lookup a min, median, max outdoor air temps in the summer in your city, and try to look at the stated performance of the two units at those temperatures, this could give you a sense of relative performance improvement.
It’s not so much the amount of insulation you have but rather which climate zone you are in that primarily determines the benefit of upgrading to a higher SEER unit. More insulation generally means you can use a smaller unit regardless of if it’s SEER 13 or 17. Climate zone gives an idea of how much runtime the unit is likely to have. You then can make a cost/benefit decision about the upgrade to see if the energy savings over the life of the unit is worth the increase in cost.
My guess is that in your climate zone it might not be worth the upgrade, but it’s hard to say for sure without also knowing your electric rates and unit tonnage.
Bill
Go with the unit with the lowest Sensible Heat Ratio (SHR). Too often the way that manufacturers get to higher SEER is at the expense of latent load management.
Peter
If this were Georgia rather than Rhode Island that would be perfect advice.
It's humid, but not THAT humid in RI, the latent loads in New England aren't that hard to control in homes that have a significant sensible load (which would probably be the case in a century old antique, even after upgrades.) In RI simply right-sizing it for the sensible load would have adequate latent control for just about any equipment out there that tests under SEER 20. (Most of those SEER 30+ mini-splits would have to be periodically switched to "DRY" or "DEHUMIDIFY" mode during the sticky weeks though.)
Of course a crummy duct design or implementation can easily defeat even the most-optimized SHR.
Thanks Dana -
But when it does get humid in RI, isn't addressing that load key? If I had to select equipment to meet "peak" load conditions, wouldn't I do that to meet both sensible and latent loads, and not favor one or the other?
Peter
The sensible heat ratio of any SEER 13-17 equipment is adequate for RI levels of humidity as long as it isn't ridiculously oversized the way 7 out of 10 air conditioners are in that area seem to be. At 2-3x oversizing the SHR hardly matters since the duty cycle is so low. Unless the sensible load is EXACTLY at the max capacity of the equipment at ~85F (typical RI 1% outside design temp) the next quarter to half-ton step larger is more than enough to manage the latent load. Even during the most tropical week in RI the outdoor dew points are below 65F about half the time or more:
https://weatherspark.com/m/26142/8/Average-Weather-in-August-in-Cranston-Rhode-Island-United-States#Sections-Humidity
https://weatherspark.com/m/26159/8/Average-Weather-in-August-in-Newport-Rhode-Island-United-States#Sections-Humidity
Unless for health reasons you absolutely need to keep the indoor RH 45% or less, the SHR of 13-17 SEER equipment just isn't going to matter in RI.
Compare that to coastal Georgia, where outdoor dew points are above 70F more than 80% of the time in the mid-summer months:
https://weatherspark.com/m/17828/8/Average-Weather-in-August-in-Savannah-Georgia-United-States#Sections-Humidity
THAT is the sort of latent load where SHR specs on 1-2 speed central air matters.
In RI just keeping the run times reasonably long is enough for anything under SEER 20. But it doesn't work with most of the SEER 25-33 modulating mini-splits- they usually have a very unfavorable SHR, and won't purge enough humidity during the muggiest of days despite nearly continuous run times. But that's what DRY/DEHUMIDIFY modes are for.