Roi and heat pump options
I had had three options offered to me by an HVAC sub:
(17.2 seer2 / 8.6 hspf2) ASZS6030, AHVE36CP1400, HKSC10, ATST-CWE
At our average electricity costs, using a calculator I found online, it looks like the payoff for the marginal efficiency improvements would be about a 75 years for the highest option and 22 for the slightly improved option. I guess that means the base is the practical option, financially.
As someone whose personal home has humidity problems (even after upgrading to a 2 stage heat pump, which I was told would help with that), I also like that more energy is devoted to dealing with latent heat on the base model (7050btu at 95F) vs the most expensive (6400btu at 95F), despite there being a lot of marketing goblety gook about it having special dehumidifier modes. At around that premium ($4600), my sub is telling me he could set me up with an ultra aire dehumidifier, which I know will do the best job.
I am thinking the best setup for the money within the options he is showing me will be the base model plus dehumidifier. Am I missing anything?
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One thing to consider is that certain equipment combinations of the ASZS6030 qualify for federal tax credits, whereas the other two models do not. Additionally, the ASZ6030 is inverter driven and has a communicating thermostat, so it's also not an apples-to-apples comparison. Daikin (Amana) does have proprietary dry mode designs, with a split coil that allows for less (but not zero) sensible cooling in dry-only rather than the dry-cooling setting. You may or may not find these differences worth the upcharge.
It also sounds like the sub is pushing you towards the base model + dehumidifier combination. If your sub has not installed many communicating systems, you may be better served by choosing what they want to install. For what it's worth, I had a Daikin Fit installed in May and have been delighted with it, by a contractor I have used for years and have been impressed by. However, I did have to call them back to correct a handful of things I spec'd in the contract that were not done. These were communication problems, not technical problems.
If this is new construction, consider using a ventilating dehumidifier as the ventilation system, and choose among the heat pumps on their own merits.
I wish the manufacturers wouldn't quote latent heat removal numbers, because it just confuses things.
No, one system doesn't have "more energy devoted to dealing with latent heat." What that means is that under the test conditions, it removed more latent heat. But the test conditions basically have no predictive value for how the system is going to perform in your house.
The amount of dehumidification that happens is determined by one thing, the coil temperature. The colder the coil, the more humidity is removed, the greater the ratio of latent heat removal to sensible heat. (Although it does plateau around 35F). To lower the coil temperature you just need to have less airflow relative to the refrigerant flow, you can do that either by lowering the air flow or increasing the refrigerant flow.
All other things being equal, a lower coil temperature will reduce the COP, because the performance of a heat pump is dependent upon the temperature difference. Manufacturers who are shooting for impressive efficiency numbers will let the coil temperature creep up.
The "dehumidification" mode in most heat pumps just means slowing the air flow down to reduce the coil temperature.
I have seen that discussed and having it available on the thermostat would be helpful because I paid my HVAC tech for a house call for him to tell me they wouldn't feel comfortable reducing the cfm and that it was as well set up as could be expected. I am just going to have to look at improving air sealing, run a dehumidifier, and see if I find a bit more aggressive / knowledgeable tech.
Yeah, the risk is if you lower the coil temperature too much you get ice buildup which can cause quite a bit of damage.
I created this spreadsheet where you can explore the impact of different coil temperatures:
https://docs.google.com/spreadsheets/d/1LONsi_1Fb6u4PsDQjzrbHn7ToX7yQHHEvr5OxExVwQc/edit?usp=sharing
What you want to look at is the Sensible Heat Ratio, SHR. The lower it is the more dehumidification. What you see is that it hits a minimum at around 35F, it actually gets higher with colder temperatures. At 40F it's not appreciably different from 35F; 40F is going to be pretty safe from freezing.
So let's say your room air is at 75F and 60% RH, typical conditions where you might want humidity removal. The minimum SHR is at 37F where it hits 56.4%. At 45F it's not much different, 57.8%. At 60F and above it's 100%. At temperatures below 37F it also rises.
So once you get down to 45F or so it's not worth going any colder. If the SHR that you get at that temperature isn't low enough to deliver comfort, no amount of adjustment is going to change that. You either need to provide supplemental dehumidification or add supplemental heat.