Adjusting Manual J/Manual S when using a whole house dehumidifier?
We have a combination of a whole-house dehumidifier and a Mitsubishi mini-split system installed. Austin TX (hot-humid). The dehu (UltraAire 98H) is wonderful, amazing how well it works even in our quite leaky house, I have a Kill-a-watt plugged in and it is not costing much to run at all. The promise that “you’ll set the temperature much higher on your HVAC than you used to” is definitely realized. Also the case that my wife and I are now much closer in the temp range we find comfortable than before (ie differences in temp perception are decreased when humidity is in the right range). We now seem to be happy with the temp at 78° in cooling season (of course high summer might be a little different).
Anyway, what is the right approach to either modeling or equipment selection when a whole house dehu is in place. How ought one to adjust? Increase summer target temp? Ignore latent loads in sizing HVAC? It does blow hot (albeit dry) air, which the HVAC has too cool, so how ought that be modeled? Perhaps Manual S already deals with this (I’ve never read it).
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The dehumidifier is just converting the latent load to a sensible load- it's still the same number of BTUs, albeit, with a bit of a kicker added on for the power dissipated by the dehumidifier's motor.
I'd make no changes to AC sizing. When it's hot out, properly designed AC will provide all the dehumidification needed. When it's mild out, the AC will have more than enough sensible capacity to offset the dehumidifier heat.
Be careful about lowering humidity further than the minimum needed for comfort. It's not a wash power wise and you can spend more money dehumidifying than you get back in reduced AC use.
Thanks, that makes complete sense (dehu convers latent to sensible, plus a bit :) and indeed we did HVAC size by total load, so that's great.
Yes, Jon R, that makes sense. Our control for the dehu is expressed in RH when I think it would be better expressed (for comfort) in dewpoint, especially across a large temperature range.
Whole thing reminds me that really we need visualizations of sensible and latent loads over the whole year, to see the 'shoulder' periods in which the latent loads are large compared to the sensible loads. And some way to vizualize the impact of over-cooling to manage latent loads. Can't say how wonderful it is, in these shoulder/rainy seasons, to be comfortable without feeling that you've frozen everyone else in the house :)
But do I also take from this that the dehu isn't mentioned in Manual J/Manual S?
>"But do I also take from this that the dehu isn't mentioned in Manual J/Manual S?"
The latent loads are counted, so the dehumidifier is thus counted even though it's converting part or all of the latent load to sensible load. The Sensible Heat Ratio of the AC begins to not matter so much at that point too.
In general, the whole house dehumidifier should not be taken into account in the ACCA manual J/S. I would suggest the dehumidifier’s airflow is accounted for in the D to ensure operational static pressures especially when using a ducted low-static mini-split. The AC if sized properly should be able to cover the sensible & latent load during peak/near peak cooling conditions. Therefore, when the dehumidifier is being used (more at part-load conditions) there will be extra AC capacity to deal with the heat added by the dehumidifier.
As for heat added by a dehumidifier… I would not think 1 to 1 and then some. For every pint of water removed approximately 1kbtuh of sensible heat is added and then ~3.5 btuh for every watt of energy. This for most dehumidifiers is 1 part latent removed for every 2 parts sensible added (1:2). In passive houses I have run into multiple designs in which the peak load on the house is less than a 0.65SHR and thus vapor compression without reheat cannot adequately cool the space. In these instances, in which a dehumidifier is to be used to supplement the AC during peak conditions, it is a bit of a balancing equation to account for the dehu heat with extra AC capacity. In these applications it is important to know the latent to sensible heat added is closer to 1:2 and must be taken into account during the design.