Dehumidifier with outdoor air

xbohdpukc · Aug 13, 2021

I saw quite a few posts, especially from @teddy bear, that insist on the benefits of having a WHD with outdoor air intake. Okay, so I am looking to add a dehumidifier to the system after this weird spring here in Houston (well, not to mention winter), where we had temperatures in 70s-80s through May and AC pretty much didn't run, which also made it insde the house not so comfy - the RH went as high as 78 before I got a portable DH.

Now I am looking to install a SantaFe Ultra 98, however I am wondering if the next one (120 I believe) is required since it has this outdoor air intake @teedy bear keeps talking about. The question is though, does it make sense in Houston at all? Why would I wnat to bring outdoor air in if it's too hot 80% of the time and too cold the rest of the time?

pecmsg · Aug 13, 2021

That air is getting in naturally so why not bring it in and condition it. That keeps the house slightly positive pressure and prevents unconditioned air from entering. It also take some of the load OFF the ac

xbohdpukc · Aug 13, 2021

pecmsg said:
That air is getting in naturally so why not bring it in and condition it. That keeps the house slightly positive pressure and prevents unconditioned air from entering. It also take some of the load OFF the ac

Well, yes, and what I don't understand is this...

Summer (which is Mar-Nov) in Houston - 90+, RH90+ as well, AC is working.
So if I turn on the outside air through the dehumidifier it will take the water out of it but it will be super hot - why would I want to blow hot air into my house while AC is trying to cool it?
I am assuming that WHD is installed return to supply, not return to return.

pecmsg · Aug 13, 2021

Why would it be “Super Hot”?
1 maybe 2* warmer but also drier.

xbohdpukc · Aug 13, 2021

pecmsg said:
Why would it be “Super Hot”?
1 maybe 2* warmer but also drier.

I guess I don't completely understand how WHD works then. Let's say my AC is at 78. Not sure if WHD is even talking to the AC to get the temp. So it will cool the outside air to XXX degrees and then warm it to YYY degrees and YYY is going to be +2 warmer than 78? Is that how it works?

Also on a different note, one of the contractors is adamant that they only install it return to return, period, that's the best thing since sliced bread.
Another contractor argues that it does not have to be hung but should be installed on the platform, because there should be a drip pan under it, while the 1st contractor argues that installing it on the platform (is that actually the right word?) will make it louder since it vibrates.

Any opinion on the opinions of these contractors will be much appreciated!

pecmsg · Aug 13, 2021

Our resident Dehumidifier GURU @teddybear will be along shortly.

teddy bear · Aug 13, 2021

xbohdpukc said:
Well, yes, and what I don't understand is this...

Summer (which is Mar-Nov) in Houston - 90+, RH90+ as well, AC is working.
So if I turn on the outside air through the dehumidifier it will take the water out of it but it will be super hot - why would I want to blow hot air into my house while AC is trying to cool it?
I am assuming that WHD is installed return to supply, not return to return.

A whole house dehumidifier is activated by a %RH humidistat. If the a/c is operating enough to maintain your %RH setting, the dehumidifier will not dehumidify the fresh air stream.

Fresh air change is needed 3-5 hours when the home occupied to purge indoor pollutants and renew oxygen. During the mild seasons with calm winds, natural fresh air infiltration may be as low as an air change in +12 hours. You can use a CO2 controller to activate fresh air ventilation which only activate fresh air when the CO2 is above 600-800 ppm CO2 indicating occupancy and minimal fresh air change. The quantities of fresh air is 100 cfm which is mimimal.
I prefer the the Ultra-Air dehus which have duct connections and fan power to move air through ducts. During windy cold weather weather you will have enough from the stack effect and wind to not need mechanical fresh air.

Keep us posted on the issues.

Regards Teddy Bear

pecmsg · Aug 13, 2021

Thank You o wise one!

xbohdpukc · Aug 13, 2021

Ok so I found a contractor who sounds reasonable and knowledgeable enough and ready to install it at an acceptable price point.

Here are a few questions,
1. He says that UltraAire 120 will be an overkill for my house of 2300 and suggest UA98, however I can't see if UA98 is a ventilating WHD - Sylvane calls 120 out specifically for its ventilation and doesn't do similar thing for 98, however 98 seems to have 2 inlets. Any idea if UA98 is a ventilating WHD as well?

2. This guy is also saying that the best way to install WHD is dedicated return to AC return, and he also says that while he can (need to get approval) to install it return to supply, he does not recommend it based on hundreds of installations they did and number of probles those with the R-S installs had. Any thoughts on this one? Should I just tell him to do R-S because "that's the right thing to do," or do I even care that much?

Thank you!

teddy bear · Aug 14, 2021

xbohdpukc said:
Ok so I found a contractor who sounds reasonable and knowledgeable enough and ready to install it at an acceptable price point.

Here are a few questions,
1. He says that UltraAire 120 will be an overkill for my house of 2300 and suggest UA98, however I can't see if UA98 is a ventilating WHD - Sylvane calls 120 out specifically for its ventilation and doesn't do similar thing for 98, however 98 seems to have 2 inlets. Any idea if UA98 is a ventilating WHD as well?

2. This guy is also saying that the best way to install WHD is dedicated return to AC return, and he also says that while he can (need to get approval) to install it return to supply, he does not recommend it based on hundreds of installations they did and number of probles those with the R-S installs had. Any thoughts on this one? Should I just tell him to do R-S because "that's the right thing to do," or do I even care that much?

Thank you!

Thanks for gettng back to the sizing issue. Without a crawlspace or basement, A 2,300 sq.f.t home with an a/c that will maintain 75^F, 50%RH during high sensible cooling loads, may even be ok with an Ultra-Aire 70H. The outside fresh air does not require a separate connection for fresh air. Use a 6" fresh air inlet duct with an adjustable damper teed to the 10"dehumidifier return duct that is best connected to the open part of the home. If the home is not occupied 24/7, an electric damper should also be in the fresh air duct to open and close the damper when occupied. This allows the Ultra-Aire 70H provide fresh air and/or dehumidify without the a/c blower running. Like I said, this ideal.

Sorry about the confusion about sizing. Is there any reason that you will need more capacity 2-3 lbs. per hour of supplemental dehumidification. Occasional 55%RH is acceptable. Your a/c will remove more moisture as the dehumidifier adds some heat to the home while supplementing the a/c with humidity removal.

Regards Teddy Bear

xbohdpukc · Aug 14, 2021

teddy bear said:
Thanks for gettng back to the sizing issue. Without a crawlspace or basement, A 2,300 sq.f.t home with an a/c that will maintain 75^F, 50%RH during high sensible cooling loads, may even be ok with an Ultra-Aire 70H. The outside fresh air does not require a separate connection for fresh air. Use a 6" fresh air inlet duct with an adjustable damper teed to the 10"dehumidifier return duct that is best connected to the open part of the home. If the home is not occupied 24/7, an electric damper should also be in the fresh air duct to open and close the damper when occupied. This allows the Ultra-Aire 70H provide fresh air and/or dehumidify without the a/c blower running. Like I said, this ideal.

Sorry about the confusion about sizing. Is there any reason that you will need more capacity 2-3 lbs. per hour of supplemental dehumidification. Occasional 55%RH is acceptable. Your a/c will remove more moisture as the dehumidifier adds some heat to the home while supplementing the a/c with humidity removal.

Regards Teddy Bear

Thanks for that. I am pretty sure I'll give him to read this, otherwise something will get lost in my translating to my layman and then to whatever language he speaks.

Any thoughts on the 2nd point of ducting it return to supply or return to return? Thanks again, this is very helpful.

teddy bear · Aug 14, 2021

xbohdpukc said:
Thanks for that. I am pretty sure I'll give him to read this, otherwise something will get lost in my translating to my layman and then to whatever language he speaks.

Any thoughts on the 2nd point of ducting it return to supply or return to return? Thanks again, this is very helpful.

This requires running the a/c blower while dehumidifying or venting, A/c blowers us as much or electricity as a dehumidifier. Never blow dry, warm air from a dehumidifir into an a/c return. The a/c will remove less moisture when the dehu is operating while the a/c is cooling. The a/c removes less moisture during dehu run with a/c.

Best is to connect dehu return to open part of the home and dehu supply to the a/c supply when ever possible. VS a/c blowers 'ON' mode slow speed offer some other possibilities.

Keep us posted on info.

Regards Teddy Bear

xbohdpukc · Aug 29, 2021

Okay, so the deed is done... Almost did not catch them connecting return-to-return, they started working in the attic while I was busy and I climbed up there to a gaping hole in the AC return... Well, all good now - it's return to return with fresh air vent.

How do I even start thinking about scheduling the ventilation programming. That's not the technical question as in how to program the DEH3000, but what does the right schedule look like? I'm in Houston, TX.

Oh, on a side note... The DEH3000 and the probe are installed on a wall a foot away from the Ecobee. The former shows 40% RH, Ecobee anywhere from 48 to 55... What gives?

mgenius33 · Aug 29, 2021

xbohdpukc said:
Also on a different note, one of the contractors is adamant that they only install it return to return, period, that's the best thing since sliced bread.
Another contractor argues that it does not have to be hung but should be installed on the platform, because there should be a drip pan under it, while the 1st contractor argues that installing it on the platform (is that actually the right word?) will make it louder since it vibrates.

Any opinion on the opinions of these contractors will be much appreciated!

Like TeddyBear suggested I find the best is a “separate return to supply”, or “return to supply”. “Return to return” can be done but the evaporator is saturated with a few lbs of water sitting on it. You’ll basically be adding moisture to the dried air that your dehum supplies.
If I’m tying into the AC systems return I’ll install a backdraft damper to prevent backflow recirculation when the AC fan is not running. You can opt to run the AC fan with the dehum but need to ensure static pressure is not too high. I just don’t like running the AC fan on the AC off cycle because, again your evaporating the water left on the coil.
As far as mounting, I find suspending is a bit quieter, but foam isolation pads can be used if mounting on a solid base.

teddy bear · Aug 29, 2021

xbohdpukc said:
Okay, so the deed is done... Almost did not catch them connecting return-to-return, they started working in the attic while I was busy and I climbed up there to a gaping hole in the AC return... Well, all good now - it's return to return with fresh air vent.

Does this mean the dehumidifier is sucking air from the a/c return and fresh air with the dry air going into the a/c supply?

How do I even start thinking about scheduling the ventilation programming. That's not the technical question as in how to program the DEH3000, but what does the right schedule look like? I'm in Houston, TX.

Fresh air when occupied would during the mild seasons of the year. This could be all the time in Houston.

Oh, on a side note... The DEH3000 and the probe are installed on a wall a foot away from the Ecobee. The former shows 40% RH, Ecobee anywhere from 48 to 55... What gives?

To check the real %RH in the space, use a metal vessel filled with water and slowly cooled to the point that the exterior of the vessel fogs while measuring the water temperature with an accurate thermometer. An example is in a 75^F space temperature, a metal vessel fogs (sweats) when the water is cooled to 55^F. This 50%RH.

It is common to have problem with %RH meters. Mark the meters with a correction factor. Some meters can be re-calibrated.

Comment about installation. Ideal to have a separate return from the open part of the home to the dehumidifier return. Connect the fresh air to the dehumidifier return via tee to the dehumidifier from the open part of the home. The supply of the dehumidifier connects to the a/c supply duct down stream of the a/c coil. This avoids blowing the warm dry air from the dehumidifier into the a/c cooling coil if dehumidifier is called for while the a/c is cooling.

You can us back draft dampers, but the a/c cool will dry to the home in +hour without any fan. 30 mins. with a/c fan on or dehumidifier. I would not damper the a/c coil. Drying the coil and pan reduces odor from bacteria/mold growth.

Tell us about the duct connects and the actual dew point/temperature in the home.

Thanks for the support and interest in controlling the %RH in your home.

Regards Teddy Bear

Dehumidifier with outdoor air

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