Economizer and Humidity Control
Economizer and Humidity Control
(OP)
I am facing an excessive humidity issue for some packaged RTUs introducing OA and with economizer options for a commercial space. The trouble with this is, the amount of OA will then be governed by the eco. options. If the economizer control is dry-bulb type it would allow OA with all the outside RH into the space (Is that so?). I am assuming this operation leads to 100% OA at some point. Space humidity would then hit the roof? Right? If its an enthalpy type control it would have control over the humidity introduced to the space. With no reheat in either case.
I am not sure what controls are on the RTUs economizer now - but the space almost starts drenching. And this is in NJ. From what the installers tell me its an enthalpy control.
We are constantly monitoring indoor&outdoor humidity to figure out the problem. No success yet. Any ideas?
I am not sure what controls are on the RTUs economizer now - but the space almost starts drenching. And this is in NJ. From what the installers tell me its an enthalpy control.
We are constantly monitoring indoor&outdoor humidity to figure out the problem. No success yet. Any ideas?





RE: Economizer and Humidity Control
There are some things that could be wrong with the set-up that would make it run in economizer mode when it shouldn't. Check the location and calibration of the enthalpy sensors. Make sure you don't have any ponding issues underneath your outside air intake. Check the operation of the damper itself, is it closing and opening as it should?
As far as the problem you are describing, I would bet that during occupied mode the fan is in the on position, the outside air damper is at its minimum, and the economizer is not even in play. I have seen this many times when there isn't enough sensible load in the space, the outside air, or some combination of both to actually get the compressor(s) to run. So the unit sits with the fan on, dumps humid outside air into the space, and the compressor doesn't run because space temp is met. When the compressor does run it is only for a very brief time which doesn't get any of the moisture out of the space. This is very common when the unit is over-sized.
RE: Economizer and Humidity Control
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Economizer and Humidity Control
thanks for your insight. I am also podering this issue on my designs. Based on your explanation, we should always spec the comparative enthalpy option. The Lennox rep and the trane rep have both shared this with me, but I thought is was salesmanship, (I am in Dallas, TX ) and it gets really humid here. So I will spec the comparative enthalpy from now on. I am sure the contractors will complain about the cost. Is this option expensiv?
What did engineers do before the comarative method?
thanks-good weekend...
RE: Economizer and Humidity Control
RE: Economizer and Humidity Control
Once you've paid for an economizer, the comparative enthalpy is a relatively small increment in price. It's also much cheaper than dealing with the problems that led to your original post, assuming it is the economizer causing them. Keep in mind that getting it spec'd and the contractor supplying it is just the first part of the equation. It still needs to be installed and set-up properly.
I am not sure which code Dallas is under but the International Energy Code gives you some flexibility on the use of economizers. Check section 803.2.6
RE: Economizer and Humidity Control
There are a few options we are planning to put to test. I think the system has the simple enthalpy type control. If at all anything - dumping in const. vol OA will definitely work. Only thing is - the system loses its energy efficiency.
Thanks once again - Ncdesign
RE: Economizer and Humidity Control
RE: Economizer and Humidity Control
With that being said demand control ventilation is a very good idea because during normal times (not economizer mode) it can usually allow you to use less outside air. You just have to make sure your CO2 sensors are kept calibrated.
RE: Economizer and Humidity Control
If you had outdoor air with a dewpoint higher than 55F and less than 62.6F it would have an enthalpy less than or equal to the return air condition stated, yet would be pumping moisture into the space as this outdoor air has a higher dewpoint than the return air.
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Economizer and Humidity Control
AbbyNormal your post made me pull my pysch chart back out and take a look at economizer operation again. In fact I'm going to have to go back and talk to a couple of my manufacturer's reps about exactly how their economizers control on standard packaged equipment. Thank you and my apologies to liliput1.
RE: Economizer and Humidity Control
Take the "V" out of HVAC and you are left with a HAC(k) job.