I have a friend/client with dry conditions in his condo. He's on the top floor of a 17-story building.  He's always had dry conditions, but after a major renovation made necessary by water damage (ironic, I know) from a busted fire sprinkler pipe, he had a high-end steam humidifier added to his HVAC system.  The installer says it's putting out 20 gallons of water per day into this condo unit that's about 21,000 ft3 total.  But the humidity conditions in this unit don't seem to have changed with the addition of this humidifier.  It's still too dry, and that's a big problem.

I did a blower door test at the outside balcony door.  I got a ACH50 of about 9.5, which doesn't seem that crazy high to me.  But of course how I'd convert that into natural air changes is beyond me.

I'm not 100% sure that this humidifier is working properly, but that's another line of inquiry I suppose.

I've got some questions, and any input would be appreciated.  Is there a reasonable way to measure or estimate the natural air changes for a 17th floor unit?

How common are dry conditions in a high-rise?  What do most people do about it?

If this humidifier is working properly and pumping out 20 gallons a day, but all that moisture is being lost, would you expect that I could measure a humidity difference in the unit between the side with all the HVAC supply (where the humidifier is dumping its moisture) and the side with the returns?  I know that the vapor pressure should equalize fairly quickly, but with the blower fan running and humidity being dumped into this condo should I be able to measure a humidity difference in different areas?  

The HVAC contractor is thinking that there's just too much air leakage, and he's thinking that the best solution is to put in another humidifier (at a cost of more than $2,000) to overwhelm the air leakage.  I'm not convinced that air leakage is the problem, but I'm unsure of what my next test or line of inquiry should be.

Any input is appreciated.  Thanks

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Real short - too much air leakage with outside conditions being very dry = low humidity inside

Add on to this the stack effect (plus the wind up here) & you can throw out those NA change numbers that most normally use

Only time you would notice a humidity increase is by the register when the heat is on. As for how long it sticks around would be a WAG

One other item is how well sealed are those ducts & where are they located - you could be losing a ton of moisture into spaces you don't want it 

@Steve, I concur with Sean's comments. Your client is just throwing money out the window trying to humidify a space with leakage that high. I recall a client who had installed four steam humidifiers, yet experienced high 20's to low 30's RH% during cold weather. His mechanical contractor was totally baffled. I ordered a blower door test, which came out to 12 ACH50. I had to explain to the guy what that meant, and why installing additional humidification equipment would be a further waste of resources.

Sean wrote: "how well sealed are those ducts & where are they located - you could be losing a ton of moisture into spaces you don't want it"

... and/or drawing in more dry (outside) air through return-side leaks to the extent they communicate with infiltration paths.

BTW, the Sherman & Grimsrud formula (from ASHRAE Fundamentals, http://tinyurl.com/2587o9a) is the gold standard for converting from blower door pressure to 'natural' pressure. Most modeling software implements this formula in lieu of the crude n-factor maps, which have long since been depreciated. But Sherman-Grimsrud only accounts for stack effect in buildings up to three stories.

Thanks for the replies.  David, do you think I should be able to see any humidity difference in different areas of this condo?  It's a steam system that's running constantly, with the blower fan running. Packaged RTU, on the roof (where else, right?)  Maybe I'm obsessing too much on this aspect of the problem, but it seems to me that if this humidifier is working properly then I ought to be able to see some indication of that, somewhere.

During the blower door test I didn't note a significant amount of duct leakage to the outside, but I don't know how that's affected by all the ventilation going on in the building's common areas.

Yes, you should be able to measure differences in RH, especially at the supply, as Sean said. But keep in mind that RH is not the same as absolute humidity, as I'm sure you know, so you must normalize for differences in temperature between your measurement locations by converting RH to dew point. Here's a handy online calculator for that: http://dpcalc.org

That said, I think trying to find the more humid spots is a waste of time. The impact of the humidifier throughout the apartment at best will be highly unpredictable, depending on localized infiltration paths. And it may not just be air leakage that's pulling down the humidity. If there's not a proper vapor barrier, vapor drive during cold weather will push additional moisture out through any and all permeable envelope components.

Do a duct test. Total Duct Leakage! Measure RH at return and supply. Verify humidifier is on. If that humidifier is working you could be pumping humid hot air into building cavities.  A recipe for mold and mildew

Simply put, high humidity in a home occurs when moisture comes into a home and cannot get out. The main way that your home’s humidity level is controlled is through your air conditioning system.

Apart from your AC not properly removing humidity from the air circulating inside your home, here are three of the most common ways excess moisture gets into your house and causes humidity:

Everyday activities. You may be surprised to learn that many of your daily activities may be introducing excess moisture into your house. Everyday actions like cooking, washing the dishes, running the washing machine, taking a shower, sweating and even breathing can cause moisture to build in your home—especially if you’ve got a big family.

Leaks. Moisture could also be entering your house through leaks and cracks in or around your home. To find the culprit for these types of issues, look around for leaky pipes, a cracked roof tile or a loose zinc joint or fitting.

Rising damp. Rising damp is a rarer, but more serious cause of high humidity in a home. Rising damp occurs when moisture from the ground rises through the pores in the bricks and masonry. Keep an eye out for symptoms of rising damp, such as mold, rotting window frames, flaky plaster or damp or wet patches on your walls.

If the unit has a RTU,  is it by any chance bring in fresh air from outside that has not been humidified?  In many of those high rise designs - they may be some kind of ERV that is supposed to capture and transfer some of the the humidity from the units... perhaps there is a failure of systems on the roof?

I use low cost dataloggers (Logtags see THEMOWORKS.COM) that capture both Temp and RH.  You might see if you can log the temp & RH coming out of the ducts from the RTU with the humidifier turned off... and measure some of the more distant rooms and even the outside air on the balcony.   

The building operator would need to be presented with the conditions in the unit... if the problem is on the roof  -- the unit owner/renter isn't going to be able to change stuff upstairs.

Also if you have thermal camera... and you do blower door test... did you see air leaks elsewhere?  Sometimes the interface between the unit and the hallway is loaded up with leaks where the pipes go through the bulkhead.   There may be leaks from outside into the hallway space above the ceiling... perhaps that's resulted in sprinkler damage.....

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