humidity issue in 5 story building (one family house)

[fineartsnyc]

NYC climate. 95 DB and 74 WB Outdoor Temp.
we have a Mitsubishi Citi multi R-2 Series (heat recovery system) with BC controller, sub controller and (Lossnay) ERV system on the upper floor
3 fire places with FA.
Fresh air is introduced to the space thru the ERV system (as designed). I was told by the MC that the ERV is off.
One exhaust fan for 6 toilets on the roof.
all air handlers are floor standing units.
CENTRALIZED CONTROLLER AG-150
MA Remote Controller (PAR-21MAA) for each air handler.
Control 4 as a third party

8:00 am (12 hours after the set temperature changed from daytime 71 degrees up to night time 77 degrees and 1 hour before set to change back to daytime as we said before per the preset programming via Control 4.)

toilets exhaust fan has been turned off/ ERV is off

We took Relative Humidity Readings this morning and the following are our findings.
Readings at the follow locations:
Cellar front 76% back 68%
Basement front 70% back 68%
1st front 69% back 69%
2nd front 68% back 69%
3rd front 67% back 69%
4th front 69% back 69%
5th front 70% back 70%
Stair Bulkhead 72%
Stair hall 70% (consistently on most all floors)

Base point of the outside is:
Outside at front yard 76%
Last time we were taking readings, even with the toilet exhaust fan running, we measured 41% at 11 am after the systems scheduled changes.
Please advise because these seem like the changes in humidity could have ill effects on the many materials inside of the house.

 

[EnergyProfessional]

If you live in humid areas, as you do, you NEED a Dedicated Outdoor Air System (DOAS) that includes ERV and a cooling coil to dehumidify. this also applies to water source heatpumps as well:
[URL unfurl="true"]http://www.innovativeenergyengineering.com/Pages/HVAC/Refrigeration.html[/url]
i would recommend a digital scroll compressor for a small system, unless you have chilled water. You don't want a staging DX system since the DAT varies too much, de-humidfying less. Turning on the ERV will help a bit since it pre-conditions the air and subtracts some humidity. But you still want to cool.

But i see you have residential application, the above applies more to commercial with constant ventilation.

Since this is residential, I assume they don't ventilate constantly? is that exhaust fan running constantly? Then this might be the culprit. Whenever it exhausts, the house "draws" unconditioned outside air in. Maybe you should exhaust through the ERV and also only run exhaust based on bathroom light (since it is residential). Need some low-voltages or some other wiring so that when either bathroom is occupied the ERV (=exhaust) runs. Obviously the ERV needs to be sized for 6 times 50 cfm or whatever your code requires for exhaust.

 

[fineartsnyc]

Thank you. This is a split DX coil (Variable Refrigerant Flow). The toilets exhaust fan is powered from light switch. the ERV is connected to toilet exhaust fan & fresh air to air handlers.
The ERV designed to run on Heat Exchange or Bypass mode. the humidity didn't change much after the exhaust fan turned off.

 

[Drazen]

i made some very provisional calculations with some generic assumptions and would say that you are lucky for not having even higher relative humidity having no dehumidifier.

first measure to rectify your issue is to raise your indoor temperature setting. 71 degrees is overly too low and unhealthy anyhow.

your daily setting should be 77-78 degrees. i. e. you can use day-round temperature.

that should lower rh by 10% (very approximately) and save your health as well.

the issue with many moders systems is that they are offering too much buttons to you without giving you idea what stands behind it.

 

[EnergyProfessional]

you said you have 6 bathrooms, are they on the same exhaust/ERV? what are flowrates? and are you in bypass now?

Is the house tight or leaky? Obviously the humidity comes from untreated outside air. The questions where does that come into the house and how much?

Not sure if your VRF units have a humidity function. In that they reduce fan speed and cool the air more to dehumidify if the RH is above a certain level. You should check that.

 

[fineartsnyc]

Thanks Herrkaleun, yes all bathrooms on the same exhaust fan 700 cfm exhaust and 300 cfm fresh air to air handler.
the ERV is not working yet. I believe its in the bypass mode.
its a new construction and well insulated.
the problem occur when the thermostat set for 71 F.
Could it be a control issue? that the coil is not removing the latent heat?

 

[EnergyProfessional]

what do you mean by air handler? And do you also have a 700 cfm ERV? Maybe a schematic would help. I think i would just use an ERV to exhaust air from the bathrooms and supply fresh air to the living/bedrooms room. but with 6 bathrooms and 700 cfm I don't see how you don't use a separate cooling coil to dehumidify.

700 cfm seems a lot for 6 residential bathrooms, especially since you bring in 700 cfm whenever just one bathroom is used. not sure if it has a timer, or how often someone uses the bathroom. but add up the daily run-time hours and you know how many air changes you get.

the 400 cfm (700-300) will be drawn in uncontrolled through all the house cracks. so you basically get 700 cfm unconditioned air. This explains the humidity.

Did you or someone else design that? This is like a small commercial office building what you have there. I thought i read above it is residential, but not sure anymore. Who came up with 700 cfm?

 

[ChasBean1]

“I was told by MC that the ERV is off.”

“700 cfm exhaust and 300 cfm fresh air.”

Sometimes the little clues matter. You need to supply makeup air that exceeds the exhaust volume (by 5% or more) and dehumidify whenever outdoor air dew point is above 58°F or so…

 

[fineartsnyc]

Thanks for your answer.
The fresh air from the ERV is connected to the return duct to each air handler. 5 floors + basement+ cellar. it's 1 family house 6 ppls (residential building). the exhaust fan works with light its not constantly. I believe its a control issue or when the ERV will work. the condition with gets better.

 

[tys90]

71F dry bulb at 70% relative humidity has about the same moisture content as 75F and 60% RH, which would be considered on the edge of "comfortable" in most settings.

The ERV will definiately help. As HerrKaLeun mentioned, the VRF fan coils I've seen do have a "dehumidification" setting usually. It just slows down the fan speed to lower the discharge air temperature. Many of the VRF fan coil units have a low sensible heat ratio too so between the ERV and that, it should help. Exhausting all the restrooms when only one is in use isn't a good design in this case though.

Usually setting a space to 71F in a residential situation will cause the cooling to run long enough to dehumidify properly but if the equipment is too oversized it could be a cycling issue. You could get small inline duct dehumidifiers if you want total control over your humidity, Honeywell makes a good one called the TrueDry.

 

[MintJulep]

You have a building with a percieved humidity problem, so you go in and measure humidity.

I guess that's a nice sounding thing to tell the owner.

But it's completely useless because it tells you NOTHING about the elements of the system that can affect the interior humidity.

What is the cooling system doing?

What is the ventilation system doing?

What is the outdoor temperature and humidity?

What are the building loads?

Get a bunch of data loggers, instrument the system, look at what the system is doing over time.