The new Honeywell Vision Pro M# TH8321R1001 set up dehumidification to a/c with high fan then next step set to reheat. With this setup when humidity is high it will run the a/c to dehumidify and also run heat at the same time to temper the air. Keep in mind if it's cold and rainy outside you'll probably have to add a head pressure control to the a/c condenser such as an ICM 325H. This is one of the most effective ways to dehumidify, but is an energy hog. there are 4-5 other ways to dehumidify with this thermostat

 

You can use the Vision Pro or Prestige IAQ, but it does have drawbacks.

The Thermostat will need to be set into the commercial mode for the reheat option (ISU 908) to be available.

According to the product data manual:

"Reheat (ISU 908): This option allows heating to run during
dehumidification to help maintain a comfortable temperature. If
only cooling stage 1 is used, during the “off” cycle, both cooling
and heating run at the same time as needed to dehumidify
without overcooling. This option cannot be used in the Heat
mode. This option is effective only if using a system with the ACoil
located before the heating coil (heat exchanger). This
feature requires a conventional forced air heating system (gas,
oil, or electric) in the application."

So the customer would not be allowed to be in heating mode in order for this option to function.

According to documentation, both do not require an EIM or the optional supply/ discharge sensors. However, if he plans on using the cooling during the winter, uses the overcool limit (ISU 907), and the low speed fan option (ISU 900), it would be a good idea to install the supply and return sensors. With the sensors installed, you can then activate option (ISU 511) to monitor evap coil temperature and set a temperature cut out limit to help prevent a freezeup.

Here's a simple reheat put together using a modulating valve on a ducted hot water coil and humidifier control on a Mitsubishi split.

 

Describe wet and cold weather. Also what is the indoor humidity level in your home? There may be other causes of the high indoor humidity.
Regards TB

 

What about install a contactor to engage an outlet on dehumidification and then just buy and install an off the shelf dehumidifier?

 

Why not just use a whole house dehumidifier?
Like one of these. http://www.aprilaire.com/?znfAction=ProductsCat&category=dehumid

 

I am not looking into adding any new equipment, except for some type of control for heat mode.
It's a long story
This house is only 2 years old. Had a huge mold problem in the basement.
There was no dehumification system in the basement. There is a first floor HVAC system that is ducted only for the first floor.
Another contractor, last year, installed a HRV unit and did not connect intake air into the HVAC unit that does the first floor above.
It was dumping intake air only in to the basement. Seems as though the HRV unit made things worse, not better.

I installed a new 1.5 ton Carrier AC system for the 1600 sqft basement area (very well insulated) that will dehumidify
during the humid summer months. Customer wants me to investigate dehumidification in heat mode.

 

This can be done if the heating coil is down stream of the cooling coil, but it will be an energy pig. Put a dehumidistat in the heating coil control circuit. When the %RH exceeds the setting, heat is activated. When the temp exceeds the a/c set point, cooling is activated. In a few minutes the %RH will drop because of the heating the air moisture collecting on the coil. The moisture will be collected on the a/c coil. After a couple lbs. of moisture is collected on the coil, moisture start to drip down the drain. Mission accomplished.
The problem is that a significant portion of the moisture collected on the coil will evaporate back into the basement instead of going down the drain. The process will start all over again. Reheat has code prohibited in most part of the county. It will work. The operating cost will be 2-3X higher compared to a small high efficiency dehumidifier like the Santa Fe Compact.
For keeping the space dry during low dew point weather (<50^F Dew Point), operate the hrv. The fresh air ventilation is needed whenever the space is occupied.
Keep us posted. It can be a learning experience.
Regards TB

 

Teddy Bear,
I currently have a regular heat/cool thermostat for the space. I installed a separate Johnson Control humidistat (common, NO and NC terminals) and connected to Dh end R terminal
which I found out only works in cooling mode. This might be a dumb question, but can I make this Johnson Humidistat work in heat mode or do I need another humidistat for the heat mode portion?
I

 

Infinity is a complex control. I suggest allowing the Inf to control the %RH by over cooling. By raising the temp with a separate dehumidistat add heat, the infin controller will operate to reduce moisture at a low fan speed.
Maybe there are other suggestions. I would get a good dehumidifier and forget about reheat. Reheat will be energy intensive.
REgards TB

 

The customer believes that a HVAC system should also dehumidify in heat mode especially
when it's rainy and cold outside and the relative humidity is high.
What's your opinion? Is their a control out there that can run the condenser and heat mode at the same time
to dehumidify?

Does the customer understand this concept? Normally, humidity is desirable in the winter. Absolute humidity remains the same on heating. It is reduced upon cooling if lost as condensate.

The RH at 68F is about 40% when you heat a closed chamber that starts off at 100% RH at 45F. The absolute humidity remains the same.

If indoor air is at 50% at 75F, it holds more moisture than air can hold at 45F, so when its cooled to 45F it will a steam. So, in this case, air exchange with outside will push out humidity.

What can be an issue is when its a high in humidity outside and its only a bit chilly. Right now, its 58F and 80% humidity in Los Angeles. When its raised to 70F, you're pushing around 60%. In this case, a portable dehumidifier maybe ideal. It is a heat pump that recovers latent heat from removed moisture and convert it to sensible heat. It will add overall heat in the amount of power consumed by the equipment.

When you want cooling, removal of humidity and rejection of heat outside is desirable, but when you the run A/C during heating season, its a net loss. Amount of latent heat waste depends on source of water vapor. If it's something within the house, the furnace is supplying the energy to evaporate the water, only to have it dumped back out side.

A portable dehumidifier sitting near air return and running does whats needed.

 

Keep the regular thermostat connected. Connect your Johnson control to turn on the A/C when the humidity gets high, and leave the regular thermostat bring on the Heat when the basement gets too cool from the A/C running to dehumidify.

 

But then, this wastes energy.

 

Thanks for all replies!
I understand the comments regarding that heat mode and humidity control (with outside condenser) is an energy hog.
Customer does understand the energy use involved. They are a lot more concerned about the mold that was just remediated.
They want to protect their investment more than anything right now.
The house is probably worth more than 2.5 million in a high end area of Massachusetts.

I hear the argument for a wjhole house humidifier. I will definitely look into one in the future.

Beenthere, I know you know controls. Right now, the Humidistat is wired, in cool mode, in between the R circuit and DH (FV4 Carrrier air handler, jumper taken off).
I would need to send power to the Y circuit when the W gets 24 volts, but only in heat mode and humidistat calling.
Seems like I would need some type of relay to accomplish this in heat mode or have another humidistat just for heat mode.

 

You just want it so the humidistat brings on the A/C only, and let the thermostat bring on the heat as needed. That way you don't over heat the house on the higher humidity days, with only a little heating load.

 

Still think the standalone dehumidifier is the best idea.

 

agreed, but that's not what he asked. He knows it's going to use alot of energy, but he or his customer is going to be paying the bills, not you

 

We take care of some art storage facilities and all units are setup with reheat along with modulating Nortecs. Energy conservation is not a concern but only that the temperature and humidity meet "their" stringent requirements.

Then again, it's too expensive for them to properly insulate the exposed brick walls and so panic ensues on a cold day when they begin to condense.

 

By the time you add in the low ambient control and redesign the system to dehumidify (i.e. move furnace to after evaporator coil and take measures to prevent condensation), it would be cheaper and easier to just install a standalone dehumidifier.

 

Its an air handler, not a gas or oil furnace. So the electric heat strips are already after the coil.

 

Dehumidistat wired in parallel to thermostat, set to desired humidity level, thermostat will call for whatever it wants and humidity will be controlled automatically.

 

Wire in a humidistat to turn on ac with head pressure controls txv and cc heater. Can't remove humidity without using energy.

 

Conclusion: if the customer has money to burn, sell him a whole house dehumidifier install. I'd rather see you get some $$ on that install than see the $$ getting used to harm public welfare by wasting resources and polluting the air for the hell of it.



When energy is free, its easy to just talk about it. I did some number crunching using engineering datasheet for a R410A Ruud 3 ton 60K 80 AFUE gas pack and an April Aire 150H dehumidifier.

Using the AprilAire whole house dehumidifier:
120v recept. to machine: 850W
2.2kW of heat from water vapor recovered
3kW returned to inside the house as sensible heat
It dumps out about 6.25pints of water per hour along with about 100W of loss.

If you run it for one hour:
It adds a total of 2500 BTU for every hour from the power used by the machine. It "filters out" about 7500 BTU of heat from moisture in air, then spits the water outside via condensate drain. In the winter, this is win-win.

Now... using the A/C system (Ruud gas pak just for example)
2.2kW electrical input
60,000 BTU/hr gas

In one hour: you consume 2.2kWh and 0.6 therms of gas.
Heat added to space: about 9,000 BTU

Buy 60,000BTU
Throw away 12,000 BTU out the flue
So, you get 48,000 BTU furnace output.
How about then paying the power company for 2.2kWh to run the heat pump to throw out most of the gas you just paid for?

For every hour you run it...
2.2kWh (about 7500 BTU) is bought from electric company
0.6 therms is bought from gas company (60,000 BTU)
It pees out 7.25 pints
You get earth shattering 8,700 BTU heating output using only 67,500 BTU (89% gas, 11% electric).

Since you get 8,700 BTU that's conducive to the purpose. So, the removal of water in itself uses 8,000 BTU per pint. You're probably in 10 to 15 cents per pint of water removed, mostly on the cost of gas.


Compare with whole house dehumidifier 0.8kW input and 6.25pint per hour removal. This is in the order of 400BTU/pt. If gas was half the cost of 'lectricity, then the financial impact would be about that of 200BTU/hr.

I'm no tree bugger, but what I described above is sufficient to say the re-heat setup is a reckless waste of resources. It's like using the engine to produce 8 pounds of reverse thrust along with 10 pounds of forward thrust to get 2 lbs of forward thrust.

 

FYI:
The FV4 fan coil has the ECM motor so it is able to slow down the fan to 80% of it's normal speed setting for dehumidification. This is accomplished with wiring in the DH and R circuit to a humidistat.

 

If the outdoor dew point is low enough, just switching to fresh air (e.g. by opening a window) might work well. In that case, you could call the efficiency penalty a necessary evil, but a HRV can reduce those losses.

 

Is the hrv still installed? It should be able to maintain humidity in the winter no problem.

 

Yes. still there and working.

 

It would probably end up more efficient to raise the heating setpoint (with maximum outdoor air) in order to drop the relative humidity under that condition than to do the "full throttle on the gas and ride the brake" approach. Calculate it and if the resulting setpoint is still reasonable, it's a good temporary solution.

 

But if the customer has money to burn, then why not get them a whole house dehumidifier?