Re-Heat

Jed_2

Thanks, Bill, that is helpful. The system I'm thinking of requires about 1200 CFM of ventilation air (light commercial building), with an independant duct system. Primary heat is Hi Velocity mini tube outlets, and 1150 CFM AHU. It is in a Cold Climate region. So, if I just make up the difference between design set point temp and the EFF% deficiency, all should be fine?

Jed

Jed_2

System Design Procedures

Bill, is there a good guideline for determining if re-heat is required for a given system, when HRV's are part of the equation? How does one determine the required re-heat needed?

TIA

Jed

Unknown

ER/HRv's

The typical ER/HR ventilator is about 80 % efficient. They usually run on low speed 24/7, moving about 100-200 cfm of air. The main system fan is not usually used. They can be manually or automatically set to high speed by a humidistat, VisionPro IAQ, or a manual control, usually to help clear the air during a party, for example. A reheat may be needed in a system where you have baseboard or radiator heating, and are using the air conditioning ductwork as a distribution system for the fresh air in the winter. Of course, the ductwork must be adequately insulated. Since the ER/HR unit is only 80% efficient, the air could be too cool for comfort on an extremely cold day. An hydronic or electric reheat is sometimes added to temper the air, so that it feels comfortable. The key word is temper. You don't need to heat the air, just take the chill off, and only on extreme cold days. Electric may be more practical than hydronic, if the ducts are in the attic. Also, an ER/HR may have its own independent ductwork as well. The reheat manufacturer will provide you with sizing based on duct size, airflow and delta t you want. You will need a control that will activate the reheat at a certain outside temp. The reheat may only be necessary in climates where you get extended periods of very cold weather.

I hope this answers your question.

Unknown

The ER/HR units I based my response on...

Only produce about 250 CFM max, adequate for resi use. On a light commercial building (with rooftop units?), ventilation air is usually controlled by an economizer or enthalpy control, with stage 1 of cooling being 100% outside air, throttling back to a fixed minimum, based on humidity & temperature. Stage 2 starts mechanical cooling. The "package" is available as an accessory from the OEM, or as an add on from a controls company like Honeywell or Johnson. Fresh air is either a fixed or variable amount, usually controlled by a CO2 sensor in the occupied space. Commercial sized ER/HR units exist, I think one manufacturer is Field Controls. I think you will be OK, just keep in mind that a resi ER/HR only produces about 1/4 of the CFM you need.

Jed_2

Thanks

These are Comm HRV,s (Imperial/Greentek) 700 CFM and 1200 CFM, for a new church/grange hall; intermittent occupancy. If I size by volume rather than PP, and use at least a .5 ACH, it shouldn't overpower the CHAHU.