Changing furnace out to air handling unit

chief

Hello looking for advice. I currently have a gas fired 2stage furnace heating my 1st and 2nd floors of the house. The furnace is getting to be 20 hrs old and I want to replace it with an air handling unit and hot water coil. I installed a hot water mod con 4 years ago to take care of the remodeled basement spaces and had it sized to handle the whole house load. What is the best way to control the air handling unit? Variable speed AHU? Modulating zone valve for the water?

SWEI

Assuming you are using outdoor reset on the mod/con, you will need to size the coil using the design water temp for the rest of the system. A variable speed (constant pressure) fan would be my minimum suggestion, as it will prevent over-pressurization and whistling when registers are closed off.

RobG

What SWEI said above. What kind of modcon is it. There may be control strategy options as well. I am going under the assumption that you did a heat loss on the home?

chief

The mod con is a buderus gb142 series with outdoor reset. Heat loss was calculated.

Rich_49

As Swei said the fan should be constant speed . You need to condition all the air in the house all the time . VAV is nonsense , use the ODR on the boiler and possibly an off the shelf ODR mixing valve like the Taco I Series . Determine what air temps the home needs through the ducted stuff at various conditions and set the reset ratio on the valve .
Short of the ODR mixing valve you could purchase a variable speed pump like a Bumble Bee at present and shortly a VT 2218 and put the sensor in the airstream right in front of the coil and use it in set point mode , this will vary the flow and thus the output based on return air and supply air temps .

unclejohn

I guess that this AH will also do the cooling as well. You are just going to set it up for a fan speed off of the "G" circuit. And let the tstat shut the unit on and off. Carrier may have a AH with a hydronic coil already in it and may have fan speed selections for heat. Most will not but will give you a selection of hi med or low for fan only. You will have to set the tstat up as electric heat to get the fan to come on with the call for heat. Then power your zone valve off the "W" circuit.

Harvey Ramer

Before you tear the old furnace out, use it to figure out what kind of airflow your duct system supports. That will help in picking the correct air handler and H2O coil.

chief

Thank you for the information. This helped us make the decision to go with a constant volume system and a zone valve. We will rely on and make adjustments to the odr if necessary.