controlling 2-mod cons w/3 temps

Greg_23

desinging a system for a 12k sq.ft.home with 6k slab,6k staple up and 2-HI-V heating cooling air handlers. Using 2-Buderus GB's tied together with a large indirect for domestic. Trying to come up with the best way to control this system and achieve the most effeciency. Any thoughts?

Glenn Sossin_2

Are the Air Handlers Back up

or are they primary heating source for some area? Based on the descriptions you gave, they seem like they are back up units? Zoned with dampers so in effect they would be 2nd stage heat units?

Perhaps I can give you an idea, just want to clarify the function of the airhandlers.

Greg_23

controlling 2 mod-cons

air handlers are for a/c and heat back-up.

Glenn Sossin_2

I would suggest Teknet 4

Since the air handlers are primarily a back up for the radiant, it would seem logical that they would only be called on as you approach outdoor design conditions - meaning, your system supply temps would already be at their highest values using the outdoor reset.

Look at the common rooms the AH services along with the radiant. Choose the radiant room with the highest heat loss, put a 2 stage thermostat there. Chances are, the supply temps for the staple up (which should be relatively high) could be used to provide enough back up heat from the air handlers to satisfy the 2nd stage heat request.

You will want to take into account the air handler speed settings for heating and the coil data for various supply temps and flow rates. Usually, we try to set the air handler for its lowest CFM for heating to limit the btu load of the coils. This is because we usually find the coils, are 2-3 times the capacity of the heat loss for the areas they service.

Use an injection control, to create another ODR curve for the 6K slab.

So basically, you have a low temp mixing control for the 6k slab, you have a 2nd higher temp for the staple up along with the fan coils (ODR of the boiler control), and you have the DHW/demand temp for your indirect. Depending on the control system you use, you may even have a setpoint demand, a fixed value other than DHW, that you could use for the forced air back up.

In a job of this size, I would be using Teknet 4, Use a 423 to control/stepfire the two boilers with the ODR set for the staple up design temp, and a 440 mixing control for the lower slab temp.

In a house of this size, with 2 air handlers, I would assume you are using a series of dampers to control heat/cooling output? If so, then you will have multiple 2 stage stats, each controlling its own damper while powering up a common circulator. Use a Tekmar 335 to control up to 4 different zones/dampers to turn on a common circulator and send a demand to the boiler.

The Teknet 4 system also allows for grouping of various thermostats to determine the temperature of a room for cooling. Very nice feature.

I have done several jobs this way. Hope this helped - maybe gave you an idea.

Glenn