He is designing a VVT (Change over-bypass) single duct, with parallel fan electric reheat box. Thats what I have never seen. I have seen reheat coils on a VVT. But if you are doing change over at the unit when do I use the fans? All I can figure is allow the fans to run in zones needing heat when the unit is cooling.
I have not seen that configuration to much. I am not sure if these are Pressure Dependent or Pressure Independent. Carrier used to have an option with their older Gen 2 VVT. You could configure them as a Fan Powered Mixing Box series or parallel. I believe the sequence would be the VVT zone controller will energize the fan relay as 1st stage heat when the heating demand rises 1.5 Deg above the occupied heating setpoint. There would be 1.5 Deg increments from 1st stage up to 3rd stage depending on the exact zone damper configuration. When the heating demand drops at least .5 to 1 Deg below the occupied heating setpoint, the zone controller will deenergize the fan relay.
How are you planning to do the RTU changeover between heating and cooling modes, by polling the zone dampers or looking at the zone damper size and deviation from setpoint or something else?
I've done this but I don't have a sequence written up. It was a retro on a Carrier VVT (Gen II) I believe. I basically allowed cooling with any vav terminal load greater than 50% and then just let the reheat boxes do the heating as needed unless the average terminal load was less than -75% and no zone was greater than 50% then I allowed the rtu/ah to run in heating until the average terminal load rose to greater than -50%. Reheat stage one - fan, stage 2 one element, stage 3 two elements. If the highest terminal load is less than 25% I would cycle cooling back off.
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Does anyone have a sequence for a VVT with fan powered reheat? Have not seen them around here, but a customer of an engineer I call on wants one.
The parallel fan powered VAV terminal is a common system design. In this configuration the cooling air valve is first modulated to a predetermined minimum position (it can be completely closed). Then the terminal fan and heat are energized consecutively as the temperature in the space continues to drop. In this configuration, the primary air does not pass through the terminal unit's fan.
When no heat is needed, the local parallel fan is off and a back draft damper is closed to prevent cool air entry into the return plenum. When little or no air is flowing to the VAV zone, and the zone temperature drops below setpoint, the local parallel fan is turned on and the back draft damper opens. Warm recirculated plenum air is then mixed with the minimum flow of cool primary air and delivered to the zone at a predetermined minimum constant air volume. Additional heat can also be provided, when specified, by a heating coil located at the leaving air side of the unit.
A major benefit of parallel fan powered terminal units is that the secondary fan motor runs only when primary air tempering is required. Also, the terminal fan requires no special interlock with the central air handler because it sits outside the primary airstream. Another benefit is that the heat of the plenum (due mainly to lighting) can be used for zone tempering.
Is this a Carrier VVT System? Or are you using the term VVT with some other controls manufacturer. OK
ebusch I know what you are saying. I will say this I have done many VVT / zone damper systems with larger DDC controls. It usually involves some tweaking They usually have a VAV controller that I have to use as a VVT controller, and need to do some custom programming to get the changeover and polling setup.
Recently I have done a few Carrier 3V and Honeywell RapidZone jobs in light commercial projects that did not need a full blown DDC system. The Carrier and Honeywell are designed for VVT and light commercial. They are also priced right in terms of cost per zone. I have not used Trane in a few years but my experience was also good with Trane VariTrac. All three have decent user interfaces as well.