Averaging temperature sensors for two mode of heating

[davster01]

I am designing a approx. 20000 sq. ft. building that will be served by 8 roof top units (RTUs) in ASHRAE climate Zone 6. The RTUs have not been selected yet.

Let us look at one RTU. This typical roof top unit might serve 4 closed offices, and 2 boardrooms.

In the shoulder seasons (Spring and Fall, we want to use electrical baseboard heating as the first stage of heating (minimal amount of baseboard will be installed) to prevent overheating the office spaces not calling for heat. We would like the RTU to fire up (gas-fired) its first stage of heating only if the room temperature in 4 out of the 6 rooms is not warm enough after 10 minutes of electrical baseboard heating ouput in those rooms.

Then if room temperature in 4 out of 6 rooms is not satisfied within an additional 10 minutes, the second stage of heating at the RTU would kick in.

My question is the following: is there technologies that can accomodate such control sequence and duel heating mode? Do you have experience with such an arrangement? We want to minimize cost of the Controls and are not willing to go with a full blown BAS. As such there is no need for a centralized user interface that would tie-in data from all 8 RTUs.

I have seen some applications for Variable Volume Temperature (VVT) applications. But our applications is a Constant Volume application so I am not sure if this could work.

The way I see it is that you have one subcontroller per room that receives one temperature sensor input from the room and sends a signal output to energize a relay that powers the electrical baseboard (if temperature remains below setpoint for 5 minutes). This subcontroller also sends a temperature sensor reading and a relay state to a Controller. This controller takes readings and electric baseboard relay statuses from all rooms before making a decision to call for heat or not. This controller has therefore averaging capability and might even be able to prioritize certain rooms over others. This controller would process all input from the subcontroller located in each respective room. Then the controller would either de-energize the electrical baseboard or not (it would be nice to have this option) and call for heat at the RTU.

For cooling, a simpler scheme would be required although the need for a voting system remains.

Hopefully this is clear (enough).

Thank you
davster01

 

[MintJulep]

And during those 10 minutes the RTUs are doing what? Dumping cold air into the rooms?

Your strategy seems guaranteed to produce a continual stream of comfort complaints from occupants.

 

[cdxx139]

I dont have experience with this sysetm, but it doesnt seem like an ideal control situation.

I have used VVT and it IS a constant volume system (referring to the supply fan). The additional cost would be the zone dampers, but it would help with the occupant comfort part of ther project, and they can be interlocked with the electric fin tube.



knowledge is power

 

[Waramanga]

I cant see why you are so adverse to a mini BAS. Using unit controllers locks you into a fixed control strategy with very little flexibility. A simple networked controller system with a few field panels in each RTU will save you money in the end (and probably up front), providing you can get a good vendor/installer/programmer onto teh job. You might initialy save the client maximum 15k and end up costing yourself 50k on fixes, call outs and bad blood etc.

 

[davster01]

Point well taken Waramanga. Do you have a suggestion as to what type of mini BAS to go with? And is 15K really all that would be required? My boss seemed to think more than that around 30k...

Thanks for the input

 

[KiwiMace]

Pick something like an Automated Logic, Tridium or Delta controls with a built-in web server. One decent supervisory controller with a front-end will save you a ton of money in setup and maintenance.

Apart from reinforcing what MintJulip said about outdoor air, I'd also say that sharing a constant volume RTU into meeting rooms is a road to disaster. People expect to walk in the morning and put 50 people around a table that seats 10, and instantly meet the new load. You'll need supplemental cooling, most commonly this is seen as a fitout solution with a DX ceiling unit chugging away dumping cold air on everyone standing at the walls.

Consider: Dividing your RTU's into the office and meeting rooms separately and VAV the meeting rooms. Putting more offices onto less RTU's so that the low load heating can be met by the minimum turndown on the furnace.