PLC as 365 day furnace controller?

[roxley]

All,
Does anyone know if a PLC could be used to control a building's heating plant? My Lodge rents out the building to tenants beyond it's normal meeting schedule, and I'm trying to integrate a PLC so I can remotely program turning on and off the heat as tenants sign up to use the space.
Any ideas, suggestions, or actual programs would be greatly appreciated!
I find it hard to believe that this hasn't been done before. Please help!
Randy

 

[Tripper]

google ddc systems (designed for that)

 

[504bloke]

All,
Does anyone know if a PLC could be used to control a building's heating plant? My Lodge rents out the building to tenants beyond it's normal meeting schedule, and I'm trying to integrate a PLC so I can remotely program turning on and off the heat as tenants sign up to use the space.
Any ideas, suggestions, or actual programs would be greatly appreciated!
I find it hard to believe that this hasn't been done before. Please help!
Randy

Yes a PLC could be used.

Have you a PLC brand in mind ?

 

[leitmotif]

A lot depends on what you want to control is this both heat and cool
do you want to control humidity CO2 makeup air etc etc.

You can start off with very simple single pole thermostat to turn on heater to very exotic and complicated to control half the world if you want. All depends on how many dollars you want to throw at it.

One of the issues I think you should consider is
how is the average HVAC guy going to service this control setup when he has never seen it does not have manuals etc etc. I would consider using standard HVAC controls that the average service guy is used to.

Dan Bentler

 

[Lancie1]

Rosley,

As Dan said, an important factor is the difficulty of maintaining a custom system. Another problem you will find in interfacing a PLC to a HVAC system is that the HVAC controls are normally running on 24 volts AC, and there are not many PLCs designed to allow 24 VAC inputs directly.

For one project I did for a church, I got around both problems by using interposing relays between the PLC and the HVAC 24 vac control wiring. I set up the wiring so that the PLC could be removed and carried off and the HVAC system would revert back to using the existing thermostat. In fact the thermostats (10 separate thermostats in this large church facility) were still controlling based on temperature, but the PLC was controlling which unit started next, and the time between allowed unit starts, in order to control the power demand factor.

The wiring is attached for this project for the first two units, and each additional unit wiring is identical to the first one. You add two relays for each additional HVAC thermostat, and an input and output is required on the PLC for each thermostat. The ZEN plc with an expansion module was used to get 10 inputs and 10 outputs. As long as the relays are left wired, the PLC can be removed and the system will still work. Normally Closed contacts are used on the Output Relays (R2, R4, R6, and so on)to make it fail-safe.

I have a parts list and the program also, if you need them.

 

[leitmotif]

Lancie

In essance you added the PLC to give you sequenced starting of what I assume to be heat pumps or large resistive heaters to reduce your demand charge. Are you able to quantify the savings in demand charge after you installed this?

What other parameters are you controlling with the PLC?

I like the fail safe idea enabling PLC disconnect and removal without system interruption.

Dan Bentler

 

[Lancie1]

Dan,
Yes, the savings were built-in before project started. Essentially the church electric bill was based on the highest KW demand for each 15 minute period. By only allowing only one HVAC unit to start in each 15 minute period (instead of up to 10 units), the demand charge was reduced by about 20% on average. At the same time, heating and cooling was not significantly affected, because the PLC program made sure that all units got a chance to start. Once a unit was running, it was allowed to continue running until the thermostat was satisfied. The savings were strictly on the high demand charge (caused by many of the units trying to start up during the same 15-minute period), not on any attempt to reduce the amount of heating or cooling.

I thought that a similar scheme might be used for Randy's Lodge furnace control, keeping the existing thermostats but putting a PLC Ouput in between the thermostat outputs and the HVAC contactors. Because of the need for remote operation, a higher-end PLC would be needed. The ZEN is about as low-end as you can go and does not have the remote networking capability. However it is not difficult to use the same scheme on practically any PLC with Ethernet communications capability.

 

[leitmotif]

Demand charge is often not chump change and can add up fast. this is really true in case of hard starting loads such as compressors.

Dan Bentler

 

[Lancie1]

Dan,

Well I am embarrassed again. CRS reared its ugly head. It was not 10 units but 11, and not 15 minutes but 5 minutes. It has been nearly 5 years ago and that is too long for my aged short-term memory.

Here is a link to the original thread "I need a furnace startup ladder program" from member Randalo in February 2004:

http://www.plctalk.net/qanda/showthread.php?p=42266&mode=threaded&highlight=randalo#post42266http://www.plctalk.net/qanda/showthread.php?t=7012&highlight=church+heating

 

[monkeyhead]

All,
I'm trying to integrate a PLC so I can remotely program turning on and off the heat as tenants sign up to use the space.

Something like this may be all you need for a simple application of turning a single point on or off:

http://www.controlbyweb.com/products.html

PLC just seems like overkill. Some of the Home automation products may work... or like tripper pointed out maybe you'll have some luck with the DDC products:

http://www.ddc-online.org/manufacturers/index.aspx

 

[Lancie1]

Good call, Monkeyhead.

That X-300 Web-enabled thermostat would be just the item needed to remotely control a single-stage heating furnace. It and a power supply would be about $239.95. Add some thermostat wire, some electrican's labor to remove the old thermostat and wire in the X-300 to the existing furnace controls, and an internet connection to the X-300 thermostat, and that should do it.

A possible road block might be if the lodge building owners object to having their existing heating system modified. They could have a maintenance contract with a local company, with a warranty on the equipment, and so on.

 

[leitmotif]

The X 300 may suffice to meet the need posted by OP. More than one way to skin a cat

Dan Bentler