Controling tempature in a residential area

Hi Joe and others,

I am an HVAC Tech specialising in controls for the last 10 years, mostly BMS/DDC of course. I really like your idea of a PLC to control a home HVAC system. I contract myself out to the oil & gas sector for HVAC commissioning, repairs, maintenance etc and most systems offshore are controlled by a PLC (usually AB)

Have you considered the need for a VSD on the supply air fan? If you are going to start closing dampers you are going to need one.

I would also seriously consider using analog dampers, with a feedback, I normally use 0-10V stuff from Belimo.

Just a few ideas for you,

cheers

Charlie

I have already considered still using the original thermostat to control the system, and use the PLC to monitor room temperature, and control the dampers.

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It is not hard to use two relays to feed the existing thermostat into the PLC, and then let the PLC decide when and if the system actually runs. You could use an infrared sensor to detect if anyone is actually at home and have the PLC setback the On time, or delay the run. Use 3 NC relay contacts for the Fan Relay, Heating Contactor, & Cooling Contactor, so if PLC is disabled, or removed, the HVAC runs normally.

Lancie1 said:

It is not hard to use two relays to feed the existing thermostat into the PLC, and then let the PLC decide when and if the system actually runs. You could use an infrared sensor to detect if anyone is actually at home and have the PLC setback the On time, or delay the run. Use 3 NC relay contacts for the Fan Relay, Heating Contactor, & Cooling Contactor, so if PLC is disabled, or removed, the HVAC runs normally.

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That is a good suggestion about using infrared sensors. Would you need one in every room?

also this is probably dumb question but it sense someone if they were sitting watching TV?

Have you considered the need for a VSD on the supply air fan? If you are going to start closing dampers you are going to need one.

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the thought has crossed my mind, but then get the problem of low air flow across the evaporator coil. then I thought if to many dampers close will still have the problem of low air flow. Figured I could measure the air flow if it gets to low start opining dampers until I get an propitiate air flow

I would also seriously consider using analog dampers, with a feedback, I normally use 0-10V stuff from Belimo.

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this could be a possibility. I will have to look into this further

That is a good suggestion about using infrared sensors. Would you need one in every room? Also this is probably dumb question but [does] it sense someone if they were sitting watching TV?

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Joe, I installed an infrared motion-sensor light switch in my bathroom. It has two adjustments, Sensitivity, and Time Delay. The time delay can be set to keep the light on between "motions" in the room. It fits in a standard wall box and works very well. I have been using it for years. We older guys that get up frequently in the night really need one of these! It saves fumbling for the switch at night in the dark. You just walk in and the light turns on. Walk out, and you have enough time to get back to bed, and 3 minutes later it turns off. It avoids having to leave the light on all the time.

For HVAC control, you would need to get a little more sophisticated. There are "occupancy" sensors available to use in controlling various fire and burglar alarm systems. You COULD put one in each high-use room, with inputs back to the PLC. If any one is ON, the PLC assumes the home is "occupied". I would put in a time delay, so that if any sensor is triggered in a 60-minute period, the "occupied" relay stays on.

If you have an alarm system, you could use a contact the alarm-set keyswitch to also set the "occupied" relay to occupied or unoccupied.

You could just mount a simple On/Off selector switch near the normal entrance/exit door.

Joe

Off the top of my head there are two occupancy sensors
One is motion - these are not recommended for "sitting areas" ie live room bathroom etc. The most common are wall mounted. The other is infrared these I believe can come in wall mount or ceiling mount. There is a third which combines the two these also are in wall mount and ceiling mount and are I believe recommended for bathrooms.

Have not dealt much with the infrared version so cannot answer your question regarding sitting ie how long would it take for sensor to decide you are part of background IR level?? Lithonia may be good source for info OR you can do as I did steal the directions from a box at Home Depot be careful though that is where they hide the little magnetic theft detector - believe me I know.

Dan Bentler

Dan,

It depends on the Sensitivity and Time settings on the infrared sensor. If you set it too sensitive, it will trigger from air movement from the vents, or a cat in the room. If the time is 3 minutes, then if it sees any little flick of the hand in a 3 minute period, it stays on, resets the timer and starts the 3 minutes over again. It is hard to stand in a room and not trigger it. If you make an effort to stay still you can fool it, but it is not easy.

Alaric said:

Home thermostats incorporate something called a heat anticipator. People are surprisingly sensitive to heat variations in a home and what happens is that people feel comfortable before the temperature sensor actually registers that the air temperature has reached the comfort value. So a very small current is passed through the bi-metallic spring to warm it up a little in antipation of reaching the desired temperature. The heat anticipator can be adjusted by a service technician. Modern digital thermostats adapt to anticipate the heat and don't need adjustment. In any heating control system that you design you should build in some kind of anticipator, otherwise your room will feel either drafty or too great of a hot/cold swing to be really comfortable.

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Yeah your right, People are surprisingly sensitive to heat variations in a home... the modern digital home thermostats adapt to anticipate the heat and don't need adjustment...



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