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David Nelson

SLC HVAC control

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hi all, i am using an slc 5/04 to contol all of my home's functions. my question is this. my fan motor has a high , med, and low, lead. if i were to blow more air over the heat strip by speeding up the blower would the unit put out more heat? or is the resistance of the heat strip going to limit the current no matter the speed? i guess in theory if you blow more air and keep the heat strip cooler it would use more current to heat it up...but i wanted to use the plc and some relays to change blower speed depending on the demand for heat or the amount of variance from the setpoint. not real familar with the type wire used to make heat strips. thanks guys
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I've been down the road you're considering, only with much larger heat strips. The trick is finding that sweet spot... getting the most heat transfer. Move the air too fast and you don't get the air warm enough... too slow and you get good transfer of heat but not the full amount the strips are capable of. If you have an anemometer (wind speed gauge) and a temp. probe you can try different motor speeds and find out for sure if your units cfm is at its optimum. btw... the heat output of your strips is based on voltage and resistance... not ambient temp. Edited by Ron_Smith
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The answer is no, the heater will not put out more heat if the blower speed increases. Heaters are made from nickel chromium wire (nichrome) with a fixed resistance (for practical purposes). A fixed resistance across a stable voltage will produce a given current, which results in a fixed output of heat (power in units of watts). Although the heat output is constant, the temperature of the air passing over the heated element will change with the speed of the blower. Lower speed air has longer over the heater to pick up heat and will be warmer than high speed air . Typical home heating uses (relatively) low capacity baseboard electric heaters that heat the air by conduction and heat the room by convection and are controlled on-off. Gas fired furnaces use a blower to distribute air heated from passing over a heat exchanger. Where would you put electric heaters that would require a blower? That sounds like central electric heat, which doesn't make much sense. In the US, electric heat is more expensive per BTU than gas or oil heat. If you electrically heat a central heater, and try to control room temperature with a blower that distributes air, that infers wasting heat at low demand, which no one who works for a living can afford to do. What turns the heater on or off? What kind of layout is this? Where are the electric elements? Dan
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thanks guys, i think i will stick to 1 speed for now

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The reason you have a multi-speed motor is because central heating systems that are also equipped with an air conditioner will often require a different speed for heating -vs- cooling. The furnace heat exchanger on my home furnace operates on medium speed while the AC operates on high speed. This is because in the area where I live different air velocities are required for proper heat transfer depending on whether I am heating or cooling. The air speed is dictated by the design of the burner heat exchanger and/or AC compressor and evaporator. Were I to run the AC with a med speed blower the evaporator coil would ice up. Were I to run the furnace with a high speed blower the air exiting the ducts would be cool and the house would feel drafty. I wouldn't go messing with the factory settings. If you feel you are not getting enough air flow at the ducts at the farthest end of your trunk line then you may be able to gain an acceptable performace boost by putting in a higher HP motor instead of running the motor faster. Most home furnaces are equiped with a 1/3 HP blower motor but can easily accomodate a 1/2 HP motor (check the current rating of your furance control). That's a 67% increase in power, and it makes a big difference without speeding up the blower. Air has mass and it takes energy (read HP) to move it, not speed. The higher HP motor can move more mass against the duct resistance with less slippage - in practice this does equate to a slightly higher air flow, but its not the same as just speeding up the motor. A furnace motor typically runs around $45US. If you wanted to give it a try then also use an anemometer and a thermometer at the exit duct for comparrison, as already suggested. Edited by Alaric
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great info. i just wanted to try an us my plc to get the best bang for my buck. but if it will cause too many headaches i will just leave it alone
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