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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I want a digital line voltage thermostat for cooling only to

Customer Question

I want a digital line voltage thermostat for cooling only to control a duct fan used in a solar atrium. The 120V fan should come on when the temperature exceeds set point (e.g. 80 deg F) and stay on until the temperature drops below set point.
JA: Just to clarify, do you think this is a larger HVAC problem, or something specific to the thermostat?
Customer: I want to buy a thermostat for this application. I prefer digital to control set point better.
JA: Do you plan on doing the work yourself?
Customer: Yes
JA: Anything else we should know to help you best?
Customer: I have a analog line thermostat (not Honeywell) which is very erratic switching on and off usually overshooting set point in switching on and switching off.
Submitted: 12 months ago.
Category: HVAC
Expert:  Phil replied 12 months ago.


Digital electronic thermostat will not necessarily solve your problem, I will be happy to work with you on this but perhaps first we should investigate the current situation.

it is quite likely that the DIFFERENTIAL setting on your current thermostat is simply set too wide... are you aware of that setting.

Also tell me if the current thermostat has a coil of thick tubing attached to it, or if it has a sensing bulb attached to a long thin copper tube.

The location of the thermostat and its sensing element is also very important.

(precise temperature control, within 1/10 deg F had been one of my specialties before I retired)... in your case we can achieve + or 2 degrees F cost effectively with a mechanical line voltage thermostat.

Digital line voltage gets very hard to find very expensive and will generally not perform any better than a well placed properly selected mechanical thermostat.

We can go from there.