Amps in Thermostat Wire? Furnace run-time monitoring?

Alexandre

Hello,

I am trying to monitor furnace run time in residences to estimate gas consumption. This is part of a study and is going into people's homes so I am trying to make the installation as non-invasive as possible. Because we cannot install gas sub meters I am hoping to monitor run time for staging and modulating furnaces using current switches around the thermostat wires, but am not having luck.

Does anyone know how many amps are flowing through the R-to-W or R-to-Y circuits when a thermostat is calling for heat. I have even double looped the wire around my current switch and am not having any luck!

That is my main question. If people have ideas about alternative options for monitoring that could be helpful as well, such as perhaps monitoring the voltage between the two terminals rather than current? We are restricted from doing a lot of things, so some solution using the control wires would be preferable.

Thanks!

Alex

ratio

I believe those are typically in the 1-and-change and under range for electromechanical controls. Electronic controls could be significantly less. You should be able to see the current with an amp clamp, you may need 10 wraps to bring it up to an easily seen signal.

Jamie Hall

As @ratio says, even older mechanical controls will be only slightly more than one amp, and may be significantly less. Modern electronic controls are voltage, not current, controlled, and the current in the thermostat wires will be in the milliamp range in a lot of cases. You will do much better to determine in each setting which pair of wires is shorted when the thermostat is calling (it may vary) and monitor the voltage on that pair. It will be, very close to zero when the 'stat is calling, and may be anywhere from 5 volts up when it isn't, depending on the control. You should also be aware that some 'stats "steal" current for power supply purposes.

Firecontrol933

No one way to easily do this across all different models of equipment by monitoring the thermostat wires because of previously mentioned differences.

Maybe rethink what you are monitoring. How about changes in temperature output verses time?

A temperature data logger dropped into a supply outlet works great............as long as you can retrieve it, because they can be costly.

Gordy

Just buy a thermostat that logs heat, or AC run times.

Yes they cover the whole period of a call, and do not reflect actual burner on times. However if there is a heat call then there is a requirement for btus to the space. Regardless of whether burners are on , or off until the call has ended.

Mine logs rounded off to the nearest minute. Daily, and previous day. With a total log rounded to the nearest hour.

JUGHNE

I have thought about doing this for some projects.
I was thinking of using a 24 VAC powered hour counter.
If connected right at the gas valve terminals it would give you the time that the full fire was actually on.

If connected to the tstat W and common it would give you the time the tstat calls for heat which includes the prepurge and ignition sequence time before the main gas valve actually opens.
And if off on high limit for a few minutes and then auto reset restarts that would not give you the actual full burner run time as the T-stat is still calling for heat. (Shouldn't cycle on high limit as normal operation but we have all found it to happen.....filter....air flow etc.)

These could be mounted at/inside the furnace or with at least 18 gauge wire could be outside if the length is not too excessive.
Any weather proof device is about double of inside. It could be mounted in a WP bell box outside. just 2 screws on a blank plate would let you peek inside for a reading without entering the house.

If you knew the orifice sizes of the furnace burner and the inches of pressure on the manifold the BTU could be calculated (might actually be the input on the nameplate) and your hour meter would complete the equation for total consumption of BTUH per month/day etc .

Also, the same would apply to the AC contactor in the outside unit if you want to know of run times for compressor.

hot_rod

An hour meter was my thought also. i wonder how well these wireless versions work?

http://www.getdata.it/prodotti.php?idcategoria=36&idprodotto=236&idfamiglia=2&lang=eng

The construction and rental industry use some also.

JUGHNE

My Mother's apartment in the city had a device that measured the "on time" of her furnace gas valve and was wirelessly transmitted to the apartment complex office. The complex (1970's) obviously had only one gas meter. So each apartment has their run time and I would guess a share of the balance of the "house usage", that being hot water heaters, common area heating etc. Any astute management would pass all these costs along to the tenants. (I probably would anyway).

PS: also probably the cost/deprecation for the monitoring system itself.

SWEI

A reasonably high impedance across the line can be used in place of current.

JUGHNE

Do you think the hour counter would qualify as high impedance if connected across the gas valve?

hot_rod

Our solar controllers have a heat metering function that is fairly simple. Possibly one of these could be used without messing with any wiring, which sound like a concern?

Basically if you know a delta T and a flow rate you can calculate energy in BTU or KWH.

So on the solar systems two sensors read S&R and you input the gpm from the flow meter. The control also calculates for pumps running on a variable speed function.

I'm not sure how you would calculate, or measure the CFM on a forced air system, but I think a calculation could be created to get a close number.

I'd donate a controller to the cause if you think it is workable, it could easily be made as a portable device in a case that would not require any wiring other that taping sensors onto the distribution, and plugging it into a 120V receptacle.

SWEI

JUGHNE said:

Do you think the hour counter would qualify as high impedance if connected across the gas valve?

An electronic hour meter probably would. Not so sure about the mechanical ones.