Usage Meter For HP Defrost

I recently had a 4 ton heat pump installed at my home. My thermostat has a set point based on outside temperature where it switches from the heat pump to backup fossil fuel heat. The set point should be determined by the relative costs of the two heat sources at a given temp. One factor that makes this more difficult is the heat pump will go into defrost cycle whenever it determines it is necessary. Essentially this runs an AC cycle.

The HP has a low voltage terminal that energizes during the defrost cycle. This can be used to activate backup heat during a defrost cycle.

I am wondering if there is an easy way to attach a time meter to this terminal so I can record how long it has been active. I would use this information in my calculation to determine the optimal temperature to kick in the fossil fuel backup.

Thanks,

Sean

I'm not sure over how long of a period you are looking to record this data or how much you are looking to spend, but something like this might work for you:

**broken link removed**

I'd like to be able to peridically check the time of use, perhaps each day or maybe each week. What I was envisioning was an LCD clock like a kitchen timer that I could look at and see the running total (hours/minutes/seconds), and perhaps reset periodically. When I notice that it is defrosting excessively, I'll know that it isn't operating efficiently.

If all you want is total running time and you don't care about the frequency or duration of cycles I might suggest the following approach. Buy a cheap old analog clock like this one:

**broken link removed**

Splice a wire to the terminal that indicates the defrost cycle. That wire will be the input to a circuit that will essentially be a switch to turn the clock on and off.

Set the clock to 12-O'Clock and hook everything up. When you come back some time later, whatever time the clock says will be the total time the heat pump was in the defrost cycle.

If you used a slightly different clock, say one that runs off of a battery, you could use a much simpler transistor circuit to turn the clock on and off. If you go with the one that runs on AC mains power the circuit would be slightly more complicated.

You could just buy a standard issue timer unit. About every electronics and commercial parts supply company has a wide range of designs to choose from. Figure around $15 - $100 depending on how fancy you want.

tcmtech said:

You could just buy a standard issue timer unit. About every electronics and commercial parts supply company has a wide range of designs to choose from. Figure around $15 - $100 depending on how fancy you want.

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Can you provide a link to somewhere? I'm searching for timer, but they don't appear to have an input that I can tie into the HP.

Thanks,

Sean

Digikey, Allied Electronics, Mouser, MSC, Grainger, Newark, eBay.

Just do a general search for timer or hour meter on their sites.