Jump to content

How to monitor heating system status: looking for ideas

Recommended Posts

I'm looking for ideas how to monitor the status of my heating system. I need to monitor the status of four 120v circ pumps.


Details of the system:


High efficiency boiler with a domestic hot water loop seperate from the primary loop. The heat zones are controlled by an external control unit (TACO three zone). There is also a primary loop pump.


I need to monitor the status of all four pumps (as well as a number of other parameters I have access to), and the problem is that there are no monitor points on the zone control panel or boiler. Heating industry standard is mechanical relays to control the 120V pumps directly. Surprisingly, there is an isolated, digital (PWM) output on the boiler showing fuel rate. This is used when ganging boilers.


What is available to me: red LEDs that reflect the zone status (only covers the zone pumps, not the primary or the DHW loop) and access to the 120V lines for all of the pumps.


I am looking for ideas to monitor the status (powered or not) of all of the pumps, to interface to an MSP430 being used for gathering data to send to a linux box. For obvious reasons, the 120V must be isolated from the microcontroller. This is where I am looking for ideas.


The four obvious ones are: Optoisolator, relay, current transducer, and transformer


Optoisoator is not ideal due to the need for either a large current limiting resistor or other control circuitry for the input, as well as additional surge protection.


Relay is not ideal due to cost (120V coil relays are not cheap), size, and the waste power involved


Transformer is not ideal for similar reasons


Current transducers are also pricey, and a fair bit of conditioning is needed on the output.



Any of these would work (and I have parts on hand for any of them) but I am looking for other ideas, in particular anything that I can put right into the control box. I have also considered using a fiber optic setup, but, as that is not something I deal with at the design level, don't know what components are available at reasonable cost, in particular that will be practical to use for the sending end.



Link to post
Share on other sites

If the AC current input correlates to the state of the system, then you can hack a solution by wrapping a helical coil around the power cord, and rectifying (and quite likely dividing as well) the induced voltage. Then you can use an ADC to capture the resultant waveform, and measure the amplitude.


I've used this as a way to non-intrusively monitor a washing machine. Each stage of the cycle uses different current, which can be monitored and reported.

Link to post
Share on other sites

... then you can hack a solution by wrapping a helical coil around the power cord, and rectifying (and quite likely dividing as well) the induced voltage....


Just thinking about this more: With appropriate circuitry, you could even power a device with the induced EMF.  I think I'm going to try that sometime Q1 with one of my solar-powered wireless boards.  Of course, I will remove the solar and replace with some rudimentary rectification and voltage limiting circuitry.

Link to post
Share on other sites

Isn't there a catch though - you have to somehow wrap the hot wire but not the return wire, which will induce an opposite EMF?

Someone told me this last time, so I cut a slot in the insulation (it was a flat cable).  I didn't think to test if it would work with the ground line as well, but for single phase AC I bet there's a workaround regardless of the cable type.  I'll look into it.

Link to post
Share on other sites

I'm fairly certain that is the case but it doesn't sound hard to do. Someday I'd love to build a current monitor for all my large appliances with minimal footprint inside the circuit breaker like this. Tapping into the appliances' power at the endpoints with cheap WSNs would be fine too though.


Sent from my Galaxy Note II with Tapatalk 4



Link to post
Share on other sites

The joys of knob and tube wiring: live wire over here, return wire who knows? Probably connected to a different breaker.


And don't touch the insulation, it turns to cursed mummy dust.


Power monitoring is another of those items to put down on my home automation... list? burden?


Maybe project *is* the right word, since it will obviously never get finished.

Link to post
Share on other sites

Thanks for the discussion. Looks like, at this point, things lean toward inductive current sensing. Might give me the opportunity to do other monitoring, as well. When I rewired my electronics bench (picked it up at a yard sale-- commercial assembly and test bench, two upper shelves, lower shelf, large work surface, ten duplex outlets built in, yada yada yada...), broke out the hot from the feed to one shelf and gave a single line and a ten turn loop for clipping on the ammeter (the ten turn loop is real nice for monitoring lower power devices)


Now, to price out current transformers. Don't need split core, as I have access to all of the wiring. Heat was first thing I did when I bought the house, after basic electrical and fixing the holes through the exterior walls to the outside. All new, all by me with the help of friends. Passed inspection the first shot, as did all of the electrical work replacing the knob and tube, some of which was bare wire. Might not have been bare in 1910, but it was when I bought the place, and had been for a long time.

Link to post
Share on other sites

Good job. I reckon with knob and tube the options are leave it be, or pull the whole lot out. No middle ground.


My last place (1910) had live bare wires terminating in the basement rafters and secured by hammering in a nail and bending it over. Nice workmanship!


While fitting insulation to cold water pipes I managed to touch one of these bare wires to the pipe, which sparked and blew a hole, spraying water everywhere... a really great combination of hazards. Oh, and I was up a ladder.


Just thinking about it frightens the crap out of me.

Link to post
Share on other sites

The worst was a junction in the wall. Romex had been run at some point, casing slit and the conductors pulled apart and skinned. The old K&T was twisted to the conductors and wrapped with tape. No boxes, No physical protection. Found it when I was working on another circuit after I thought all of the remaining K&T was dead and I was trying to figure out why the wall showed hot with a sensor over most of the surface.


I may go all out and buy a few extra current xformers and monitor current in a few key lines (AC, dehumidifier, office, etc) as well. When I rewired, most of what I did is set up for access without cracking the panel. Might even set up for power stealing from a current xformer to maintain supply for the MSP. Gotta see if I have a line that has regular, stable draw for that, tho.


Again, thanks for the input, and if anyone has other ideas, they would be appreciated.

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...