Installing 24 volt WiFi Thermostat on millivolt stoves

James76

New Member
Jan 29, 2017
1
Huntsville
Hey folks, I recently was successful is installing the Honeywell RTH6500 on my napolean gas stove.

One thing I was not anticipating was the unit was cycling on 5 to 6 times per hour even though the desired temp was being maintained. Most times the stove would only come on for a few mins. It is my understanding that short cycling is not good for gas stoves.

I've done some research and that heating cycle rates settings may be the culprit. Seeing that there appears to be folks who have successfully installed programmable thermo's on their stoves I am wondering if anyone else has run into this problem or has any suggestions?

Thanks
 

Arti

Feeling the Heat
Feb 14, 2014
386
South West Wisconsin
Hey folks, I recently was successful is installing the Honeywell RTH6500 on my napolean gas stove.

One thing I was not anticipating was the unit was cycling on 5 to 6 times per hour even though the desired temp was being maintained. Most times the stove would only come on for a few mins. It is my understanding that short cycling is not good for gas stoves.

I've done some research and that heating cycle rates settings may be the culprit. Seeing that there appears to be folks who have successfully installed programmable thermo's on their stoves I am wondering if anyone else has run into this problem or has any suggestions?

Thanks
I believe it is Function number 5 in the Setup Menu that sets the cycles per hour.
This thermostat is called a comfort tstat it doesn't really heat to a set point it calculates how many minutes the unit needs to run an hour to maintain a temperature and then breaks this up into 3,5,9, or 1 cycles per hour and then runs so many minutes per cycle. If you look in the install manual and find the setting for steam or hot water it will cut the cycle rate down for you.

Also you may want to look at the smart response settings.
I'm not familiar with this particular model thermostat but it may have an adjustable swing temp setting in it although most honeywell thermostats don't. They maintain within approx 1/2 degree of set temp, I find most wood/pellet stoves live a 2 or 3 degree swing.
 

RavingZeus

New Member
Feb 22, 2017
19
Montreal
Harman has a technical bulletin on how to use their stoves with a T-stat. They suggest to make the set temp on the T-stat to 95 Deg and then set the stove to the desired room temp.
I connect the Lowe's Iris T-stat with swing setting set to 2 degrees and connect the 2 wires in series to one of the room probe wires on my workshop's Harman Advance. Then I can manually turn the stove on and off with my iPhone or use a schedule and see what the temp in the room is on my iPhone. It works very well unless the stove runs out of pellets, then I have to reset the stove. :)

Hi, I have a Harman P43

If i refer to the Technical bulletin of Harman Tstat need to be connected in serie with the probe.
If I use the schematic in the first post to connect a WIFI Tsat with the relay and a 24volt power source, do I still need to connect the room probe in serie? If yes, were do I put the probe in serie exactly in the diagram?

Thanks for your help
 

bmwmaw

Member
Dec 20, 2011
3
Eastern Panhandle, WV
Yes, but when temp is achieved on the stove it will error #4 and go to shut down mode. It will start back up when the call comes back and the error will go away till the tstat is satisfied again. No harm will come to the stove, it's just that Harman uses a thermistor that always has a circuit. When the tstat opens the circuit opens and the error shows up.
I have a PF100 furnace with the Harmon wall control. So according to Harman's set back thermostat install I have to wire the Tstat in series with the wall control. Do I have to use the wall control knob? I'd rather just use the programmable Tstat. Are there any advantages/disadvantages to either method?
 

bmwmaw

Member
Dec 20, 2011
3
Eastern Panhandle, WV
That's fine so long as the run to the stove is short from the tstat. You cannot reliably send a millivolt signal very far that's why we use 24v
Thanks for the good info - just to confirm again, I don't need the relay as long as I remove the R and Rc jumper and am not running very far? I think my Tstat is only going to be maybe 10-15 feet from the pellet furnace.
 

Nigel459

Feeling the Heat
Oct 24, 2017
295
Ontario, Canada
Why not just hook up a 24 Volt transformer to RC And C this will power the thermostat just fine!
Take the jumper out from RC And RH, Hook up a wire from RH and W back to the pellet stove to make the stove work and you are done. No need for relays etc.
Found this on another site.
Just wanted to reply to this thread to say: The above worked great for me with a Honeywell (Wifi RTH6580) thermostat.

Wanted to update as I have been searching and searching for something to corroborate my findings (i.e. you might not need the relay, since there's one built in to the thermostat!) and finally stumbled across Arti's posts. If there's a way to update the first post of this thread, which says you need a relay, that would have saved me some time.

Thanks @Arti! Cheers all and happy thermostatting :)
 
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maihem.rc

New Member
Feb 26, 2019
1
Knightdale, NC
Many stoves use millivolt jacks on their stoves. This poses a problem when you want to use a thermostat that had functions etc...especially things like Wifi.

Basic heat only thermostats, such as mercury filled or the ye old slider style basically just complete a circuit when the temp falls below a specific set point. They don't need to be powered. As soon as you start using thermostats that have digital readouts, have clock, are programmable or connects to WiFi portals, one must power the thermostat. Most thermostats run on 24 volts. Your doorbell on your house may run on 24 volts too. Most central heating equipment will have some kind of transformer that powers the thermostat via the Rh and W wire connections. However,these connections will only power the thermostat when the unit is calling for heat, and that's what the C wire on the thermostat is for...to provide a common neutral wire for when the tstat is NOT calling for heat.

Many pellet and has stoves utilize milli-volt powered tstat jacks. The reasoning is that room heaters such as these should not be used as primary heating systems and as such and controls for them such as tstats should NOT be located too far away from the unit. Milli-volt current will not travel to far away so that is why they are used. What we don't want to happen is to have the stov calling for heat too soon after the unit has gone in to shut down mode. We call it short cycling and it can mess up the logic in the control board of the stove. Not permentaly, but perhaps enough to force the owner to unplug the unit and restart the system.

It the pictures below I have made a powered relay with a 24v transformer to allow a pellet stove with a milli-volt tstat jack to be used with a 24v C wire powered tstat. The one I am using is also WiFi enabled.

I chose the Cheaper one from lowes
Honeywell 7-Day Programmable Thermostat
Item #: 171234 | Model #: RTH6580WF1001

The relay is a RIB UC12 and the transformer I got from Radio Shack.

The wires coming out of the j box to the left are not being used. The orange and the yellow coming out of the bottom go to the stove, the red wire goes to the Rh on the tstat and there is one white wire for the C connection and one for the W connection.

It works like a champ. Here are some benefits...

You can turn the stove on and off from work for your kids, start the stove on the drive home from work and have the house warm when you get home. Monitor the stove in a rental unit or a time share vacation home.

Good stuff!

View attachment 112141 View attachment 112140
Thank you. Got a Harman Accentra 52i in September of 2016 and immediately set out to control it with a nest thermostat. Your wiring diagram and list of parts was exactly what I needed. The only thing I did differently was to set two boxes under the house for the high voltage transformer to be separated at much as possible from the low/milli voltage per electrical code as one of my coworkers is an electrician and advised as such to avoid any issues. I will try to remember to crawl back under the house to post pictures for anyone that is interested.
Has worked flawlessly since install and makes my wife very happy that we can remotely control and/or schedule heat. We heat with the 52i all winter long and only have the HVAC system set to circulate air 15 minutes of every hour when we are home to distribute heat in our two story house.
Sorry I did not give you props earlier.