New Honeywell Heat Only Thermostat keeps radiators on constantly. Model T87K1007

dsivie

Have a weird situation in a condo that I own. Building has oil fired boiler and fin-tube baseboard hot water radiator heat. It seems like a very simple system. While remodeling, I decided to replace the decades old mechanical thermostat with a somewhat more modern Honeywell mechanical. There are only two wires in use and I connected them to the "R" and "W" terminals of the new thermostat. For the first few weeks, everything worked fine. If I set the thermostat to 70 degrees, the room became roughly 70 degrees. However, the other day, I came home and the place was over 80 degrees. For some reason, the thermostat is constantly calling for heat, even if I set it at 50. You can still hear the click when the thermostat is supposed to stop calling for heat, but it has no effect on the actual call for heat. The only way to stop the radiator getting hot is to remove the control unit of the thermostat. I unwired everything again and did some tests. When you short the two wires, you get heat and when you unattach them the heat stops. There must be something wrong with the control unit of the thermostat. Has anyone had this happen? Model T87K1007

This is so weird, because it worked perfectly for two weeks and then just stopped. Could the power stealing nature of these thermostats wreak some havoc at the zone valves in the boiler room? Is it worth me checking out the boiler room?

Oh, that red wire was not connected to anything previously. Also, I decided to put a multi-meter in AC voltage mode between the white and black wires. It read between 7 and 8 volts. Does this mean anything?

mattmia2

it looks like that t-stat is rated up to 1 a. What is the current draw of whatever it is controlling? If it is more that the t-stat is rated for it could have welded the relay closed.

dsivie

Well, Is there a way to find out what the current draw is? Would I have to get into the boiler room? I may have to ask building management to let me in there. Are there thermostats rated for higher than 1 AMP?

mattmia2

you could set the meter to the 10a ac or 20a ac range and put the probes in the appropriate jacks in the meter for high current measurement and use the meter probes to close w and r, it will read what the control draws.

Is the t-stat just opening a zone valve for your apartment?

dsivie

I think it is opening a zone valve for my unit. See, there is one large oil fired boiler in the boiler room of this 12 unit building. Why it works right, the call to heat happens and then you can immediately feel the heat of the hot water flowing through the fin tube system.

HVACNUT

That looks like 14 AWG wire. Are you sure its 24 volt controls and not 120v?
Is there a junction box behind the thermostat that was spackled over?
Can you post a pic of the old thermostat and the boiler controls?

dsivie

That is definitely not 14 gauge wire. It is in the 20-22 gauge wire range. I verified with my wire stripping tool. I stupidly threw the old one out, but it was a very old totally mechanical Honeywell (non-mercury). AC Voltage across the black and white wires is about 8 volts.

STEVEusaPA

You probably have an issue in the basement, or the wiring itself is shorted out.
If you remove the wires from the base itself, does the overheating stop? If it does, it's the thermostat.
If it doesn't, it's something else-from the wires, hanging up zone valve, etc.

HVACNUT

Are you measuring voltage while the wires are connected to the thermostat terminals or (hopefully) the wires themselves?
Wire to wire, without a short, it should read 24v. Where is 8v coming from?
If jumping the black and white makes and breaks the circuit, then I'd say it's a bad sub base.

And that's 16 ga. at best.

dsivie

STEVEusaPA said:

You probably have an issue in the basement, or the wiring itself is shorted out.
If you remove the wires from the base itself, does the overheating stop? If it does, it's the thermostat.
If it doesn't, it's something else-from the wires, hanging up zone valve, etc.

If I remove the wires from the block, heating stops immediately. In fact, I don't even have to remove the wires from the block. I can just pull the control unit off the base and heating stops (like in the first photo) If I manually touch the wires together, then heating starts. When I take the wires apart, heating stops. Thats what is so weird. It behaves like I would expect. Maybe my thermostat control unit went bad?

mattmia2

The t-stat is clearly bad (unless in power stealing it is drawing enough current to close the valve). It is never a great idea to use an electronic t-stat without a separate wire to power it.

dsivie

mattmia2 said:

The t-stat is clearly bad (unless in power stealing it is drawing enough current to close the valve). It is never a great idea to use an electronic t-stat without a separate wire to power it.

Are there still pure mechanical, heat only thermostats available? Like that use a simple expanding metal strip mechanism to call for heat?

Jamie Hall

Did you throw out your old one? Why did you swap it out?

The answer to your question is e-bay. Lots of them, some used, some NOS.

dsivie

Jamie Hall said:

Did you throw out your old one? Why did you swap it out?

The answer to your question is e-bay. Lots of them, some used, some NOS.

I threw it out for a dumb reason. The color did not match the new wall paint. But, just to clarify - if I disconnect the two thermostat wires from the block and then setup a multimeter in AC voltage mode and then touch one probe to each wire, should I read between 20 and 30 volts?

mattmia2

Yes, assuming it is a standard system you should read 24vac between the wires with nothing connected to them. If it is some controller that uses some other voltage then that could be the issue, the t-stat could have been working until its battery was depleted then it malfunctioned.

HVACNUT

Why would you substitute digital for a bi metal helix?
A simple Honeywell battery operated digital would be more accurate.

mattmia2

Slightly more accurate doesn't matter much if the battery goes dead while you're away.

dsivie

mattmia2 said:

Yes, assuming it is a standard system you should read 24vac between the wires with nothing connected to them. If it is some controller that uses some other voltage then that could be the issue, the t-stat could have been working until its battery was depleted then it malfunctioned.

I'm really confused honestly. I bought another Honeywell T87K1007 and went down and looked at the TACO controller. With the new thermostat in and the temp all the way down, the call for heat light and zone valve open light are now blinking on and off. I wonder if this is the unit attempting to steal power and charge itself? Anyway, I'm going to set it at 70 degrees and leave it overnight and see what happens. I tell you, I keep reading 8 volts AC across the wires. When I put the multimeter in AC current mode, I read 1.6 Amps across the 2 wires. I'm actually worried I damaged the zone valves or TACO zone control board.

Jamie Hall

Whoa. This is a power stealing thermostat -- but it is NOT set up as some other power stealing thermostats are, to take power from the common. With a Taco zone controller (or, for that matter a zone valve), it should have only R and W connected (Y is used for the odd instance of a power close zone valve).

There is no way that you should be reading 1.6 amps across R and W -- and no way you should be reading only 8 volts. Both of those numbers are telling me that there is something wrong either with the wiring from the controller to the thermostat or in the controller. Therefore... you need to start back at the controller. Take the thermostat wires (R and W) off the controller. There should be no heat call. Voltage should be 24 volts more or less. Now jumper those terminals there. There should be a heat call. Take the jumper off and short the terminals with your ammeter. You should get a heat call, and some reasonable current -- perhaps a few tenths of an amp.

If that passes, with the wires still off the controller and disconnected from the terminals in the thermostat base plate, measure the resistance between the two wires. It should be at least 1 megohm. Probably a good bit more, if your meter goes high enough.

If that passes, measure the resistance (wires still disconnected) between R and W on the base; that should also be a megohm or greater.

If that passes, connect the wires to the controller. You should get exactly the same readings at the thermostat ends of the wires that you got when you were working at the controller.

If that passes, connect the wires to base and work between the R and W terminals. There should be no change.

Now pick up the thermostat and measure the resistance between R and W on the thermostat when there is a call for heat; should be zero. and when there isn't; it should be significantly higher (but not a megohm -- your meter will be trying to charge the battery).

Somewhere in that sequence you should find an anomaly.

mattmia2

If it is reading 1.6 a that is about 50% more than what the relay in that thermostat is rated for.

Do you happen to know the model of the Taco zone controller? Others here will know if some of them use a nonstandard control voltage and require a mechanical t-stat or a t-stat with a separate power source.

STEVEusaPA

You also shouldn't have replaced the thermostat without shutting off power to the control.
Your original T87 (not an original mercury T87) is kind of junk. I've had a few bad out of the box.
T87 is not a power stealing thermostat.

dsivie

mattmia2 said:

If it is reading 1.6 a that is about 50% more than what the relay in that thermostat is rated for.

Do you happen to know the model of the Taco zone controller? Others here will know if some of them use a nonstandard control voltage and require a mechanical t-stat or a t-stat with a separate power source.

Yes, I got into the boiler room. The model is Taco ZVC406-EXP. I have a question. This is a 1955 era building and the previous thermostat must have been there for decades. Is it possible that some screwy messed up wiring just happened to work OK with the old mechanical thermostat, but is highly non-standard and problematic for newer systems?

mattmia2

@STEVEusaPA According to Honewell's page for that submodel of stat it is "battery assisted power stealing"
https://customer.honeywell.com/en-US/Pages/Product.aspx?cat=HonECC Catalog&pid=T87K1007/S&category=&rank=&v1=Sort.1.Product.Rank&asc=1

I suspect what is happening is that the Taco control for whatever reason doesn't provide 24vac because it just expects a contact closure and over time the internal battery went dead.

STEVEusaPA

It's more likely all the on/off you performed with the wiring, did something.

HVACNUT

> @mattmia2 said:
> Slightly more accurate doesn't matter much if the battery goes dead while you're away.


Replace them at ST and DST. Just like the batteries in your smoke detectors.

And I could have installed a battery powered digital 6 hours ago and we'd all be at the bar by now.

dsivie

HVACNUT said:

> @mattmia2 said:

> Slightly more accurate doesn't matter much if the battery goes dead while you're away.





Replace them at ST and DST. Just like the batteries in your smoke detectors.



And I could have installed a battery powered digital 6 hours ago and we'd all be at the bar by now.

Well, this is for a rental condo, so I would like to make sure it stays working. But honestly, if a simple battery powered thermostat fixes the problem, I'd go for it. I would just buy the most expensive, longest lasting lithium ion battery and put it in there. It would have to last years

Jamie Hall

First, have you followed the troubleshooting routine I suggested? Or something like it?

Second, is there a really good reason you have to have a fancy electronic thermostat in there? Battery powered or not? What is wrong with a more or less bulletproof mechanical thermostat?

dsivie

Jamie Hall said:

First, have you followed the troubleshooting routine I suggested? Or something like it?

Second, is there a really good reason you have to have a fancy electronic thermostat in there? Battery powered or not? What is wrong with a more or less bulletproof mechanical thermostat?

I haven't tried all your troubleshooting steps yet. I want to, but it is a little dicey. See, I own one unit inside a larger condo building. I have a key to the boiler room, but no one has ever said if owners are allowed to go in their themselves and touch anything. I went into the boiler room tonight, just to look at the Taco controller. It is interesting. I have definitely isolated my zone and thermostat wires. It seems they exist in three possible states:

Zone light T Stat Call OFF: When the thermostat is removed from the wall and the wires are disconnected.

Zone light T Stat Call ON: When the thermostat wires are shorted or the thermostat is actually calling for heat (room temp lower than thermostat setting)

Zone light T Stat Call LIGHT BLINKING ON AND OFF: When the thermostat is connected and the temperature in the room is higher than the thermostat setting. This is the anomalous case. No heat should be delivered in this scenario, but it is clear the zone valve is delivering some heat as it blinks on and off.

My next step should be to try your troubleshooting steps, but because of the permission issues, I may have to call a pro

Dave H_2

Looking at the Taco ZVC controller, are the lights on the front with a plastic cover or on the side with a metal cover?

Open up the cover or look around the controller for a bag of resistors (1k ohm, .5 watt). Put one resistor across TT terminal of the controller.

It has been experienced with some thermostats that "weird" stuff happens. It's never pinpointed to "over heat" or "underheat". Both could happen, one happens, sometimes, and then never when you look at it.

Its the power stealing type stats that keeps the switch either closed or open in the thermostat.

Dave H.

dsivie

Dave H_2 said:

Looking at the Taco ZVC controller, are the lights on the front with a plastic cover or on the side with a metal cover?

Open up the cover or look around the controller for a bag of resistors (1k ohm, .5 watt). Put one resistor across TT terminal of the controller.

It has been experienced with some thermostats that "weird" stuff happens. It's never pinpointed to "over heat" or "underheat". Both could happen, one happens, sometimes, and then never when you look at it.

Its the power stealing type stats that keeps the switch either closed or open in the thermostat.

Dave H.

The lights are on the plastic. Attached is a photo. My zone is 5 and it is currently calling for heat as the thermostat is set higher than room temp. There is a bag of resistors on top of the Taco

Dave H_2

Put one of those resistors inside on the terminals W and C.

Dave H.

STEVEusaPA

Forgot it had an internal non replaceable battery.

rick in Alaska

I have never had god luck with that thermostat, even right out of the box. I guess If i run in to any more having issues, I will at least remember this thread and look in to the power robbing issue. But, I won't install any more.
Rick

dsivie

Well. I returned the Honeywell thermostat and I bought a pure mechanical - heat only thermostat. I wish I could have through the troubleshooting steps mentioned here, but I just got nervous about my liability for possibly damaging equipment. I'm probably not even supposed to be in the boiler room of our multi-unit building let alone taking the cover off of the zone control unit. I'm hoping the mechanical thermo just keeps a consistent temp so I can finish renovating and getting ready for rental. The only thing I don't know is what to set the anticipator to. Out of the box, it is set at 1.2 A. Any simple way to know what to set this to?

mattmia2

Set it to what you measured before which was 1.6 amp. if it runs cycles that are too long (gets too hot then too cold) you can bump it don a bit, if it isn't 24v the anticipator might not produce enough heat (the anticipator is a resistor that heats the element in the t-stat to make it shut off sooner to anticipate the residual heat in the heating system, in your case the hot radiation)