I've installed a Honeywell RTH7400 thermostat on a heat pump system with electric resistance auxiliary heat. The thermostat is properly wired and programmed but would not operate properly. It would run the heat pump for less than a minute and shut the system down.

After an hour with Honeywell's India call center we found the following. Pulling the 'C' wire from the thermostat base permits the system to run normally. In other words, the thermostat runs fine on internal 'AA' battery backup but not AC power.

The tech says the thermostat is rated for 24-30VAC at 1A and questioned power quality which is not an issue. I ran a wire pair to the baseplate and measure 28.3 volts which sags to 26.5 with relay closures.

For some reason this thermostat does not like to see AC power... which the old dual mercury switch model never minded.

Look here see page 12 number 4

http://customer.honeywell.com/techlit/pdf/PackedLit/69-1900EF.pdf

Something is wrong somewhere. Post your wiring from your heat unit to the thermostat and how you connected it.

hvac1000,

Thanks for your quick response. Seven wires run from the heat pump with backup electric heat to (old) Janitrol mercury switch HPT18-60 thermostat as follows:

Wire--->Thermostat
Yel Y
Grn G
Red R
Org O
Brn C
Blu E
Wht W2

Using jumpers and volt/ohm meter I've established function for these:

Yel Heat pump
Grn Fan
Red 24VAC
Org Cooling changeover valve
Brn AC Common
Blu Emergency heat
Wht Aux heat

Honeywell RTH7400 thermostat wired per the manual:

Yel Y
Grn G
Red R
Org O
Brn C
Blu E
Wht Aux

R and Rc are jumpered.

Symptoms: With C wire disconnected, thermostat works properly but on AA battery backup power. With C wire connected, Emer heat does not function. Normal heat runs for less than a minute, systems tries to select aux heat and shuts down. This repeats after 5min compressor safety time.

Honeywell tech support says the problem is with AC power quality and says unit requires 1 amp 24 to 30VAC. I measured voltage under thermostat load at 28.3V with sags to 26.5 with relay closure in heatpump. AC mains power quality is good with no spikes or dips.

If you can steer me to a wiring or other error I would be most appreciative.

hvac1000,

Thanks for your quick response. Seven wires run from the heat pump with backup electric heat to (old) Janitrol mercury switch HPT18-60 thermostat as follows:

Wire--->Thermostat
Yel Y
Grn G
Red R
Org O
Brn C
Blu E
Wht W2

Using jumpers and volt/ohm meter I've established function for these:

Yel Heat pump
Grn Fan
Red 24VAC
Org Cooling changeover valve
Brn AC Common
Blu Emergency heat
Wht Aux heat

Honeywell RTH7400 thermostat wired per the manual:

Yel Y
Grn G
Red R
Org O
Brn C
Blu E
Wht Aux

R and Rc are jumpered.

Symptoms: With C wire disconnected, thermostat works properly but on AA battery backup power. With C wire connected, Emer heat does not function. Normal heat runs for less than a minute, systems tries to select aux heat and shuts down. This repeats after 5min compressor safety time.

Honeywell tech support says the problem is with AC power quality and says unit requires 1 amp 24 to 30VAC. I measured voltage under thermostat load at 28.3V with sags to 26.5 with relay closure in heatpump. AC mains power quality is good with no spikes or dips.

If you can steer me to a wiring or other error I would be most appreciative.

From instructions on page 12 number 3 red triangle

If your old thermostat had separate O and B wires, attach the B wire to the C terminal. If another wire is attached to the C terminal, stop now and contact a qualified contractor for help.

Remember yellow is for cooling and on your second post you said you used a ohm meter and determined that the old thermostat used it for the heat pump. That part of your diagnosis in incorrect.

If this does not work you will need to follow the wires to the equipment and find exactly where the wires go. A ohm meter is of no use but you can look at the landing area in the furnace and see exactly what color goes where. I will work with you on this but I need factual information so follow the wires and post back what color lands on what terminal on the equipment.

hvac1000,

Wish you could have been here... the best part was bringing the heat pump fan/coil cover down from the attic, carefully placing it on the copy machine and making the attached copy of the schematic. Referencing the terminal block (TB) on the left side, the thermostat wires are connected as follows:

Thermostat-->Terminal Block
Red R
Grn G
Org O
Wht W2 (internally jumpered to W1/W2, E and OTC)
Blu E (jumpered as above)
Brn OTC (jumpered as above)
Yel OTC2
Blk Unused

Of course there are also wires running to the compressor which are not shown above. A good wool cap and a LED headlamp have come in handy here.

See note 3 on the right side lower of the diagram. Do you still have the install instructions hanging on the unit or in the unit housing?

See note 5 and confirm wiring of emergency heat relay.

The wiring for the O terminal is not addressed on the diagram? It might be on the item mentioned in number 3 above.

hvac1000,

Good suggestion, I've found the instruction manual and starting to go over the schematic.

You got lucky finding the manual. The main diagram you posted gives some info but not all.

There is a possibility that there is a short in the wiring. If there are metal studs, it could even be in the wall.

Try connecting the tsat to Rc/RH and (C). Leave all other connections open.
Try with and without batteries and measure the voltage between these terminals.

If the voltage is OK and the stat is operational (agreed it won't control anything), then try connecting one wire at a time.

If you still suspect the wiring, you can always connact the stat directly to the furnace.

Don't forget that the outside unint t-stat wiring could have shorts too.

The shorts can be to ground or another terminal.

I recently troubleshooted a system where a 4 conductor wire was run for an utside unit that required two, and there were two shorted pairs.

Gentlemen,

I found the problem in the upstairs system... a 24-year old wiring error by the installing technician. In the air handler, the thermostat common wire (C) was connected to aux heat (W1/W2) on the terminal block. This meant that whenever the thermostat asked for aux heat, the thermostat common line would jump up to 24V.

For 24 years the old mercury thermostat didn't mind this since it had only passive components. The Honeywell electronic thermostat did not like it all when what should be AC ground banged up to 24V - and it went tilt. This accounts for the fact that it ran okay on battery backup with the hot wire (R) disconnected.

Now to figure out what's going on with the downstairs systems. Thank you for your advice.

Great. It is always a good idea to address the drawings if you can when all else fails. LOL

Your welcome. I had to also deal with the idiot that splices the t-stat wire and changed the colors. So, you know that was tough.