Check out some similar questions!

HVAC888 · Jan 30, 2007, 03:40 PM

Hi, I'm not a HVAC guy, so please don't read anything into my user name. I was thinking of HVAC when I created my user name.:)

I've followed all the faq and answers here, and am pretty sure I understand how this stuff works, but would like confirmation on the following:

So on my 20 year old York furnace,

I measured 36Vac at the Red and Black (common) terminals at the furnace and at the TH and TR terminals on the gas valve. Shouldn't it be around 24V?

There is a terminal strip inside the furnace, labeled R, C, W, Y, and G. The a/c is connected to C and Y (which is right), and all other wires are connected accordingly. But nothing is connected to W, is this right? Does it matter?

The white wire from the termostat connects directly to the gas valve (TH) though, and a black wire connected to (TR) so I believe that is right.

I have the furnace guy coming on Friday to fix a recent service call, and I will probably pick his brain, but I would like to know if the service guy actually knows what he's doing..

Thanks for all replies.

NorthernHeat · Jan 30, 2007, 03:58 PM

It sounds as if something was jumpered out in the past. Just as long as none of the safeties where removed it should be fine. I don't know why the voltage would be 50% high, I see 22-30 VAC all the time and it makes no difference. Other than the higher than expected voltage is everything working all right? Is your meter a good one measuring true RMS voltage? Are you in the US, 60 hz, 120 V AC?

HVAC888 · Jan 31, 2007, 07:42 AM

Hi, thanks for the quick reply.

I used a Fluke 112 True RMS meter. I'm in Canada, so the voltage is the same.

The terminal strip is the exposed part of a Printed Circuit Board which has a metal cover. I think there may be fuses? Relays? Under this cover?

Above this metal box, is a smaller black plastic encased box. I'm thinking this is the transformer, as there are two screw terminals with two black wires coming off them which are the ones described in my original post.

The gas valve turns on/off, the burners come on and a few minutes later, the fan turns on. So the fan and limit controller works, but the thermostat doesn't seem to be controlling the temp. It seems that it is "preset" for a certain temp. Maybe the fan limit control has the high-low range too narrow for the thermostat?

NorthernHeat · Jan 31, 2007, 04:20 PM

The thermostat is just a simple switch on/off. Is it that the furnace doesn't run enough or does it over heat the house? You never really said in your first post what the problem is exactly.

HVAC888 · Jan 31, 2007, 10:02 PM

The problem is that we have the furnace serviced and the ducts cleaned, and had a humidifer installed recently. But while observing the furnace guy working, I sort of gasped. He knew what he was doing, but he seemed very careless, sloppy almost in the way he worked.

It seems when the furnace guy installed a second transformer for the humidifer, he fried it. So he installed another one, and that was fine. He also put in a new fan limit control.

A few days later I wanted to install a programmable thermostat. I read your faq (excellent info by the way), and connected up the wires, R-R, G-G, etc. Tried the heat on test, nothing happened after 10 minutes. Usually I can hear the furnace fire up and the fan kick on.

I replaced the old thermostat back and everything seemed to work as usual. I ran through your checks and that's when I measured 36V.

I also found that the humidifier transformer had no voltage on the secondary, and the electronic air filter was not on. Hence the service call.

With the service call coming, I'll have him install the thermostat. Hopefully there won't be any problems.

My main reason here is to get a better understanding, so I can question the service guy, and make sure I have him cover all the issues.

Thanks for you helpful information.

nmwirez · Mar 6, 2007, 11:21 AM

With the service call coming, I'll have him install the thermostat. Hopefully there won't be any problems.

My main reason here is to get a better understanding, so I can question the service guy, and make sure I have him cover all the issues.

I would be concerned that a pro HVAC guy would fry the secondary of a xfmr in the first place. Incidentally, C is common, R is the 24v from the transformer secondary in the furnace. If you can find the furnace xfmr and disconnect the secondary momentarily to take a reading....it should be 24Vac. If it does then there is a backfeed condition on that circuit. Hire a licensed professional who knows low voltage control systems. JMO nm

HVAC888 · Mar 6, 2007, 02:25 PM

Nm, thanks for the reply.

The service guy that came was an older person who said he had over 25 years experience. So I felt a lot more comfortable.

He found that a primary lead had come loose from the marrett which had two other primary leads already. All this was hidden behind the cooling transformer.

This loose wire was for the newly installed transformer by the first service call. :eek:

So anyway the wire was put back, and all marretts were taped up (should have been done in the first place).

He measured 26Vac, so everything is in order and now working great.

Anyway, for the thermostat, I installed it myself. I traced the 5th wire from the thermostat back to the furnace and found that it was from the heating transformer. So I removed the jumper from R-Rh and now it works properly.

Now the furnace has three 24V transformers. 1 for the humidifier, 1 for heating and 1 for cooling... Boy talk about complicated... :)

nmwirez · Mar 6, 2007, 03:06 PM

Fantastic, all is well then... nm:)

T-Top · Mar 6, 2007, 07:50 PM

If the service tech did not cut the jumper on the fan limit control you will get bleed through voltage on the low voltage side if you read your voltage at the gas valve with it on (test from one side of gas valve and to equipment ground) I bet you get a 115 volt reading. I have seen this happen and work on only the older style furnaces. The jumper from limit to fan will have to be cut WITH POWER OFF.

HVAC888 · Mar 7, 2007, 07:30 AM

Originally Posted by T-Top

If the service tech did not cut the jumper on the fan limit control you will get bleed thru voltage on the low voltage side if you read your voltage at the gas valve with it on (test from one side of gas valve and to equipment ground) I bet you get a 115 volt reading. I have seen this happen and work on only the older style furnaces. The jumper from limit to fan will have to be cut WITH POWER OFF.

When the first service guy blew the 24V transformer he was installing for the humidifer, it damaged the fan limit control. :eek: So he installed a new one, and I saw him cut the jumper for low-voltage operation.:)