Baby It's Cold Out There!

[CJrun]

So we bought a new/old house and the AC worked just fine, but naturally we didn't test the heat, as we bought it in the summer and we live in Florida.  It has an old gas/electric HVAC system (Central Furnace, model number DCHG-F036N100B), that I figure we will have to replace soon, but I'm hoping I can get some use out of it first.  The temp is dropping into the 40s tonight and so I tried cranking up the heat.  Nothing.  I followed the instructions (there is no pilot, just an electric igniter).  Nothing happened.  Gas was in the ON position, (now off).  I don't have a clue as to where to start with this thing.  I'll attach a few pictures and I will appreciate any thoughts.  Thanks!

Edited November 1, 2014 by CJrun

[BryanS]

Not to be smart, but do you have gas tank. If so, has it been filled. It happens more often than you think where people call and a tech gets there to find no gas or the tank is locked out. You may want to go make sure the gas is on outside as well. Also if this thread gets deleted it is because you didn't put your model in the title.

Oh wait. I see on your data plate you have natural gas, so you wouldn't have a tank. I haven't messed with that much so I don't know where you shutoff is.

[BryanS]

I see DSI on you wiring diagram, so you have direct spark ignition I believe. Are you getting any spark at all? Is this a heat pump with gas backup? Typically, if that is the case, then your gas furnace won't come on until the thermostat sense the heat pump can't keep up. Also you could kick it on by switching it into emergency heat if heat pump is your main heat source.

[CJrun]

I'm not seeing a way to edit the post and add the MN to the title.

 

Meanwhile, this is a Forced Air Furnace With Cooling Unit, according to the label, so I don't believe there is a Heat Pump.

 

Yep, we are plumbed into natural gas and the water heater is working fine, so I presume gas is making it to the furnace.

 

How do I find out if I am getting spark?

 

Thanks!

[BryanS]

I believe you can usually hear a clicking like its trying to ignite. I'm not really the best person to give you a step by step troubleshooting of gas furnaces. For the most part I know how they work, but I haven't worked on many at all.

The sequence for a hot surface ignition style is as follows:

1.Thermostat calls for heat.

2. CPU verfies pressure switch contacts are open.

3. Inducer fan runs a prepurge cycle.

4.pressure switch contacts close (proves inducer fan is running/adequate airflow through the heat exhcanger)

5.HSI is energized amd warms up ( in your case I would imagine direct spark ignition starts)

6. Gas valve relay contacts close applying power to the gas valve.

7. HSI ignites the gas ( direct spark ignition).

8. Flame sensor has a few seconds to prove flame.

9.Ignitor deenergizes soon as flame is proven.

10.Once flame is provem, the blower motor is energized 25 seconds after the gas valve opens.

11. Thermostat is satisfied , cuts off gas valve.

12. Inducer fan purges combustion gas

13. Blower motor continues to run for 180 seconds.

This sequence is from a set notes when I was in HVAC school. Yours should run pretty similar to this. I guess observe your furnace and see what steps you to. Depending on the age you may not have an inducer fan if I'm not mistaken. Usually, it just a black motor behind your main access panel and the pressure switch is a round switch with little black hoses and 2 wires. If you have a heat pump, then your indoor unit and outdoor unit would be on when you put it to heat. You would be getting heat as well out the vent. I thought Florida uses a lot of heat pumps. Gas and heat pump wouldn't be running at the same time.

[CJrun]

This is an older unit; the label has 1988 on it, though that may not mean anything.  I don't see anything that looks like a CPU.  I hear no clicking, as I might from a gas range when you turn the knob on.  I assume I am getting no spark, but don't no where to begin to test whether it's the igniter itself or something upstream.  Are there things like thermo-couples, or fuses?  Is that the pressure switch, sitting behind the gas valve in the first photo?  Do I test it for resistance?

 

There is no inside unit, as the air handler is built in.  Ductwork runs under the house from the unit to floor vents.

 

When there is light, and a few more degrees outside, I will poke around some more and update this post.  I have a lot on my plate tonight and tomorrow, as we are also getting ready to have the house ready for tenting/termite treatment, Monday morning.  I may not keep up with responses to this thread as often as I should, as we need to be ready to get into a motel on Monday morning.  We would be calling in a pro on Monday morning, but on Monday morning, that unit is going under a tent, with the rest of the house.  For now, for tomorrow, some easy/obvious (to experts!) trouble-shooting tips would be greatly appreciated.  Is there something that always goes out with these units that I have some chance of being able to repair/replace on a Sunday?

 

Thanks again!  And Brrrr.

[BryanS]

From what I can tell you have a venter motor and flame sensor by the schematic. Venter motor is probably an inducer fan I would imagine. I don't see any fuses. I do see a few limits in there. No board from what I can tell. Just the DSI which is probably more a less a terminal block. I don't know for sure. Usually, I can figure units out when I get there, but in this case I don't know much about this exact model. Not to mention, I can't just run the model as easy as an appliance.

Edited November 1, 2014 by BryanS

[CJrun]

So, the house and this unit are now tented.  When the tent comes off, do I test the pressure switch to see if the contacts are open?  Do I check the venter motor for AC power?  Do I check the DSI for AC power?  Does the igniter fire repeatedly/constantly?

 

We'll call in a pro, but I would like to understand this unit better and make my own judgment as to whether it needs to be replaced immediately or we can take some time to decide what we want to do.  Thanks again, folks.

[BryanS]

You can remove the hose and blow through the pressure switch while testing for ohms to see if the contacts open and close. You should be able to hear the inducer fan run, but you can check for power. Should be 120v. As for testing the DSI, I would assume you just check voltage going in. The one gas furnace I worked on with direct apark ignition I could hear it sparking and it just wasn't getting gas. It had a bad gas valve. Voltage to the valve but no gas. It was a business and the other furnace ran fine so I know he had gas in the tank.

[CJrun]

Ah-hah!  There's a pressure switch to check, but also a gas valve relay to be checked.  In my first photo, I'm guessing the pressure switch is on the left.  Where would the gas valve relay be? I'll try to get some better photos when the tent comes off, tomorrow.  And we probably call in a pro if I can't figure out something sooner.

 

Any chance somebody has a diagram that is more typical than the schematic, that may help me figure out what the various parts are?  I can tell what and how things are connected from the schematic, but do the parts themselves look like and where are they located?  Is the venter/inducer fan, the squirrel-cage fan?  These questions are better answered after I take better photos.  To post better photos, I will have to re-start this post/thread.  The image link in the Reply window always freezes up on me.

 

Thanks again, folks.

[BryanS]

Your top picture is the gas valve. Typically your pressure switch is just a circular disc with two hoses hooked to it. One hose should run to your inducer fan and I think the other is from the heat exhcanger. It's basically reading a difference in pressure between the exhaust fan and heat exchanger. It prevents the unit from running if your exhaust vent is blocked. If you don't have anything running at all during heat mode then I would start looking at your thermostat first. It may not even be sending a signal down to kick the heat on.

[CJrun]

Well, more tomorrow when the biohazard tent is removed.  For now, just curious: Thermostats sometimes function properly on the cooling side, but fail on the heat side?  Or more likely a wiring issue causing the heat side to not communicate with the thermostat?

 

Thanks.