Furnace failure, please help

bio_guy

Furnace quit on Monday evening. I took my time to investigate because there are lots of other things going on. It is an old Ducane horizontal furnace in the attic with a Honeywell controller (ST912GC, maybe) Fan Timer and SV95D1 Smartvalve,

After a call for heat, the combustion blower spins up and the furnace apparently ignites normally, but the circulating blower never engages. After a relatively short time, everything shuts off and I get no AC step-down volts at the terminal strip. After quite a long time, apparently it resets and can go through it again. The blower motor spins freely. The safety switch on the blower access door seems to be working, opens and closes.

Where do I go from here, check the motor capacitor, trouble-shoot the control board? I don't have a meter with a capacitor check, but I suppose I could buy one. Is there anything that I am overlooking that is easy to check?

That is the end of the furnace problem description. Thanks for reading. What follows is why I am not panicked to fix it.

Thanks for looking

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Don't send too much sympathy my way. Yes, It is cold in S. Louisiana, but I have mini-split heat pumps. I think I was a little below the design temp Monday evening and it got a little chilly in the house with the compressors spinning very fast indeed. At the time of installation, gas was relatively expensive, but price dropped very quickly after the first half winter with them so we went back to using the furnace and ducts for heat for the most part. Part of that is cost and part comfort and convenience. At the time, the controls for the Mitsu mini splits could not do a set-back, only on-off. I like to sleep cool, but have it warm when my feet hit the floor so that does not work.

Ironically, I was thinking that it might be time to recalculate cost to heat with the heat pumps vs. furnace :-) No matter how that looks, I'd probably fix the furnace if it does not cost much. I can get power to it and run it with a portable genset rather than run all the heat pumps and there is still the (lack of) set-back thing.

Again, thanks

Big Ed_4

First , you need to check if proper voltage is going to blower motor ...

mattmia2

you would only need an ac voltmeter to check the cap. Does the blower motor hum but not turn or nothing at all? Does it run if you set the t-stat to fan on?

pecmsg

mattmia2 said:

you would only need an ac voltmeter to check the cap. Does the blower motor hum but not turn or nothing at all? Does it run if you set the t-stat to fan on?

A little knowledge and PPE is also required.

Bronxtech

How about the temp. Limit switches that feed / supply juice for the blower control.?

JUGHNE

Put "Fan On" switch to on....if fan runs, then see if heat will stay on with the manual fan switched on.

bio_guy

Thanks, the blower does not start from a manual call for the fan. I did not note any humming nor any singing, nor whistling. I am pretty sure I asked for fan only while in the attic so I had a better chance of hearing compared to listening over the rest of the ruckus. I remember to try that first thing I am face to it again.

I can check for line voltage to the motor, I guess. I don't know much about capacitor-start or -run motors although I did prime myself a little on a fractional HP, vent fan motor in my garage last spring. I can't comment on any other switches as I've only had the covers off the control board cover and the blower at the return. I am respectful but confident with line voltage. I've done a little work with the three phase open delta power supplied at my home. This is obviously single phase 120 so nothing complicated. I just have to draw myself up and think a second time before I touch anything. Unfortunately, the toggle switch box for this furnace is in a spot that would be easy to roll over and trip while in the very place I am trouble-shooting. I'll have to do something about that before I touch it again since I am about to escalate my exposure.

What next, check for proper voltage at the motor during the start up?

If 120 true, what after that? Capacitor?
If no voltage what next?

SuperTech

If you have no voltage to the fan motor with the fan switch on and the capacitor tests within the acceptable range (usually +or-6% of the microfarad rating) the next thing to check is the 24 volt side of the furnace.  Check for voltage between the G and C (common) terminals on the control board or fan relay. If you have around 24 volts there and no power to motor then the relay or board is bad. 

Some techs take a shortcut and remove the wire for the heating or cooling fan speed and connect it to a hot 120 volt terminal or wire to see if the motor starts. I don't recommend this. It's best to use a meter and check everything the right way. 

mattmia2

SuperTech said:

Some techs take a shortcut and remove the wire for the heating or cooling fan speed and connect it to a hot 120 volt terminal or wire to see if the motor starts. I don't recommend this. It's best to use a meter and check everything the right way. 

That is just way more work than using a meter...

EBEBRATT-Ed

@bio_guy

Check the motor wiring and the capacitor.

unclejohn

I'm one of those guys that pull the wire off and connect it to line voltage and then I know instantly if I have a good motor or not. 

bio_guy

Thanks, that all makes a lot of sense to me. You've cut my research time to nothing. I am lucky to a have a more or less complete backup system in place so this can be leisurely :-)

bio_guy

Well, I took my leisurely time with this. We had the nationwide cold snap breathing down our necks right before Christmas. 18k of 51kBtu of the heat pumps was not working and 9000 of that is in a utility room that we don't usually bother heating. I thought that the furnace was the quickest path to sufficient heat. Back up in the attic, I found that the only thing that would fire up was the inducer blower rather than just the AH motor not spinning up after ignition. I spent quite bit of time figuring out how the beast works, rectification sensor are a cool idea. I did not like the volt readings coming out of the smartvalve to the ignitor under load. I was a bit puzzled so I called and experienced friend.

He said to stop screwing around trying to figure out how it all works and just check the flame roll-out sensor and the pressure switch for the inducer and if that all checks out, look for a bird's nest on the flue. Sure enough, the new problem was the pressure switch to prove the inducer blower. It was just stuck; with that straightened out, I just bypassed the AH blower control part on the board so we had an on/off furnace for the first, coldest night. (I did not want to run the blower continuously so I just turned it on/off at the breaker. It needed to run for long, long periods anyway. I had planned on running out on early Friday, Christmas eve, but I remembered that I had a pretty universal relay at work that I had on hand in case I needed one to fix one of my low-temperature cut-outs (different designs) on our laboratory refrigerators (DPDT with 120 and 24V coil connections). Using that was going to be quicker so I decided to control the AH blower with the 120V inducer power so the former spins up early. I grabbed my crimper some crip-on connectors, wire and a couple of wire nuts and made up some relatively neat patch harnesses before I crawled through the attic again.

It works pretty well, I think. At some point, I guess I'll have to decide if I am going to leave it be, buy a new control board, or find the AH relay on the board and bypass it properly (soldering on the board or add some connections for an off board replacement relay.) I can hear the AH relay clicking, I think, but the contacts are probably be burned out or a track on the board is cooked.

I did not check the heat pump installer's heat calculations before the cold snap. I see now that his recommendation was for about 32.5 kBtu heat and the design temp around here is about freezing. We went way below that with a lot of wind. At the temps we saw, I might have to derate output by 10% or so leaving us with about 30k, or 22 discounting the utility room heat. We would not have frozen but the house would have been pretty cool and drafty with no heat pumps in three rooms.

leonz

AH maybe it is time to retire the beast and install a new one??

Are you anywhere near Franklin, La.?

Jamie Hall

You not only have to consider the derating of the heat pump with cold source air -- and 10% is generous; it was probably more like 20 to 30% -- but you also have to remember that the heat loss of the house is more or less linearly related to the temperature difference between inside and outside.

So -- if your system is designed for 32 outside and 72 inside, what that really means is that it is designed for a 40 degree temperature differential at full rated load. So if it got down to say 10 outside, it wouldn't be able to maintain more than 50 inside -- even before it was derated for source.

This is why I've never cared for the notion of designing for less extreme conditions than the 99% degree day. True. It doesn't get that cold that often -- but...

bio_guy

Jamie, I am not certain that I understand everything that you are expressing, but I am looking at real test data for these systems where an independent agent varied conditions the "outdoor" (test chamber) temperature, fan speed of the indoor unit, and the compressor speed (they wrote "indirectly....," and I think that they mean by changing the differential between the set point and actual "indoor" temp in their test chamber). I eyeballed the graphs to come up with the derating. As another example from their data, a unit with heating output, steady state, 70 F return , high fan, putting out approximately 19kBtu at 40 F yielded approximately 12,000 at 10 F and 7,000 at -10.

Since some of my rooms are smaller than the minimum wall unit available, I ended up with about 52kBTU for the house which should be plenty when (if) it gets into the mid 20s here. Temps might might get a little uneven from room to room if I were running everything flat out if we had record lows with wind.

bio_guy

Leonz, I am not sure what beast, although probably the furnace. I can't see replacing it since we don't get much use out of it in this part of the country, especially since I now know how it works and how to fix it. Anything higher efficiency probably can't go in the attic 'cause the condensate will freeze and the ROI would be way too long to be economic. If I am going to continue using that system for heat, I should probably go up and re-seal and otherwise check the duct work, I am sure it has some leaks.

The busted minisplit system is 1 (24kBtu) of three compressors (other two 20k) and three indoor units, 3x6kBtu). They have all been operating for about 12 years. Repairs to that include a reversal valve (warranty replacement), two indoor blower motors and two boards on the smaller outdoor units, same board on both. The first board replacement I bought a new one. Second time around, I took it to a local electronics repair shop that does some work on VFDs. He fixed it for about half the cost of a new one. Some very big chips were replaced.

Choosing what to heat with will take some calculations and thought. One thing that I think most people do not take into account in a situation like this, heat pumps vs. dumb furnace, is the cost wear on the equipment -- lifetime cost for dumb furnace is probably a lot less than a heat pump.

The reason I went to mini splits was that some of the duct runs were terrible, down a central-located chase from the attic to the open crawl under the house to the back of the house. When they first added central HVAC to the 1940s house (I suspect in about 1950), they did a decent job dropping the ceiling from 9 to 8' in the central hallway, and ran all the hard duct supply through there. Too bad the furnace was not indoors as well, but at least the bulk of the ducts were. Later, the furnace was moved about 10 feet and some flex added to the original. That probably all happened with some room additions and wall adding/moving.

P.S. Not very near Franklin. I work in NOLA and live just off Jefferson Hwy. very near (downriver from) the Huey P.

Jamie Hall

The derating aspect of my comment really is a ballpark -- and it's important to remember that various systems will vary in that respect.

What I was really driving at was not that, but the concept of maintaining a given temperature differential between inside the house conditions and outside the house conditions -- and it applies to any heating or cooling system. Put simply, if a given system is designed to maintain a 40 degree differential under certain wind conditions -- let's say still air for simplicity -- that is what it will be capable of doing. Period. If the outside air is at 30, it can maintain 70. If it drops to 0, it can maintain 40, and no amount of hope is going to change that.

mattmia2

Neither the board nor the relay is likely burned, there was likely little solder between the 2 and the combination of the mechanical strain from opening and closing and the current probably broke the solder joints at the load leads for the relay. sometimes i wrap some stranded wire around the lead and solder the whole thing when i fix something like that

mattmia2

bio_guy said:

Choosing what to heat with will take some calculations and thought. One thing that I think most people do not take into account in a situation like this, heat pumps vs. dumb furnace, is the cost wear on the equipment -- lifetime cost for dumb furnace is probably a lot less than a heat pump.

Well designed and properly designed split systems last for decades. Not sure that well designed is easy to find with the mantra of make it cheaper driving every decision.