[cadesmarais]

I have a Coleman 3300L823 (23Kw /78,000 BTU) furnace with 4 elements stacked vertically (bottom-up 1, 2, 3, 4) in the heater box above the blower .

Sequencers are working correctly. The elements are all in good shape. However, when the weather gets around 0 degrees F, the furnace has a hard time keeping up with the natural heat loss of the house as elements 2, 3 & 4 are being switched on & off by the bi-metal limit switches connected in series to each element. It is a boderline situation, as the elements will cycle in & out as the limit switches detect 'a little too much heat'.

These 4 limit switches are designed, I believe, to prevent an overheating element or elements from overheating the furnace and potentially cause a fire.

The limit switches are not adjustable. I have tried spacing the limit switches to give them more a little more latitude or tolerance to radiated heat, but to no avail as the radiance from the lower elements reaches the next level of elements & the limit switches then knocks off elements 2, 3 & 4, sometimes only leaving the lowest element working.

Other than bypassing some of the limit switches (defeating their purpose, and somewhat risky), any ideas/thoughts to solve this annoying problem.

Thanks in advance.

André (cadesmarais)

[KC13]

You are right, the limit switches only come into play if the unit is running too hot. Make sure the filter is clean, registers and returns aren't blocked, air conditioning coil isn't clogged with dirt (if equipped so), and correct blower speed is selected.

[hvac1000]

Move more air across the elements to for the heat out. IE increase the blower speed.

If this is in a mobile home high static of the ducts can be a problem.

[cadesmarais]

Gents,. a further question - on a forced air electric furnace, what is a reasonable temperature differential when heating between return & supply sides?

Testing it with 3 of four elements on, I noted 70-74 deg F on the return and close to 130 deg F on the supply... ie: just after the air handler.

Thanks (CADesmarais)

[hvac1000]

Temp rise should be posted on the unit in degrees.

On a side note: I am glad I will not have to pay your electric bill.

[cadesmarais]

Hi again HVAC1001. Seems I can't rate your response before 'spreading ratings around' (whatever that means!)

- No indication of temp rise indications on the unit or in the manual. Must say this unit is from the mid-eighties, so a lot of what's expected today of manufaturers was not then.

... about electricity bills & rates. Pointe-Claire (Montréal), Québec is darn cold early this year, but the hydro rates are also low... CAN $0.054 /Kwh, which at today's exchange is about US$0.043 /Kwh. In any event tomorrow's forecast is for a milder 30-32 F (~ 0 C).

Best regards,. André (CADesmarais)

[hvac1000]

50-60 degrees rise is a bit high in my opinion for electric. Drop one of the heaters out and take another reading.

[cadesmarais]

Hi again.

An update FYI.

The air handler was definitely looking for more fresh air. After some testing (removing a filter), I added an adjustable 4"x12" register just after the filter /before the evaporator coil and this settled the return air temperature so that the limit switches do not cut off power. I had to wait for a few colder days to confirm the problem solved.

The filter bypass register will allow some unfiltered air through, about 12%, but it will recycle and eventually get filtered.

Not a perfect solution, but it works!

Many thanks to KC13 and HVAC100 for your tips and helping push the troubleshooting through to solving the problem.

Best regards,. André (CADesmarais)

[KC13]

Well, okay, but this sounds more like symptom treatment that problem solving.

[hvac1000]

At least it will keep working without droping out.

[cadesmarais]

Hi.

I agree. Hey - but it works!

As I see it, the root problem is the furnace design where furnace elements are too close together and limit switches pick-up radiated heat from other lower elements.

Also, adding the high efficiency filter last year has only made a borderline situation worse.

Replacing the furnace, adding/enlarging several air returns or putting in a blower that runs a higher cfm (than manufacturer specified) are other solutions, but more $$$ to spend and noisier air flow I don't want for now.

This air handler unit has served me well (since installed in '86) except in these somewhat extreme temprature conditions.

Bear in mind that the air flow system in this house was built ('61) for oil heat forced air, which was converted to a heat pump/electric furnace in the mid-eighties. In such conversions, the air returns are almost always a problem because of the lower differentials.

Thanks again for your help.

Best regards,

... André (CADesmarais)

[hvac1000]

Return air is always a problem period. The only systems that have a guarantee return air performance is the old gravity systems. The old men (dead now) new that it had to be huge for the system to work. LOL