Drolet HeatmaxII variable blower speed

[andym]

As some of you know I installed a HeatmaxII this spring. I finally have got around to wI ring up some of the aftermarket controls mentioned in the large thread.
I purchased a used Totaline P251-0094H head pressure controller. (It has the built in transformer.) I understand it would normally be connected to an ICM379 or ICM377 sensor. My furnace already has an RTD probe that reads the plenum temp.
My question is this: can I use the RTD sensor for the speed controller? I don't know enough about the different sensor types. I would really like to use the RTD because I know it is precise and accurate.

 

[brenndatomu]

The problem is that the resistance range of the sensor has to match what the controller is designed to be used with...and "tapping in" to that RTD circuit could very well have nasty un-intended consequences, especially with the Drolet controls, things very well may not play well together! I would use the sensor it (TL) comes with...they are plenty accurate enough for the intended purposes.
Just remember to wire in a 5 or 10k potentiometer inline so that you can dial in the operating range you want...you couldn't do that with the RTD as it would screw with everything else too (if it worked at all)
Once electronics let their smoke out, it can't be put back in.

 

[andym]

That's what I suspected. Do I put the sensor inside the plenum?

 

[brenndatomu]

I have tried it taped to the outside, and currently have it inside...I like it inside a little better I think...more responsive.

 

[andym]

That makes sense. My plan is to unhook the 1, 2, and 3 fan speeds. I'll hook the speed controller to the 3rd speed wire. This should make it variable speed up to around 150° when the factory control activates the high speed. (Currently the fan comes on at low speed at 120. I'm not positive, but I think it bumps up the speed every 10 degrees thereafter. It uses all 4 speeds as opposed to the older models that only used 1 out of the 4.) The way I have it planned, if the controller malfunctions for some reason the factory high speed relay will still come on at 150 or 160.

 

[brenndatomu]

The way I have it planned, if the controller malfunctions for some reason the factory high speed relay will still come on at 150 or 160.

I'd wire in a relay so that if the high speed does get activated, it kills power to the speed controller...or at least to that 3rd speed tap...that way there can never be power to two at once...

 

[andym]

I'd wire in a relay so that if the high speed does get activated, it kills power to the speed controller...or at least to that 3rd speed tap...that way there can never be power to two at once...

The factory control has four separate relays. One for each speed, with five wires going to the motor. You can hear the relays click as the temp rises or falls. Same way as an electric golf cart.

I'll have three of those wires going into the speed controller with one wire going out. Or I might just do the two lower speeds. The factory setting works great for the higher speeds. I'm tring to of course minimize the on/off cycling. I'm also wanting to try and keep the plenum temps a bit higher on short burns.

 

[brenndatomu]

I'll have three of those wires going into the speed controller with one wire going out

Sounds like you have a handle on it.

 

[andym]

I'm back and I need some help. My variable speed controller is not doing anything. I need to check it with the multimeter when it's running to confirm it's getting power. You can see which controller I'm using in the OP. I don't remember which thermistor I purchased, either the ICM 379 or 377. I believe it was the 377 because it was harder to find, yet matched the temps I wanted more closely. I have the 5k 10 turn potentiometer wired in line.
What is the correct resistance range I'm looking for? The controller will be activated at 150°, and I would like the fan to cut out around 130°.

 

[brenndatomu]

What is the correct resistance range I'm looking for?

Resistance of what?

 

[andym]

Resistance of what?

Oops! The resistance that the head pressure controller is looking for from the thermistor. The other threads mention using the potentiometer to "correct" the resistance of the thermistor.

 

[brenndatomu]

I'll hafta find the chart that came with the one I have...it showed the temp to resistance curve...might be able to google it if you don't want to wait on me...

 

[brenndatomu]

I think this will get you in the ballpark...they all use the same temp sensor(s) I believe.
You can tweak it with the pot to your taste after it's working...

 

[brenndatomu]

The controller will be activated at 150°, and I would like the fan to cut out around 130°.

That's about 30-40* warmer than I'm running things...its cold out and things are rockin and rollin if I see plenum temps in the 120's.
IIRC, there is a 10* span between cut out and cut in temps...

 

[Case1030]

For my Tundra I have fan cutout at around -95f, and turn on at 110f I believe. The ICM won't put full blow on until 125f.

I think that's the most effective way to run it... the heat stuck in the plenum won't heat your house.

 

[andym]

With the way the Heatmax2 is designed I really can't turn the blower on at that low a temperature without some fancy wiring and relays. I'm trying to vary the blower speed within the factory range. In my observation the fire burns better when the plenum temps are allowed to climb a little. The heat is not disappearing, it's still heating the house just at much lower volume. Between the chart and my multimeter I should be able to get it.

 

[Case1030]

With the way the Heatmax2 is designed I really can't turn the blower on at that low a temperature without some fancy wiring and relays. I'm trying to vary the blower speed within the factory range. In my observation the fire burns better when the plenum temps are allowed to climb a little. The heat is not disappearing, it's still heating the house just at much lower volume. Between the chart and my multimeter I should be able to get it.

All it takes if I remember correctly- 3.9 ohm resistance on the RTD to lower the preset relays. It won't change the display f° but it definitely lowered mine down to 95f off.