Drolet HeatmaxII variable blower speed

andym

Member
Feb 6, 2020
142
Hicksville, Ohio
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

Minister of Fire
Aug 21, 2013
4,998
NE Ohio
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.
 

brenndatomu

Minister of Fire
Aug 21, 2013
4,998
NE Ohio
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

Member
Feb 6, 2020
142
Hicksville, Ohio
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

Minister of Fire
Aug 21, 2013
4,998
NE Ohio
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...
 
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andym

Member
Feb 6, 2020
142
Hicksville, Ohio
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

Minister of Fire
Aug 21, 2013
4,998
NE Ohio
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.