watt resister

[superheat3211]

anyone know what a watt resistor does on a rhemm electric heat air handler. Its wired into the electric heat sequencers. After studing it it look like it energizes the sequencers but unsure why it's needed.

[sktn77a]

It prevents all of the backup strips from switching on as soon as the thermostat calls for Aux/Emer heat. So you don't have all of the resistance heating elements coming on unless its absolutely necessary.

[WhoIsThat?]

Doesn't it heat the bimetallic relay in the sequencer, and thereby give you the sequencer time delay?
You have a pic or a schematic?

[Shophound]

"Watt restrictor" is what I've heard it called. Delays onset of subsequent heat strips so initial set of strips can possibly assist heat pump in satisfying the stat.

[WhoIsThat?]

"Watt restrictor" is what I've heard it called. Delays onset of subsequent heat strips so initial set of strips can possibly assist heat pump in satisfying the stat.

I thought they did it so a 24 kw, 100A load wasn't dumped on the elec. service all at once, which would cause the lights to blink and get the HO upset.

[Shophound]

I thought they did it so a 24 kw, 100A load wasn't dumped on the elec. service all at once, which would cause the lights to blink and get the HO upset.

There's no inrush current with a strictly resistive load, such as electric heat strips. If the circuit can handle all strips being on at the same time staged in sequentially, it could handle all strips banging on at the same time.

24 kw? That's 81,000 BTUH on strip heat. I think the homeowner might be griping about something else.

[amd]

It prevents all of the backup strips from switching on as soon as the thermostat calls for Aux/Emer heat. So you don't have all of the resistance heating elements coming on unless its absolutely necessary.

Is "watt resistor" a fancy term for heat sequencer?

[docholiday]

No "Smart heat" from Carrier is a fancy way of saying "watt resistor". The watt resistor limits the amount of electric heat coming on when the return temp is warm. (keeps your watts down, hence the name).

[superheat3211]

Thanks for the info. I thought that 's what it was for , but haven't run into anthing like that before.. Always thought the sequencers did that function of staging on heat.

[WhoIsThat?]

There's no inrush current with a strictly resistive load, such as electric heat strips. If the circuit can handle all strips being on at the same time staged in sequentially, it could handle all strips banging on at the same time.

24 kw? That's 81,000 BTUH on strip heat. I think the homeowner might be griping about something else.

100A dumped on my resi service will drop the voltage by 2.4v. For incandescents this is a 3% change in brightness. I'd think it'd be noticeable. 20A five times, no.
For 12 kw the brightness change would be 2%.

[chilliwilly]

Thanks for the info. I thought that 's what it was for , but haven't run into anthing like that before.. Always thought the sequencers did that function of staging on heat.

Heres my tuppence ha'penny.

A *watt resistor" sometimes called a "power resistor" is just like any other resistor exept it can handle more current than the type you see on a pcb thats probably only rated between 1/16 - 1/4 of a watt. And the power resistor can be rated up to 1500 watts at variable resistances.

I can only imagine in the case of heat strips they will be connected in series with the strip elements to maintain a steady continous current flow through the strip element. So that when the set temperature is satisfied the elements aren't switched off they remain in the circuit when the resistors are switched in, allowing the overall power of the elements to be reduced maintaining the temperature by slugging the current passing through them. And in turn giving a two stage setting by default.

Then if the temperature drops lower than satisfied, the resistor is switched out of the circuit but the element remains in the circuit and the current flowing through the element is increased thus increasing the temperature.

I'm suprised that this system is still being used and hasn't been superseded by a triac network in this day and age.

Just my tuppence ha'penny.

[WhoIsThat?]

I'm suprised that this system is still being used and hasn't been superseded by a triac network in this day and age.

Because they wanted a non-electronic time delay; electronic delays would be sensitive to the EMI/RFI generated by all the relays switching heavy current, inductive and non-inductive loads.

And vacuum tubes are still used, somewhere.

[chilliwilly]

Because they wanted a non-electronic time delay; electronic delays would be sensitive to the EMI/RFI generated by all the relays switching heavy current, inductive and non-inductive loads.

And vacuum tubes are still used, somewhere.

Couldn't they put an emc/rfi filter on the system like they do in any other electronic control?