Modulating electric duct heater with SCR and Thermostat.

[ChaChaMan]

Does anybody have experience using electric duct heaters that can modulate using an SCR. This is supposedely an improvement to staging. I am proposing a 12 KW duct heater in a residence. I wasn't sure if I sould divide it in 2 or 3 stages or specify modulation using an SCR.


My questions are :
1. Does modulation (pulsating power) affect the quality of the rest of the power in the house ? : ex lights, computers and other electronics.

2.What thermostat exists on the market that can generate the analog output 0-10vdc for these electric duct heaters as well as control a 2 stage heat pump. The duct heaters are complementary to the heat pump while it is being used to heat or act exclusively as emergency heat when the heat pump is shut down. If the thermostat can do dehumidification with electric reheat too that would be ideal.

3 What else is there to know about these devices ?

Thx in advance and regards

 

[cry22]

Know that the thermostat has an Auto-On-OFF and does not operate both the compressor and the reheat, it switches heat pump operation between compressor and electric heating coil. It does not control two components (compressor and heater) at the same time.

Dehumidification and reheat are not allowed by ASHRAE 90.1 (simultaneous heating and cooling) unless it is a process requiring reheat such as DOAS 100% OA applications.

I do not have a clue about power quality, may be some elctrical engineer can pitch in. Why are you concerened with power quality? have you heard of such problems using SCR?

SCR is widely used CRAC units. We tend to spec multi-stage heat when accurate control is not needed.

 

[tys90]

You can get multistage thermostats with emergency heat options. The fan has to be capable of two-speed operation as the dehumidification mode will run the fan at low speed for greater latent capacity.

Also, 90.1 allows reheat of up to 30% of the design airflow, although I doubt you have a variable volume system if this is a residential heat pump.

 

[ChaChaMan]

Tys90...
Just so you know, the fan is a plenum fan with a VFD with up to 8 preset speeds.

Do you know of a thermostat model by name ?

I don't understand what you mean by 'Also, 90.1 allows reheat of up to 30% of the design airflow' why do they base it on flow rather than on cooling or enthalpy ?

 

[mauricestoker]

The amount is given based on flow as it is referenced to VAV zone application. The 30% peak flow, 62.1 outside air flow, 300 CFM, 0.4 CFM/SF, and any higher amount that the AHJ will sign off as being energy reducing, are given as reheat limitations. I'm working today in a building full of single fan dual ducts which is a whole lot of simultaneous cooling and heating. I guess some AHJ would sign anything put in front of him.

If you are looking to maintain constant supply tempeature regardless of heat pump operation, you may want to look at Neptronics. I've used the SCR controller with an in-duct temperature detector and its been running two years with no problems. Humidity control was installed separately though stat and unitary controller.

 

[tys90]

ChaChaMan - mauricestoker gave a summary of the section of ASHRAE 90.1 I was referring to. However, you said this is a residential application, in which case, 90.1 does not apply to low-rise residential buildings.

I would talk to the local heat pump rep. to see what he recommends.

 

[DRWeig]

http://www.viconics.com

-- look for SCR power controls.

These are applied often in commercial/industrial situations, they work well.

Up to you to come up with rest of control system, though...

Never heard of noise problems -- it's best to follow proper grounding procedure, and run the wiring from the load side of the controller to the heating elements in metal conduit (should be a short run, eh?). Heaters themselves are inside of steel duct, so radiated noise isn't much of an issue.

Noise impressed back into the house wiring is minimal, or it wouldn't meet CE or FCC requirements.

Good on ya,

Goober Dave

 

[QualityTime]

As mentioned above, try Neptronic. This will control your SCR duct heater. The one file is for a remote temperature sensor (if you want such an animal) and the other file is the controller itself which also has an onboard temperature sensor. Talk to them if they have another controller that can dow what you are looking to do with respect to cooling

http://www.neptronic.com/CenterPage/Controls/PDF/English/TRO5404.pdf

http://www.neptronic.com/controls/PDF/stc8.pdf

 

[ChaChaMan]

Guys...thanks for your comments !

This is what I've found so far.

There isn't any thermosat capable of controlling a 2 stage heat pump with auxilliary heat (Emergency Heat)with a 0-10VDC output required for the electric duct heater with SCR.

Be very kind of someone to prove me wrong.

Regards

 

[DRWeig]

ChaChaMan,

There are a million varieties of thermostat nowadays, but I don't find exactly the one you need.

However, you can do it without resorting to a PLC or anything. Get a heat pump 2-stage version that gives you a dry contact output for emergency heat. Use that contact to enable a proportional or PID analog-out thermostat or single-loop controller. Your controller will have to have its own temperature sensor -- but it's not necessary to have it in the space, you could control the heat output to maintain a fixed supply air temperature perhaps?

So you'd be cascading one thermostat's action from another, but they're all readily available...

I'd be interested as well if anyone finds a proportional-out on the E-heat of a heat pump thermostat!

Good on ya,

Goober Dave

 

[mauricestoker]

I've encountered a similar problem and ended up using an Orion unitary controller for humidification requirements.

If you don't want to spring for the unitary controller, the duct heater with SCR control can be used standalone from the t-stat by including the in-duct detector. It does not need to control your heat pump, only to provide desired discharge temperature.

 

[ChaChaMan]

Thanks maurice,

what do you mean by in-duct detector?
Is that a thermostat or temperature sensor in the duct ?
Do you have a specific device model # I can quote to someone ?

What value did you use as a discharge temperature ?

Thanks

 

[mauricestoker]

The duct sensor is for temperature.

http://www.neptronic.com/Index.aspx

Since I had used it before, they have another item on the page which might interest you, the 5404.

For what I needed, I had a 54* F discharge temperature to provide redundancy to preheat, and protect coils in case of normal electric failure. The 5-1/2' of snow and normal power failure we went through last winter tested it pretty well, no failures or blips.

 

[QualityTime]

The information I posted earlier is for a controller (which has its own on board temperature sensor...similar to what you would install in your house) and an external sensor. In my case, they are wired to each other because I had to control the output of the duct heater by measuring the discharge temperature in the duct. I therefore disabled the on board temperature sensor in the controller. The duct heater has a high temperature bimetallic safety cut out on it rated for 125 oF. You need to have something in the duct to measure and control the heater output. Unless you have something controlling it the duct heater will ramp up to 100% output whenever there is call for heat and it could get extremely hot and become dangerous. You can set the duct output to be anywhere up to say 100 oF or even more. You don't want to have the bimetallic safety cut out working all the time because it will fail at one point in time...just when you need it.

As far as for what temperature you set it on the duct discharge it is up to you. If you set it for 85 oF the room will eventually get to 85 oF because you are most likely continuously returning heated air.

You actually can get a controller that will work your two stage heat pump and work the duct heater as a supplemental heating system. Talk to Johnson Controls. They have the controllers and the stat you are looking for. It is a matter of programming. Appended is something I just did that is similar to what you are trying to do. You can see how i set it up via a process flow diagram

 

[ChaChaMan]

If anybody cares to comment I am uploading a system schematic. I am new at this, feel free to comment or correct my schematic.

This design revolves around the use of three thermostats.

T1 has a room temp sensor and controls the heat pump and at times signals the need for aux-heat.
T2 has a room temp sensor and controls duct heater_2
T3, enabled by the aux-heat signal from T1, has a duct temp sensor and controls duct heater_1



Regards

 

[ChaChaMan]

Here is the uploaded schematic !!!

 

[DRWeig]

That looks like it's worth a shot, ChaCha.

Let us know how it works out for you!

Good on ya,

Goober Dave