Paralleling a motor speed control with a relay?

[pct88]

I have an Energy Recovery Ventilator that is normally run at low speed for pre-heated/cooled (season dependent) fresh air supply to the house.  I would like to have my HAI Omni panel increase the supply fans to full speed when the bathroom exhausts or clothes dryer is running to pull in more air to make up for what is being exhausted-  i.e. bring in makeup air through the filtered and pre-conditioned ERV rather than through seepage.
 
I currently have a manual motor speed control that I use to adjust the airflow of a FanTech inline external rotor exhaust fan whose instructions say "100% speed controllable, All units are suitable for use with solid-state speed control"  on the ERV.  If I put a relay in parallel with the two wire speed control when the relay closes the fan would go to full speed, open and it will drop back to the speed setting selected by the speed control, correct?  I would do this with a RIB relay so it is a solid short,  control the RIB with a UPB I/O module.
 
This would at least give me high/low speed control of the blowers.
 
Has anyone tried this approach?  When the relay closes it would look like a momentary short across the speed controller; perhaps the noise choke in there could maintain enough current to damage the triac?  
 
I have three Fantech units each with standalone speed control; they work just fine with no motor buzzing.  I have one underslab venting Fantech running at very low speed for years with no problem (i.e. the motor hasn't burned up on the one running at about 20% speed much of the year).

 

[drozwood90]

just because I have been looking into something like this for a few years...which ERV did you get?  I keep going back and forth on ERV or HRV...for me either would be good based on what I read about ERV/HRV and where I live.
 
Are there things you would do different?  Why did you ERV instead of HRV? 
 
Good idea about changing the fan speed.  So, is your plumbed into the ducting in parallel with the furnace?  Basically, this unit will pull air from the return, then pump INTO the furnace OUTPUT?  Do you find problems with the pressure differences?  Or, do you pull from and return to the postive pressure side of the furnace (i.e. the furnace MUST run to get the fresh air pumped around the house)?
 
Thanks and sorry to sidebar your question.  Although, I am highly interested in this feature as well.  With our new build the house is VERY tight.  Based on two independent tests, I ended up running a bathroom fan (designed for this) to run a few hours a day...just to get enough seepage through the few holes that we do have in the house (bathroom fans mostly).
 
--Dan

 

[ano]

I went through this as well, and my advise is to to create a solution for a problem you don't have.  An energy audit also said my house was sealed good, and most newer houses are, but don't assume you have to bring in much outside air.  I have installed a CO2 meter and it measures the need for ventilation.  Hi humidity can also be cause for ventilation, but its very dry where I live, so I don't worry much about that.  In any case unless I have lots of people over, I've never seen ventilation a problem.
 
I looked at ERV and HRV, and no matter what they say, their efficiency is rarely over 65%, plus they have two fans you are running.  I decided instead to add two air intakes to my system, controlled by a damper. One also has a booster fan which turns on with the damper.  Instead of using this all the time, we use it to save energy by bringing cool air at night when our home is warm, and warm air in the day when our house is cold. So instead of costing money to run, this system saves us money.  The home automation system figures all this out So I don't have to. 
 
By measuring the CO2, we can decide if we need ventilation at other times, but we have NEVER required it, and our setpoint is 800 ppm CO2 which is very low.  Even if we do have to run the intake air when the temps aren't desirable, we figured out that the extra energy costs would never be enough to counter the added cost of a ERV or HRV, let alone all the electricity it uses.  It was maybe a few hundred dollars to add the air intakes and dampers. The booster fan was $50.
 
By the way, a relay in parallel with a "two wire" speed control should work fine.  If it has a neutral, this is another story.  Also, remember you still need to meet electrical codes, and mixing low-voltage and line voltage can be tricky.

 

[pct88]

I failed to mention:  the enclosed crawl space is the source of the exhaust air that pre-heats/cools the outside air.  The air flow through the crawlspace is for two purposes:  to circulate the air in there since we store all sorts of items in that space, and also to reduce the radon level.  Prior to remediation in our basement we had 30+ pC/li radon levels.  With the basement sub-slab fan running 24x7 that has been reduced to around 3pC/li.  The crawl space area has a rough concrete over vapor barrier floor (1950s) and plenty of places for radon to enter.  So circulating air in there is also required all the time.
 
The problem I run into is that when it is particularly hot and humid or cold (New Jersey) the fresh air being dumped into the return plenum can drop into the 40's or up into the 90s if the fan is running full speed.  Which brings us to the next factor:  the house originally had the forced air furnace return ducted straight into the open crawl space.  No ducts beyond that-  the entire crawl space was the return duct.  After 50 years like that we built an insulated relatively well sealed enclosure along one side of the crawlspace where the return vents headed upwards between the floors.  Within the new "duct" are insulated hot and cold water pipes.  If the outside air coming into this duct drops the temperature far enough the pipes could freeze.  So my plan is to have a three stage system:  put an HAI extended range sensor in the "duct" where the air has a chance to mix.  Program the HAI to turn off the OA supply fan below 45 degrees mixed temperature or over 85.  Low and high speed selection based on other criteria to be determined.
 
The Energy Recovery Module I am using is this:  http://lowkwh.com/images/BPE-XE-MIR_200_-_Spec.pdf
with a pair of FanTech FR100 fans, custom assembled with assorted ducting.  Of course fully instrumented with Geist temperature and humidity probes on the supply and return sides to see how much difference the unit makes.  I would cut/past graphs but I think I need to upload them as pictures, then reference the URL of the picture?

 

[ano]

I know, never as simple as it should be. :angry2:
 
I actually had a radon problem also.  I never got to 10 pC/li, but that is enough for me.  I have a SafetySiren Pro Series Radon alarm, which is a great device that gives near-realtime radon readings, and it is quite accurate, but I digress.
 
Your plan sounds valid.  I have noticed the radon drop since I've added ventilation, but I also have a sucker under my slab.  Between the two, radon is down to about 2.0 pC/Li or less.
 
> No ducts beyond that-  the entire crawl space was the return duct.
 
That can't be good for radon.
 
My ducts are in the attic, where its only 10,000 degrees here in the summer. 

 

[pct88]

I also have a Safety Siren; I then bought a refurbished (direct from the manufacturer) professional unit to compare.  The pro unit logs data, so I can actually turn off the fans and watch the radon levels increasing hour by hour, then drop again when I turn the fans on again.  I considered automating the fan speed vs. radon level, but that seemed a bit extreme.  And unnecessary.  I also bought a bunch of CO2 realtime sensor/fan controllers; same thing-  the levels I was seeing in most cases didn't justify the effort of hooking them up to actually control the fans.
 
My wife has this odd idea that if it seems stuffy in the house I should just open the windows.  Where's the fun in that when I haven't maxed out the expansion chassis of the HAI system yet?

 

[ano]

So how did the professional radon detector compare to the Safety Siren?  I think its quite accurate for what it cost.  At one time I too was going to automate my radon fan. But then I decided that risking my health for the cost of running a 60 W fan wasn't worth it.

 

[pct88]

I have a Safety Siren hanging just above my http://radon.sunnuclear.com/1027/1027.asp and the figures indicated by both after a week or so of comparison are within 0.5 pC/li which seems quite reasonable.  The 1027 logs the data over time and can dump the data through its serial port; I don't know if there is a way to continuously poll it.