coolmen

I have a home owner with maney questions for me to answer in order for me to get the job. plus for my own knowledge.

When installing a varaible spd air handler that has dehumidifiying control feature,how does this exactly work?
do you install a speciel thermostat for this? sensors?
variable spd ramps up or down according to what?
humidity removeing from the house is the homeowner biggest concern.
I have done a full load calculation and sensible is 18,000. I selected Rheems
RHKLhm36 and a condenser RAPL36 that shows 27,000 sensible.
this is based on a 92*outdoor and indoor of 73* at 1900 sq '
being over sized 37% based on actual # what effect will this have on humidity removel?
3 other contractors are saying 3 tons is needed so telling him 2.5 tons will most likly kick me out the door.

All so the specs show that when equipment was tested manual S. Why does it show 1000 cfm tested at in instead of 1200 cfm for a 3 ton?
maybe they adjusted in lab and 1000 cfm gave them the best Efficiency # or best humidity removal. Dont know but would quess for the optimum efficiency.
Thanks

flashheatingand

Your question as far as dehumidifying control is a good one. I have never had to install any external sensors in the duct work, and have never seen any sensors within the furnace. So how the system can determine that it is pushing x cfm's through the duct-work? Sure, you can control the rpm's via dip-switches, but that does not guarantee that you are moving the cfms necessary. It all comes down to existing ductwork.

I am not sold on variable speed. I think the manufacturers and various dealers are over hyping v.s.. I am not hard headed, so if someone can convince me otherwise, cool. But until then, (just my opinion), you are doing the homeowner a dis-service by steering a client towards v.s..

beenthere

First, if the sensible load is only 18,000BTUs.
You should select a unit that is closer to that then what you are proposing.

With the posted oversizing, it will have poor humidity control.

A VS blower can be set up to do a slow ramp up. This allows the evap coil to be colder, and remove more humidity.
Or, you can use a humidistat, or a thermostat with built in humidistat, to slow the blower when the humidity is above set point.
Several thermostats can do cool to dehumidify.

VS blowers, will speed up, until they are working against the preset torque value for the CFM they are set for.
Or until they reach max RPM.

During a dehumidify call, the VS blower will be slowed down by roughly 20%, to keep the coil colder, to remove more moisture.

flashheatingand

You are clearly a knowledgeable guy.

Are you saying that the v.s. blowers are determining the cfm's based on the torque on the motor? If so, that's crazy. All that extra expense, and high fallutin' claims all based on the resistance inside of the cabinet?

Wouldn't it be more economically effective to simply increase the "fan on" delay. Or, use the med high lead for cooling as opposed to the high speed wire.

Most folks find a programable t-stat challenging enough. Now we are selling stats that also have humidistats? That can be overwhelming on people who just want an appliance to work reliably. It just seems as though we are starting to sell items that are in our best interest as opposed to the customers.

westernhvac

There are definitely some good systems out there now that include good dehum setups, bryant/ carrier come to mind. The best control out there for it now IMHO is the honeywell true IAQ. Up here in SW Idaho we use it mostly for humidification but reading through the specs, it will control a variable speed fan well enough to do the opposite. Good luck, and about the sizing... most customers can appreciate a smaller condensing unit as long as you can explain why they need it and make them understand.

BTW: what was the latent load

NickTech

the unit is to be sized for total load, latent and sensible. the humidistat option on the IAQ 8000 will offer a more calibrated speed solution to humidity needs. slower air = more moisture removal. instead of blower on delay being set for only one speed to accomidate a numerous amout of jobs, the VS motor can alter the out put to what ever is needed. the torque of the motot will call for more or less cfm's depending on the set up and on the resistance in the airflow. ie and dirty filter will cause the motors load to change signalling for a ramp up to compensate for the load. the dehumidifier option will simply allow the blower to run longer after the cooling call has satisfied to help remove as moisture as possible. furthermore this tstat can also call for dehumidification, even if there isnt a call for cooling and allow the system to run at a lower capacity soley for moisture removal, which is the latent load.

mo-flo

Good tread here,i can tell you guys one thing.....listen to beenthere,he knows his stuff

beenthere

Most VS blowers are PWM controlled to ell them what speed/CFM they should be running at.
They can be thermostat controlled.

But, the blower motor still needs a way toknow if its moving the CFM its been set/told to move.

A blower running at 800RPM working against .5" static presure will move less air then a blower running at 800RPM against .9" static pressure.

So a way to know if the blower is moving its set volume of air is needed. And what way is better then applying torque laws.

Temp alone won't tell you on a heat pump or A/C.


For humidity control. A delay is not nearly as effective as slowing the blower to 80% of normal cooling speed.

A blower delay can't do anyhing if the humidity starts to rise in teh middle of the cooling cycle.

A VS blower with a stat that can slow it down, when humidity is high. Is more operational economical.

If the humidity is low. It lets the blower ramp to full set speed right away.
If the humidity is high. It keeps the blower speed slowed down, untill the humidty is back below set point.


A VS blower. Is no more complex than a Emission Control Module on a car.

beenthere

What any individual company sells to a customer. Should be based on the needs of that customer.

I have customers ask me aboout VS blowers and 2 stage A/Cs and furnaces.

Some customers are not good canidates for VS systems, with high end thermostats.

Many are.

Equipment should be sold that meets the wants and needs of that individual customer.

flashheatingand

I have never seen any sensors in a furnace to tell the module what kind of sp. the blower is working against. I understand you can set the dip switches to xcfm's, and the blower motor will do x rpm's. That I understand, but I do not see how the furnace senses that there is a shallow crawl space or low ceiling which causes a major restriction. If the duct work isn't right, the v.s isn't going to deliver what is promised.

My gripe isn't against v.s. I just believe many outfits ignore the issue of ductwork. Which is critical for the v.s. to perform as it should.

I worked for a company that installed a v.s. upflow furnace (new construction). One year later I went on a service call and the blower was bad. The culprit was a frozen indoor coil, as it thawed, the water must have shorted out the control module. The supplier did not have a module and we had to replace motor and controler. This is a detail that I think both the manufacturer and installation company have overlooked.

Another gripe is that our suppliers do not keep a lot of parts in stock for the high dollar appliances. When they break down (usually during the peak of cold season or summer), it's hard to tell a customer, not only is the part not immediately available, but they will need to pay for the premium for next day air.

beenthere

Some contractors/salesman do use and sell VS blower units as a cure all for bad duct work.

Others use all sorts of tatics to justify not selling them.

A company should stock some of the VS motors for the brand furnace/Air handler they install.

All VS blower motors can be replaced for a week with a regular blower motor for a week. No reason to make a customer wait.

That isn't the fault of the VS blower.

Just lack of education and or training on them.

Its not sensors that are used for the VS motor to know what CFM its moving.
But rather its RPM and amp draw converted to torque..

If its not drawing X amp at X voltage at X RPM, then it is not moving set X CFM.

This is programed into the module.

So the module speeds it up, until it reaches an RPM and amp draw that will cover the set CFM or close to it, within reason.

coolmen

You are correct beenthere. I only find my self selling VS or 2 stage condensers to a select few

coolmen

I found that setting the dip switches according to what part of the country your in will have the blower timed to start a slow speeds for X amount of minutes then ramp up. High humid area will run for longer minutes befor ramping up to full speed.

beenthere

I like to set up the blower speed to match the sensible and latent load as close as possible. Weather thats 1250 CM for a 3 ton, or 1075 CFM for a 3 ton.

Set the blower to go to highfull cooling speed after 30 seconds.
And install an IAQ to slow the blower if the humidity gets high.

If the customer doesn't want an IAQ stat. Then I set it for the slow ramp up also.

I prefer to slow the blower by an IAQ or humidistat, instead of a slow ramp up. Because when it gets near outdoor design temp, and the unit is still cycling, seldom is there a need to ramp up slowly.