Question about ECM motor and AHU

[Freeagent]

Our Mitsubishi heat pump uses an ADP AHU (BVRMB6230S) with variable speed motor. Literature says it maintains set CFM over wide range of esps.

I was wondering how it does that. It does not appear to have a means of measuring CFM, so must infer this from other information? I have read the ecm textbook which I found in another thread (very useful!). It says:

"The variable speed motor control is programmed to provide constant airflow. This is accomplished by converting the desired CFM to a specific amount of speed (RPM) and torque (current) delivered to the motor."We have jumpers on the speed control board for 600/800/1000/1200 cfm. If we set unit at, say 1000cfm, but esp is lower or higher than rated esp of air handler, would it control at 1000cfm? Or are those jumpers based on some expected esp?

The reason I ask this, is that our unit was set by installer at 1000cfm mainly to keep noise down (it should be at 1200cfm for 3.2ton heating capacity). But, our measured air flow (admittedly inaccurate with anemometer) says more like 1100-1200cfm.

More: Our esp is only 0.3" (0.2" on return and 0.1" on supply measured on installed magnehelic). The nameplate on the ADP AHU says 0.4" max (see attached which also says 30,000 btuh but is in fact 36,000btuh ) Elsewhere it says that esp can be as high as 0.8" and motor will adjust - confusing! We get some noise through supply and it seems to come from AHU. This is stopping us from increasing the CFM settings.

[fixitguy]

1. I'd start with the compressor rating and see if it REALLY 36K btu.. manufactures are
often off of the rating with smaller compressors.

2. Course you cant test the AC cause its too cold?? Performance testing would help..
You could get the place hot.ike 75 or more inside and if scroll compressor it shouldnt hurt to run it at 50 degrees???

3. The 100 or 2 CMF may not be an issue.. theory and practical are often quite aways
apart..

[Freeagent]

1. I'd start with the compressor rating and see if it REALLY 36K btu.. manufactures are
often off of the rating with smaller compressors.

2. Course you cant test the AC cause its too cold?? Performance testing would help..
You could get the place hot.ike 75 or more inside and if scroll compressor it shouldnt hurt to run it at 50 degrees???

3. The 100 or 2 CMF may not be an issue.. theory and practical are often quite aways
apart..

The Mitsubishi/ADP system is AHRI rated to put out 38000 btu/hr. (see attached chart which was apparently based on 1200cfm).

I can test heating capacity but need to know flow. That is partly reason for my original question about how ecm AHU controls to jumper settings.

[genduct]

Literature says it maintains set CFM over wide range of esps. I was wondering how it does that. It does not appear to have a means of measuring CFM, so must infer this from other information?

Well you're right, what the fan "knows' from the back emf is the rpm of the fan and from the amp draw it knows how much "work" it is doing while the fan is "shoveling" those pounds of air through the unit.
There are a series of possible combinations of RPM and AMP draw that will "tell the unit it is delivering the right amount of air

[Freeagent]

Well you're right, what the fan "knows' from the back emf is the rpm of the fan and from the amp draw it knows how much "work" it is doing while the fan is "shoveling" those pounds of air through the unit.
There are a series of possible combinations of RPM and AMP draw that will "tell the unit it is delivering the right amount of air


Thanks - I did some more reading and see how they could do that.

Because the control is all done based on blower, presumably the CFM figure is at the heated air temperature (for a draw through heat pump). Our unit seems to vary outlet temperature. I sometimes see it run and close off at say 86F. But I have also seen temperatures as high as 118F. 1000CFM at 70F is 1090CFM at 118F. It would seem we could get different CFM flows depending on outlet temperature.

I am concerned that 118F may be too high and CFM should be increased. I only see this overnight on Max/Min reading on thermometer. It may happen when set point changes from sleep setting to wakeup setting (7F increase). Could that temperature cause excessive pressure in outdoor unit?

I am sure it would be best to get a Pro in here to check this out, but we don't have anyone locally that has installed more than one unit!

[second opinion]

Thanks - I did some more reading and see how they could do that.

Because the control is all done based on blower, presumably the CFM figure is at the heated air temperature (for a draw through heat pump). Our unit seems to vary outlet temperature. I sometimes see it run and close off at say 86F. But I have also seen temperatures as high as 118F. 1000CFM at 70F is 1090CFM at 118F. It would seem we could get different CFM flows depending on outlet temperature.

I am concerned that 118F may be too high and CFM should be increased. I only see this overnight on Max/Min reading on thermometer. It may happen when set point changes from sleep setting to wakeup setting (7F increase). Could that temperature cause excessive pressure in outdoor unit?

I am sure it would be best to get a Pro in here to check this out, but we don't have anyone locally that has installed more than one unit!

Could the temperature difference be heat pump verses heat pump and auxiliary heat?

[Freeagent]

Could the temperature difference be heat pump verses heat pump and auxiliary heat?

We don't have any aux heat in this unit.

Back to question - Is 118F (+/- 2F) supply air at AHU exit excessive for an R-410 heat pump?

[second opinion]

We don't have any aux heat in this unit.

Back to question - Is 118F (+/- 2F) supply air at AHU exit excessive for an R-410 heat pump?

When are you seeing the 118 degree temperatures?

[Freeagent]

I just ran a test - Bumped set point up from 69F to 72F. AHU exit temperature increased to 115.3F when return reached 72F (43.3 delta). Outdoor temp is 36.5F. At 36.5F outdoors, rated capacity of this 3ton unit is 38,000 btu/hr (apparently tested at 1200cfm). But we are set at 1000cfm. (1.05x1000x43.3=45465 btu/hr!) (even more if I use enthalpy data for 40%RH air)

How can that be? Either unit is putting out more that it's design or ECM controlled flow must be less than 1000cfm? Any other explanation?

It seems that we get the high supply temperature when a big change in set point occurs - like when we go from sleep temperature to wake up. In normal operation, unit achieves set point at lower supply temperatures.

[beenthere]

Does it have any strip heaters for aux heat?

[Freeagent]

Does it have any strip heaters for aux heat?

No, no strip heaters or other aux heat. The 118F supply temp is strictly from the heat pump. Don't know what refrigerant would be at, but would that be near max pressure?

[chuckcrj]

Is this a Mitsubishi Zuba?

[Freeagent]

Is this a Mitsubishi Zuba?

Yes - That is it. As you probably know, a Canada only model (so far)

[chuckcrj]

Yes - That is it. As you probably know, a Canada only model (so far)

Yes, I wish we could get them here.

I do a lot of Mitsubishi but have no experience with the Zuba.

You might know this but it has an inverter driven compressor which will vary compressor speed for changes in demand.

How are you using your anemometer to get airflow readings? At the registers?

[chuckcrj]

I just ran a test - Bumped set point up from 69F to 72F. AHU exit temperature increased to 115.3F when return reached 72F (43.3 delta). Outdoor temp is 36.5F. At 36.5F outdoors, rated capacity of this 3ton unit is 38,000 btu/hr (apparently tested at 1200cfm). But we are set at 1000cfm. (1.05x1000x43.3=45465 btu/hr!) (even more if I use enthalpy data for 40%RH air)

How can that be? Either unit is putting out more that it's design or ECM controlled flow must be less than 1000cfm? Any other explanation?

It seems that we get the high supply temperature when a big change in set point occurs - like when we go from sleep temperature to wake up. In normal operation, unit achieves set point at lower supply temperatures.

Lowering the CFM slightly won't change the BTU output significantly, the temp rise just goes up and you are still delivering the same BTU.

[genduct]

1000CFM at 70F is 1090CFM at 118F

The secret is the pounds of air didn't change and that is what the unit is indirectly measuring

[beenthere]

You mass flow dropper you.

[Freeagent]

The secret is the pounds of air didn't change and that is what the unit is indirectly measuring

Fans capacities and curves are usually in CFM without any conditions specified, so I presume CFM means ActualCFM.

But then they measure rpm and load (which I would think would be related to mass flow?) to control a volumetric flow? 1000cfm at 70F is not same mass flow as 1000cfm at 115F.

I guess I need to think about that a bit more :)

[Freeagent]

You might know this but it has an inverter driven compressor which will vary compressor speed for changes in demand.

Yes I am aware of that - It seems that when thermostat requests a big change (say 63 to 70F), compressor must go to max and then we get teh high supply air temperature. That is reason for this thread. When I see supply temp at 118F, is that too high and should I increase the air flow.


How are you using your anemometer to get airflow readings? At the registers?

I have been convinced that if the AHU is set at 1000cfm, then that is what I will get (approximately). I did check airflows with an anemometer and got higher flows. I measured both in the return duct and in supply boot necks with registers/grilles removed. I tried to get a grid, but method is inherently inaccurate, even if repeatable. I am not using those numbers - they were obviously high when I looked at delta T. Anemometer was useful in getting relative flows from supply outlets for balancing purposes.

[genduct]

It is mass flow!
Problem is we have been told for years that the fan will deliver 1000CFMs at sea level or Denver even though density diff. True of the old approach, but the new ECMs with the controls do deliver mass as described. It may take years for this concept to take hold

[beenthere]

Fans capacities and curves are usually in CFM without any conditions specified, so I presume CFM means ActualCFM.

But then they measure rpm and load (which I would think would be related to mass flow?) to control a volumetric flow? 1000cfm at 70F is not same mass flow as 1000cfm at 115F.

I guess I need to think about that a bit more :)

In heating and cooling. When the system is running. The supply and return duct are both moving a different amount of air, in volume.

On a 100% no leakage system, in heating mode:
If a return has 1000CFM of 70°F 30%RH air coming in at the air handlers inlet. When the air is heated to 115°F its RH drops to 7.4%, and the supply discharge of the air handler is now moving 1084.75CFM. But the mass air flow through both ducts is still 74.35 pounds per minute.

Most inverter compressors have the ability to go to 100+% capacity for a short time. Often over 110% of rated capacity.

[Freeagent]

I think I now understand how the ECM controls mass flow! Good learning exercise!

The jumpers on the board used to determine the mass flow set point are given in CFM (600/800/1000/1200)! For CFM to be a mass flow, wouldn't they have to mean SCFM?

If I have jumpers set to 1000SCFM, then at 115F, actual CFM would be 1000x575/520=1105ACFM. (ignoring slight pressure difference). Am I running at 1000CFM or 1105CFM? It could make a difference to noise levels.

When HVAC pros suggest 350-450CFM/ton - Do they mean SCFM or ACFM? Maybe with cooling it doesn't make much difference?

I asked my supplier about running at 1000CFM and the high supply temperatures we sometimes see. He say not to worry about it because the outdoor unit has sensors to protect itself. I hope he is right!

[genduct]

yes S(tandard)CFMs! No one except us anal folks care that there is less Cooling CFMs coming out as did go in.

This was just a little exercise in the scientific method is all

[Freeagent]

yes S(tandard)CFMs! No one except us anal folks care that there is less Cooling CFMs coming out as did go in.

This was just a little exercise in the scientific method is all

Well, I think I feel a bit better now about that 1000CFM setting!