air handler question

[bure961]

A friend has a restaurant where he had a roof ac / heater unit installed and much to large for the area. The air flow is too much for anyone to put up with. It gets too cold or hot too quick. You can not sit anywhere near [under] the air flow. Money is a concern for him. Would a speed control like a vfd be a possible solution for this problem.

If some one has another means [idea] we would love to hear it. Replacing unit is not an option, its 2-3 years old and expensive.

 

[iwire]

If you reduce airflow below the design you will likely cause the unit to shut down on high temp in heating season and freeze the coil up in the summer.

 

[StarCat]

Methods

Methods

Cooling and Heating loads are based on numbers just like Electrical calculations.
I have to wonder just what has been done in such a situation and how the duct system compares to the unit that was installed on it. Did he get a supposed deal on some surplus piece of gear when the old one was failed? This kind of logic is 99% always a trap. Attempting to solve such a matter can lead to much larger problems depending on the degree to which the system is mismatched to the load.
If its a belted fan drive, for starters open the drive sheave and slow the fan down but realize there are limits and if the design numbers have been violated and disrespected in the large you have got a problem that can not be solved as the system must have a minimum airflow. If the unit is oversized for the duct system the FPM velocities will be too high. Some creative reducting may be possible to a degree. If the fan CFM is set too low the operating characteristics on both cooling and heating will show problems, some which can quickly damage the compressor and cause overtemp conditions on the heat side. Oversizing a cooling system will cause the conditioned space be cooled very poorly. The system won't remove humidity and will short cycle and simply not work. I'm sure you can see where this leads. There are rules of thumb basics for load calculation, but you can see where you fall at 1 CFM per square foot based on the system in question. VFDs are typically set up on systems that are larger and have zone dampers in involved.

 

[mgookin]

A friend has a restaurant where he had a roof ac / heater unit installed and much to large for the area. The air flow is too much for anyone to put up with. It gets too cold or hot too quick. You can not sit anywhere near [under] the air flow. Money is a concern for him. Would a speed control like a vfd be a possible solution for this problem.

If some one has another means [idea] we would love to hear it. Replacing unit is not an option, its 2-3 years old and expensive.

That's actually a better problem to have in a restaurant than the inverse.

Here's what I'd suggest to try the cheapest things first:
1. Install a controllable damper where he can play with the cfm. I would think this should be on the return side so you don't choke air in the AHU and cause freeze up or overheating.
2. Install a deflector at each supply where the volume of air is too much.
3. Ask an a/c guy if a fan speed control is an option.

In a dorm we had a kid from the Philippines who had never been in a/c. He was getting so sick we almost took him to the hospital. We ended up removing the supply grills and stuffing bath towels in his supply duct.
Next year a buddy of mine said his room was very hot. I told him to remove the grill in the supply and see if there's a dark blue bath towel in there!

 

[bure961]

The owner had a contractor[havc] and he installed the biggest one and large duc based on what he could charged not what the area called for. He had it oversized where he had to disconnect one of the outlets and left it in the drop ceiling cooling or heating that area.

The air blower in some resi heating/cooling units have the motor where you can connect either of the speeds.
Is this common on some commercial ahu ? Don't know how its connected belt or direct drive haven't been up there to see.

 

[topgone]

The owner had a contractor[havc] and he installed the biggest one and large duc based on what he could charged not what the area called for. He had it oversized where he had to disconnect one of the outlets and left it in the drop ceiling cooling or heating that area.

The air blower in some resi heating/cooling units have the motor where you can connect either of the speeds.
Is this common on some commercial ahu ? Don't know how its connected belt or direct drive haven't been up there to see.

@bure961,
Follow what @mgookin suggested. I have also experienced this problem when the contractor/vendor supplied a 65 TR RTU when he was given the requirement as 45 TR (availability problem). The tweak they did was to control the return air versus the intake outside air. It works.

 

[Sahib]

VFD for the blower fan is better to control air flow considering energy saving compared to inefficient damper control.

 

[iwire]

Guys, if we had an HVAC contractor recommending modifications to an electrical system we would all be shouting unqualified yet you all feel qualified to recommend modifications to an HVAC system.


It makes no sense. :happysad:

 

[Sahib]

What about a facility manager making the recommendation?

 

[Mgraw]

To reduce the velocity of the air coming out of the supply grills you could add more supply grills. To reduce the temperature of the air coming out of the supply grills you could increase/add fresh air in the return. This will also give the unit longer run times.

 

[iwire]

To reduce the velocity of the air coming out of the supply grills you could add more supply grills. To reduce the temperature of the air coming out of the supply grills you could increase/add fresh air in the return. This will also give the unit longer run times.

Now that makes sense as neither of those options change the airflow through the furnace or evaporator coil.

But what would probably make the most sense is having an HVAC contractor go out and look at it.

 

[Sahib]

Now that makes sense as neither of those options change the airflow through the furnace or evaporator coil.
.

Air flow volume should match the heat/cooling load. If it is so, changing the air flow in accordance with the heat/cooling load will not cause overheating or freezing up of the coils.

 

[iwire]

Air flow volume should match the heat/cooling load. If it is so, changing the air flow in accordance with the heat/cooling load will not cause overheating or freezing up of the coils.

Wrong.

Unless the unit is designed for varying airflow through the furnace or coils it can freeze up and it most defiantly can shut down on over temp.

 

[Sahib]

Wrong.

Unless the unit is designed for varying airflow through the furnace or coils it can freeze up and it most defiantly can shut down on over temp.

You mean no thermostatic control?

 

[iwire]

You mean no thermostatic control?

No, I mean what I said.

If you reduce the airflow of a furnace that was not designed for that in some way there is a very good chance the furnace high temperature limit will be exceeded and the unit will shut down.

 

[Mgraw]

There are some newer inverter type units that will control compressor speed and blower speed to match the load. These usually have multiple evaporator coils. I don't think that is what we have here.

 

[iwire]

There are some newer inverter type units that will control compressor speed and blower speed to match the load.

Exactly, or even old units that had three speed blowers where designed to operate with different airflows. Or staged compressors to match the cooling load.

On the other hand a modern unit, not designed to be variable air flow, and built down to a price point is not going to have much tolerance for running out of design. IMHO.

 

[Mgraw]

Exactly, or even old units that had three speed blowers where designed to operate with different airflows. Or staged compressors to match the cooling load.

On the other hand a modern unit, not designed to be variable air flow, and built down to a price point is not going to have much tolerance for running out of design. IMHO.

Agree. Too much airflow and the compressor runs hot.. Not enough airflow and liquid floods the compressor, evaporator could ice up(which causes more liquid to flood compressor). Both situations are bad for the compressor.

 

[ptonsparky]

Guys, if we had an HVAC contractor recommending modifications to an electrical system we would all be shouting unqualified yet you all feel qualified to recommend modifications to an HVAC system.


It makes no sense. :happysad:

:thumbsup:

 

[Sahib]

Retrofitting

Retrofitting

This article offers options for retrofitting variable volume supply air to existing HVAC installations etc., to your advantages.

http://liveperformance.com.au/sites...esign_guide_-_retrofitting_hvac_in_venues.pdf