air handler question

[kwired]

Nobody claimed they're "qualified", that's for sure! People learn by asking others, and that's probably the biggest reason why Mike put up this forum. One came with a question, and we tell the guy there's a solution. How he does it, it's his thing. Pray, tell me that ain't so?

You are absolutely right, many of us are knowledgeable enough to realize what problems may exist with this install, and even know of some possible solutions. IIRC OP wanted to know if changing the blower speed would solve his problems, to which many stated yes but it would likely create other problems. Other then to explain what the nature of those problems may be, that is about where the electrician advice should stop and the HVAC advice should kick in. There are some people well qualified in both areas, their input is valid on both sides of the situation.




Another possible solution not mentioned is to dump a certain amount of conditioned air outside - but that really has a negative effect on efficiency and the amount of utility bills

 

[iwire]

Nobody claimed they're "qualified", that's for sure!

By them telling the the OP to modify the equipment they are claiming to be quilified to give that advice.

As much as I know about HVAC and refrigeration work I still say the best advice is to have this restaurant call in an HVAC contractor.

People learn by asking others, and that's probably the biggest reason why Mike put up this forum. One came with a question, and we tell the guy there's a solution. How he does it, it's his thing. Pray, tell me that ain't so?

I am almost 100% certain Mike did not put up this forum to answer questions about how to modify a furnace.

The mods and I regularly toss people off this forum that are from other trades asking electrical advice. That being the case it is my opinion this forum is not the place to give out advice about other trades work.

 

[kwired]

By them telling the the OP to modify the equipment they are claiming to be quilified to give that advice.

As much as I know about HVAC and refrigeration work I still say the best advice is to have this restaurant call in an HVAC contractor.



I am almost 100% certain Mike did not put up this forum to answer questions about how to modify a furnace.

The mods and I regularly toss people off this forum that are from other trades asking electrical advice. That being the case it is my opinion this forum is not the place to give out advice about other trades work.

But there is nothing wrong with telling the OP that reducing the speed of the blower may end up overheating a heat exchanger or freezing up an evaporator coil and that more then just a blower speed change is going to be needed for successful continued operation of that unit. What those changes should be is what starts to turn this into a thread for a HVAC forum instead of an electrical forum. The initial proposed solution borders on electrical related topics making it somewhat ok to ask the question here, but the most common answers to the question sends us to HVAC related topics.

 

[iwire]

But there is nothing wrong with telling the OP that reducing the speed of the blower may end up overheating a heat exchanger or freezing up an evaporator coil and that more then just a blower speed change is going to be needed for successful continued operation of that unit. What those changes should be is what starts to turn this into a thread for a HVAC forum instead of an electrical forum. The initial proposed solution borders on electrical related topics making it somewhat ok to ask the question here, but the most common answers to the question sends us to HVAC related topics.

Is there a point here or did you just have a pressing need to restate what has been said?

 

[kwired]

Is there a point here or did you just have a pressing need to restate what has been said?

What was the point of what you stated in the first half of what I quoted from you? It had already been stated as well.

I know you don't like me much but my god grow up. Getting a little sick of some of your nasty remarks, everyone is different maybe you should accept that fact.

 

[mgookin]

Sounds like everybody's right.
There are MEP engineers out there who work multiple disciplines.
Same with the trades. We have many in town who are HVAC & Elec, all 3 MEP, etc.
Same goes for combo inspectors & cross trades in regulatory compliance.

Seems iwire's point is this is an NEC forum and advice should be limited to the NEC. Makes perfect sense. Don't need some tween complaining this site caused her unicorn to fall ill.

We members are guests subject to the established rules and standards. If the man says it's off the table then it is.

 

[kwired]

Sounds like everybody's right.
There are MEP engineers out there who work multiple disciplines.
Same with the trades. We have many in town who are HVAC & Elec, all 3 MEP, etc.
Same goes for combo inspectors & cross trades in regulatory compliance.

Seems iwire's point is this is an NEC forum and advice should be limited to the NEC. Makes perfect sense. Don't need some tween complaining this site caused her unicorn to fall ill.

We members are guests subject to the established rules and standards. If the man says it's off the table then it is.

This site is more then just NEC, though NEC is one of it's biggest focuses.

There are sub forums titled business management and safety that are other aspects of the trade, the rest of them, aside from the campfire and trading post, are more directly NEC related. This particular forum is calculations/engineering, which I would assume the engineering portion may very well cover topics that get beyond NEC or even electrical, but do understand that it shouldn't necessarily turn into a "how to" for other trades either. Some aspects of multiple trades do overlap one another at times.

 

[Sahib]

The OP's problem is the velocity of air and short run times. Adding a VFD alone is not a solution.

Higher velocity and higher noise go together. Adding blower VFD with automatic control to reduce air velocity will not cause freeze or overheat. After retrofitting it ,the OP check compressor does not shortcycle.

 

[topgone]

Higher velocity and higher noise go together. Adding blower VFD with automatic control to reduce air velocity will not cause freeze or overheat. After retrofitting it ,the OP check compressor does not shortcycle.

Kindly enlighten us electrical people how that (highlighte above) is physically possible?

 

[Mgraw]

Higher velocity and higher noise go together. Adding blower VFD with automatic control to reduce air velocity will not cause freeze or overheat. After retrofitting it ,the OP check compressor does not shortcycle.

You appear to not understand the relationship of CFMs with refrigerant pressures, temperature, superheat, sub cooling, and static pressure. These are all important to proper operation to the unit. They all have target ranges designed to make the unit operate properly. You change the CFMs and you change everything else.

 

[iwire]

Kindly enlighten us electrical people how that (highlighte above) is physically possible?

You appear to not understand the relationship of CFMs with refrigerant pressures, temperature, superheat, sub cooling, and static pressure. These are all important to proper operation to the unit. They all have target ranges designed to make the unit operate properly. You change the CFMs and you change everything else.

It's simple, you just rebuild the entire unit from the ground up.

 

[Sahib]

Kindly enlighten us electrical people how that (highlighte above) is physically possible?

By providing thermostat at suitable locations and operating blower VFD through a control algorithm, the air quantity ie CFM can be varied. The operation of compressor qdepends on this variable CFM and thermostatic action.

 

[kwired]

By providing thermostat at suitable locations and operating blower VFD through a control algorithm, the air quantity ie CFM can be varied. The operation of compressor qdepends on this variable CFM and thermostatic action.

Systems that do vary air flow also vary the compressor, refrigerant, or maybe stage multiple compressors to match the CFM needs.

Similar goes for a combustion heat exchanger, they vary the combustion rate if they are going to slow the air flow down or else the heat exchanger operates at a higher then designed temperature.

If unit wasn't designed to vary heat/cool rate then you have to consider if it is even worth the cost of doing any such conversion (or if even possible) over purchasing something already designed for it.

 

[gadfly56]

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.

In terms of operating parameters, not replacing the unit is probably the worst choice. Having eliminated your best option, the following remain:

1. Increase the load on the system. During the heating and cooling seasons, increase the % of outside air and/or divert some of the outgoing air to exhaust. This will "burn off" the excess capacity and keep the unit from excessive cycling. It will also increase his utility bill, possibly by enough that replacing the unit would be economical.

2. Redesign your air distribution. Put in deflectors to prevent direct impingement of the air stream on the patrons or increase the number of outlets, possibly installing linear diffusers. The key is to change the airflow direction and/or lower the velocity without trying to reduce fan output as that can have other unwanted effects as others have mentioned. This approach will not change your quick cycling problem. If the unit is too big, that's all there is to it. On the other hand, you should see if the thermostat is possibly in the direct path of one of the HVAC outlets. This would cause it to cycle the unit too quickly. If that's the case, redirect the airflow or relocate the thermostat so that the thermostat "sees" a better time-averaged temperature for the space.

 

[Sahib]

Systems that do vary air flow also vary the compressor, refrigerant, or maybe stage multiple compressors to match the CFM needs.Similar goes for a combustion heat exchanger, they vary the combustion rate if they are going to slow the air flow down or else the heat exchanger operates at a higher then designed temperature.If unit wasn't designed to vary heat/cool rate then you have to consider if it is even worth the cost of doing any such conversion (or if even possible) over purchasing something already designed for it.

Changing refrigerant flow or multistaging compressor is one way to match CFM needs as you said. There is one more option: switching on/off compressor by thermostat to match CFM needs and that would easier/less costlier to do in OP case.

 

[iwire]

Changing refrigerant flow or multistaging compressor is one way to match CFM needs as you said. There is one more option: switching on/off compressor by thermostat to match CFM needs and that would easier/less costlier to do in OP case.

So let's follow you so far, your recommendations are

• Install a VFD to slow the airflow
• Install some control logic to run the VFD
• Install a thermostat, I assume in the duct, to short cycle the compressors so the coils don't ice up.

How are you going to handle the furnace going over temp?

 

[gadfly56]

Changing refrigerant flow or multistaging compressor is one way to match CFM needs as you said. There is one more option: switching on/off compressor by thermostat to match CFM needs and that would easier/less costlier to do in OP case.

Please note that all of your suggestions would likely cost more than simply replacing the unit with one of the correct size. This is a restaurant, not a 50,000 square foot office building. I'd WAG it as no more than 50 tons of cooling at the outside, maybe half that, with no compressor staging. Not to mention that short cycling the compressor could be just as damaging as freezing the coil.

 

[topgone]

Please note that all of your suggestions would likely cost more than simply replacing the unit with one of the correct size. This is a restaurant, not a 50,000 square foot office building. I'd WAG it as no more than 50 tons of cooling at the outside, maybe half that, with no compressor staging. Not to mention that short cycling the compressor could be just as damaging as freezing the coil.

Dang, right! It could go both ways, either you damage the compressor or you wear out the contacts of the starter!:ashamed:
My mentor used to repeat these words to me before: "care to explain how your idea won't be costlier?"

 

[Sahib]

• Install a thermostat, I assume in the duct, to short cycle the compressors so the coils don't ice up.

Short cycling is the worst case scenario; to be confirmed by calculations.

How are you going to handle the furnace going over temp?

Same as for cooling.

 

[Sahib]

"care to explain how your idea won't be costlier?"

More information is required to decide the case of whether to retrofit or to replace.