replacement hvac

[danhotb]

Hi, All,
We are replacing our 24 year old hvac unit in our Florida townhouse. The problem is that the old unit is in a small hall closet, and the new one is going to be installed in the attic due to size (not worth tearing out a wall for 3 inches of width).
The attic is "traditional" in that the roof is not insulated, but the plane of the attic is, and it's pretty hot up there.
Is there a problem placing the hvac in the attic? Won't that reduce the efficiency of the unit or am I being paranoid? It's a new Trane ac.

[rabadger]

A lot of it depends on the contractor. Will you have access to the filter for changing? Will you have an aux. drain pan with a float that shuts down the A/C if it does not drain? will the unit sweat? If so what is he going to do at the time of installation to prevent it.

One thing is for sure. Make him do the load calculations and explain them to you.

[danh]

Duct system within conditioned envelope = good.

Standard duct leakage rates vary anywhere from 15% to 70%. Even if installed well, modern sealants don't last, modern insulating techniques don't prevent condensation & mold, etc etc.

I'd do my best to make sure that the equipment stays indoors. Unless there was a way to insulate the roofline?

dunno

[rabadger]

70% duct leakage :? If you have that much duct leakage you better have the installers finish the job.

[danh]

70% duct leakage :? If you have that much duct leakage you better have the installers finish the job.
All it takes is a windy day :)

The study I saw did over a thousand homes, and had leakage above 80% at 25 Pa for at least 80 of them.

Suddenly it becomes apparent that builders who put distribution systems in attics aren't too concerned about energy efficiency...

[rabadger]

by any chance are you confusing duct leakage with infiltration?

[danh]

by any chance are you confusing duct leakage with infiltration?
You're right in that ducts inside the conditioned envelope are not exposed to these kind of pressure differentials. But outside the conditioned envelope - crawlspaces, attics - why not?

[rabadger]

What do you mean pressure differentials? No matter where you run the ducts if they are not sealed and insulated you have larger losses. The supplies are positive pressure and returns are negitive pressure. They should not be effected by the wind outside.

The way you have used the words pressure differentials wind and duct You are confusing the terms.

Wind and air pressure can cause uncontrolled infiltration. This infiltration is a direct result of the qualifty of contruction.

Duct pressure is called static pressure wich is the resistance to air flow inside the air distribution system. It is a direct result of the duct design.

All this is taken into account when sizing equipment, and designing duct layout. After the load calculations are completed and duct layout done the equipment is picked out. You look at the equipment extended blower charts to find the amount of CFM available at the external static pressure of your design.

Do not confuse infiltration and duct loss.

[danh]

What do you mean pressure differentials? No matter where you run the ducts if they are not sealed and insulated you have larger losses. The supplies are positive pressure and returns are negitive pressure. They should not be effected by the wind outside.

Wind and air pressure can cause uncontrolled infiltration. This infiltration is a direct result of the qualifty of contruction.

Attics & crawlspaces are vented. Why would they be pressure neutral?

Wind & air pressure can only cause infiltration if exfiltration is happening elsewhere. Constant volume laws apply.

Oh I see what you mean by duct losses. You mean Reynolds number stuff. I meant exfiltration due to positive internal pressures. Exfiltration due to positive internal pressure (a function of sealing efficiency) is also dependent on air pressure in the spaces into which they leak. Meaning that stack effect & the negative pressures it generates just kinda exacerbate the problem. This is why it's so important to locate this stuff inside the conditioned envelope.

[rabadger]

When sizing equipment you need to focus on the conditioned space not the crawl space and attic. When the equipment location is determined it is up to the contractor to do the best job within reason to control duct loss.

Load calculations include the duct losses, and conditioned space inflitration of outside air. If the load calculations are accuarate and the contractor is knowledgable and able to apply equipment correctly you should not have a problem unless you already have a problem and are not addressing it with him.

Focus on the load calculations for the conditioned living space first, then address the equipment location. If you do not want the equipment in the attic tell them. Have them expalain to you why it should be loacated there and what they are going to do about added eff loss do to locating it in the unconditioned space.

[danh]

When sizing equipment you need to focus on the conditioned space not the crawl space and attic. When the equipment location is determined it is up to the contractor to do the best job within reason to control duct loss.

When danhotb started this thread, it was to ask if his replacement HVAC should go in the attic or conditioned space. I don't think that equipment sizing or load calculations are going to be run if there are no other changes to the house. He just wanted to know if attic HVAC was ok.

[Bakerdog]

This is a florida question, so cooling is probably 90% of the work. So,
why would you want to put a unit that emits heat in your conditioned space? That is kind of like opening your refrigerator door and expecting the room to get cold. In my experience, cooling systems operate extremely well in attics because the cold air wants to go down (where you want it) and the hot air wants to go back up into the attic. Sure you have warm air around your pipes but the increased pressure more than makes up for that.

[danh]

Why would you want to put a unit that emits heat in your conditioned space?

Ok, you got me on that one.

[rabadger]

1. The contractor should know the correct procedure for an attic installation.

2. They are replacing an old unit so load calculations need to be done to address any building updates etc.

3. If the unit stays in the conditioned space what part of the unit will generate enough heat when the air conditioner is on to change the unit size? I hope no one thinks the condensing unit should be indoors, and the blower motor will not do it.

4. If the attic installation is done correctly, the equipment and duct location is taken into account when the load calculations are performed, the equipment should not be effected.

I stated it before, the owner has to make sure the unit can be serviced with ease and he can change the filters when needed.