RTU's not cooling the building enough
Today and the past few ones have been scorching hot outside and the temperatures inside our facility have been suffering badly.
We have several 7.5 ton units each serving one wing of the building. On days with outside temps of up to about 95F, the building seems comfortable enough and the units seem able to cope up. But when it gets to about 100F out, the hallway temps begin to rise to about 78 to 81 and everyone will of course start complaining about it.
We clean the coils regularly and we have the HVAC contractor doing regular PM's on the units. I know that these units were not designed based on these very high temperatures but I would think they should also be able to keep the building comfortable at all times.
We have replaced older units with new ones, and the contractor have just changed them with the same sizes. With these new ones being used, we're always still having this issue of not enough cooling inside on really hot weather and we would at times end up renting portable ones to supplement the cooling.
Do you guys think we should get bigger units? I don't remember our contractor having done any load calculations inside our facility, and we'll soon need to replace another one. Would appreciate any suggestions.
A larger unit will most likely require a curb adapter if not a curb change. Also you have to take into account the higher electrical load and the existing duct work if adding more capacity.
Given your location it my be more practical to struggle through the few days a year you have to deal with excessive ambient.
Its actually simple physics not the sizes of your units.
If I want to remove 110°F heat from a condenser coil I have to have a temperature lower then 110°F.
We know this cause of the Second law of Thermodynamics, heat moves to cold.
Hence, the closer outside temperatures come to the temperature of your refrigerant in the condenser coils the less amount of heat you will remove. Since the point of an AC system is to take heat from where we don't want it and move it to where we don't care if the system can't remove the heat...........
If you're too "open" minded, your brains will fall out.
Artificial intelligence is no match for natural stupidity.
Outside air intake puts a big hit on capacity during extreme periods. You might consider making a temporary adjustment to the osa volume. Can't adjust equipment efficiency or insulation .
Hallways getting hot, how are the offices?... are there windows in these hallways? If your unit is not sized for such hot temperatures then you might try ways to lower the cooling load... like blinds on windows, turning off some lights, etc...
Lowering your OSA might help, but your employees might start complaining about air quality.
You can call me Sam
It should be a crime to be a mechanical engineer in San Diego
Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7
It's design temp meeting capacity.
Originally Posted by abcando
I would see if the hallways are primary served by a unit or two that you can identify, and upsize those units. Adding a couple of multi stage larger units allows for good humidity control when operating near setpoint, and increased capacity during the hottest days.
Yes, a larger curb, some ductwork, and possible electrical service to the new units needs to be considered.
[Avatar photo from a Florida training accident. Everyone walked away.]
2 Tim 3:16-17
RSES CMS, HVAC Electrical Specialist
AOP Forum Rules:
Just a guess, Have these condensers been split to clean? Awhile back one of our accounts was not keeping up. You could run water through it. split and clean little seeds and strands from plant life.
Do you turn the units off at night? Timeclock or control system? I agree with the other guys, just trying to throw something else out there.
Everything been tested & balanced? ..
This is not the ARP forum
Last edited by beenthere; 07-06-2012 at 04:40 PM.
Reason: removed negative political comment
Today's forecast for the Chicago area - 103F, geez! Thanks guys for the advices. We are actually a nursing home facility with about 200 bed capacity. A 7.5 ton RTU serves a wing with an average of 15 resident rooms (mostly 2 beds) and 1 or 2 offices.
There is one thermostat per wing located in each hallway. When the stats read high in the hallways, you can expect the resident rooms and offices to be warmer.
Larger curbs, some ductwork and electrical service changes seem like a lot of work, costly too, but good to be informed about it.
We have 3 units with double coils on them and I make sure we split them when cleaning including this stupidly setup smaller unit in the picture for our laundry. It makes me mad whenever we clean this one.
We also have covered most of our outside air intakes. Not good I know, but no complaints about air quality somehow. We also advised the residents to keep the blinds closed for now.
I think this is what we'll end up doing for this season again. But summer has just started a few days ago. I dread of thinking how much more hot weather we're going to have.
Originally Posted by Six
I know this is going to elicit a number of negative comments, buy I've seen water sprinklers wetting the coils to get thru extreme conditions.
is the returns ducted from the space back up to the rooftop or egg crated tiles pulling the ceiling heat back?
Record setting temperatures and equipment not designed for those temperatures are causing problems all over.