Carrier HAP VS Manual J 1

[JJS0228]

All

I have a 273 unit apartment project in Florida that the Carrier HAP program was used for the design.
We are currently having a number of problems that seem to be caused by the units potentially being over sized.
An outside consultant ran the calculations with the Manual J program and there were extreme differences between the HAP and Manuel J programs. The Manual J program has much less tonnage per unit.
Does anyone have previous experience with this??

 

[EnergyProfessional]

99.999999% of the time a design is wrong the problem is in front of the computer, not within the software.
How do you know the consultant's data are right? Did you compare assumptions for infiltration, insulation etc?
I'm not familiar with HAP, i use Trace. But with the right assumption on scheduling i wouldn't hesitate to use it for residential loads. Physics is physics and thermodynamics and heat transfer don't care about commercial or residential classification. Residential is just simplified since residential designers 9 often the contractor just looking in a table that says 1 ton per 400 ft²) isn't able to understand proper sizing.

I assume you have humidity problems? What system do you have? More detail needed for us to say anything about the actual problem.

 

[Drazen]

it is well known that hap will oversize residential systems, manual j should be much better reference point.

 

[JJS0228]

HerrKaLeun
The systems are basic splits in 1.5,2 and 2.5 tons and 14 seer.
Humidity is the problem.
The data entry for the two programs were apples to apples.

 

[EnergyProfessional]

by split you mean Mitsubishi units or an airhandler?
What method was used in HAP? CLTD, Heat balance, RTS, or what? re you sure HAP didn't have some plug load or high lighting loads like in commercial applications?

 

[cry22]

Daren is right
Actually, when humidity is a problem, under sizing is better, it keeps the compressor running all day, thus keeping space dry.
HAP and Trace are lousy programs for residential. They always oversize AC and downsize heating
Manual J is the only good program for single family residential.

 

[EnergyProfessional]

cry22: you say single family residential... you see he talks about 273 unit apartment building?

Not sure what the definition of undersizing is. I always try to correct-size

If you undersize by definition you won't meet the load on design conditions (i.e. 99% percentile). Under sizing would be against code, where code defines the OAT conditions to size for (at least in my State)

 

[cry22]

Herr
For residential, the "correct size is to undersized"
ASHRAE has a document out there addressing design in high humidity climate. It recommends under sizing cooling systems. It is proven to be true.
Even for 273 units, you wouldn't approach it the same way with Trace. Granted trace can do a descent block load on a multistory residential facility. You're not in the 300SF/ton business, you'd be in the 450/500 SF/ton, in mid-Atlantic weather for example.
For a single family home, you're looking at around 700 SF/ton, try trace for that, and it will give the usual commercial numbers.

Sorry to say Herr, but I used to think the same way 20-years ago.

 

[EnergyProfessional]

cr22: I'm not doubting your expertise and admit i haven't used Trace for residential yet. It may also depend on what method you use (RTS etc.), what design conditions you chose (99%, or 99.6% etc.) and what indoor conditions. Just with these three variables results in the same software can vary widely.

If you undersize, you reduce the airflow, but keep DAT the same? If you just reduce DX size,you get warmer DAT, so you dehumidify even less. for example, if you calculate you need 800 cfm at 55°F, you then reduce to 700 cfm / 55°F? i would think you don't reduce by going to 800 cfm / 60°F DAT?

I think I have a hard time understanding what the basic difference between commercial and residential is. Heat transfer through envelope is the same at given assembly, heat and moisture capacitance is the same, ventilation load depends on ventilation rates, solar load on glazing and orientation, internal loads like plug load etc. are treated the same. the bid difference is how frequent the loads occur. For an office i can assume the plug, lighting and people load is constant fro the scheduled time. for residential it is not. And in a 2-people residence, those two people may be in one room only, for short time. So assumption have to be made for that, that are different from commercial. I would think Manual J has some ideas for those assumptions, I couldn't really find a free publication that details manual J more. If nothing else, residential loads are a very simplified version (no multi-zone, we don't deal with complex equipment, very little ventilation etc.). so with all the right parameters and assumptions i think Trace could do the same. But again, i don't want to discredit your experience, especially since i haven't tried it out myself. what software do you use for Manual J calcs?

JJ: what are the actual temperature setpoints, and what humidity do they have? you say too humid, I don't know what that means. 50%, 70%? Do they have permanent ventilation, or does it cycle with the AC? Are the units tight construction, or have a lot of infiltration? i think there may be more to the problem than just unit sizing.

another problem I've seen with residential units is they discharge at ~60°F, while commercial AC discharge down to ~50°F or even below. So your dehumidification decreases dramatically. The AC contractor may not have charged the units properly. Just an idea, i recently observed that on a building to be part of the problem.

So you should provide more info, or field-survey what is going on. sizing of the units may be one problem, thee may be many others.

 

[317069]

It is not a 273 apartment building served by one system, it is 273 apartment project, could be number of townhouses, 273 single house or 273 units in a condo building, and in any case, as a residential occupancy, each unit has to be served by its own air duct system and considered as a single residential, I think most codes prevent serving two dwelling units by a common duct system.
The main post didn't mention the type of the system used, but from his answers, he said 1.5,2,and 2.5 tons are used, it tell us that it is an individual air duct system serving each unit separately.
Check if the HAP and Manual J are using the same calculation approach (CLTD for example)
ASHRAE handbook 1997, has solved same example by using different approaches and got different results each time.

 

[EnergyProfessional]

317069:
I realize these are single units and systems. But your envelope load and infiltration per unit is smaller than single-family home.

 

[317069]

Herrkaleun,
How do you know, you suppose it is 273 units in the same building, the post didn't mention about this point, no matter if a load is high or low, it is still considered as a single residential unit, some condo units load is higher than a unit in a townhouse or even small single home.
JJ:
can you tell us about this project, is it condo building or townhouses or single units, and what is the system used.
and do you have a humidity problem with each unit or some of them.

 

[ProcessHVAC]

I sized my house's gas furnace and dx coils system using eQuest with good results, as it has been mentioned before, HAP overestimates cooling load and under the heating load. As someone else mentioned too, I undersized the dx coils a bit to keep compressor running longer and get better humidity control, it works well although it barely keeps up during hottest days of the year (here its about 35 C DB)

 

[EnergyProfessional]

Does anyoen know if HAP can use RTS or heat balance method, like Trace?
nothing is mentioned here, so I fear it doesn't have that capability, which would explain inaccurate results.

But google found this where HAP actually undersized a lot, but only in the useless (imho) CLTD method.

317069: while he wasn't clear, it says "apartments" and 273 unit project. so I doubt he means 273 single family homes....

We really need to determine what features were used. for example infiltration increases with building height, due to wind and stack effect. Trace can account for that (if feature enabled). not ure about HAP. 273 units could be 15 storeys tall for all i care. So yes that would be different than a single family home. Infiltration being the biggest guesswork, especially in humid climates this is important. If they use a floor-space or ACH estimate, this will be wrong for appartments with relatively little perimeter wall. Better to use a cfm/ft² envelope method.

there are 1 mio things one could do wrong entering data. In most cases errors are due to the user, not the software (at least in case of Trace, I don't know HAP well to vouch for it)

 

[317069]

He wasn't clear, yes I agree, let us wait his answer then we can think of it.

 

[JJS0228]

Sorry I have been out of town.

I will attempt to answer the questions.

First of all the we have 10 4 story buildings that are concrete construction both exterior and all demising walls between units.
They are apartments
The buildings are orientated in all directions (my thought is this is one of the biggest problems but not an engineer).
Every unit type was sized with the same size unit no matter what the orientation.
They are exterior corridors entrances.
The units are 700, 1150 and 1200 square feet approximately.
The units are very tight and the only exhaust is the bath ventilation that is switched with the light.
Each unit has it's own individual system 1.5,2 and 2.5 tons.
Humidity is not consistent from unit to unit but in the mid 60s.
We do not have problems in all units or all buildings.
We verified that all units are now charged correctly ( many were not originally)
The off the coil temperature is around 55 degrees.

Here is some basic info for the units design (this is all Greek to me)

Indoor: Heating Cooling
West Palm Beach Intl AP, FL, US Indoor temperature (°F) 70 75
Elevation: 20 ft Design TD (°F) 22 15
Latitude: 27°N Relative humidity (%) 50 50
Outdoor: Heating Cooling Moisture difference (gr/lb) 14.7 59.2
Dry bulb (°F) 48 90 Infiltration:
Daily range (°F) - 13 ( L ) Method Simplified
Wet bulb (°F) - 78 Construction quality Average
Wind speed (mph) 15.0 7.5 Fireplaces 0
Thanks for the input JJS

 

[317069]

JJ: Can you put some details from a unit name plate as an example such as: maker, model.
did you talk to the consultant who ran Manual J calculation and found out that the units are oversized

 

[EnergyProfessional]

Some issues I see here:
- thermal mass (concrete) was not taken into account, this reduces peak load in cooling using heat-balance method. I assume Manual J just assumes leightweigth construction
- orientation was ignored.

Again, the problem is not the software, it seems the person in front of the computer.

 

[IRstuff]

1 ton/500 sq ft is something that the A/C salesguy can do by pencil and paper, which is a pretty standard number.

If your output humidity is truly higher than your input humidity, then there is a problem in the air handler. On the other hand, being that this post is in mid-July, where I would expect a 90% ambient humidity, getting it down to 60% might not be unreasonable.

> You didn't say how old the systems are
> You didn't indicate whether the system is delivering the correct amount of air and at what temperature is it at the registers?

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[EnergyProfessional]

I may not have my bearings straight for Florida rule of thumb... but I have a 2,200 ft² single free-standing home with 2-ton in WI. So I'm at 1,100 ft2 per ton. I would think an apartment with less perimeter and potentially no roof should need much less cooling. Even if I assume Florida requires twice the cooling than WI, accounting for you having apartments only it really seems your units could have been smaller. At least half a ton less per apartment. So 1-ton, 1.5 ton and 2-ton should have sufficed.
(none of my rule of thumb guess is to be taken seriously for design