hvac basic terms and concepts

[dtx]

Hello: trying to make sure I have some basic terms and concepts down before trying to get a new system installed...please let me know if there is an existing post with hvac basics....

1) BTUs -- is this just a metric of how much air can be blown? i thought it only had to do with the amount of heat a gas furnance could put out

2) what is the difference between a "heat-gain" calculation and a manual J or D calculation?

3) at what outdoor temperature are heat-pumps not energy efficient?

4) when I say a 3-ton AC, does the tonage only speak to the cooling or is that the amount of the air the system (heating & cooling) can move?

5) it seems SEER ratings range from 13-19 efficiency, what's the range for a heat-pump?

6) how many 6" supplies make a ton? how many 8" supplies make a ton?

7) do you have to have as many returns as supplies? ie- if you have 10 6" supplies do you need 10 6" returns (or their equivilent)?

[hvacpope]

Originally posted by dtx
Hello: trying to make sure I have some basic terms and concepts down before trying to get a new system installed...please let me know if there is an existing post with hvac basics....

1) BTUs -- is this just a metric of how much air can be blown? i thought it only had to do with the amount of heat a gas furnance could put out

2) what is the difference between a "heat-gain" calculation and a manual J or D calculation?

3) at what outdoor temperature are heat-pumps not energy efficient?

4) when I say a 3-ton AC, does the tonage only speak to the cooling or is that the amount of the air the system (heating & cooling) can move?

5) it seems SEER ratings range from 13-19 efficiency, what's the range for a heat-pump?

6) how many 6" supplies make a ton? how many 8" supplies make a ton?

7) do you have to have as many returns as supplies? ie- if you have 10 6" supplies do you need 10 6" returns (or their equivilent)?


if you were a tree, what kind of tree would you be?

[comfort comando]

lets see if i can help some.
#1 & #2 look in the dictionary for " British Thermal Units" which refers to the amount of heat the furnace can produce and "heat gain".
#3 heat pumps loose efficiency with every degree that the out door ambient temperature drops. this is an aggressive curve. i generally set my h/p's up to cut of at 40 degrees.
#4 tonnage refers to the capacity of the air conditioner or heat pump. you do need enough cfm to accommodate the tonnage of unit you have coming out of your furnace.
#5 look up "Seasonal Energy Efficiency Ratio"
#6 there is no specific amount of supply ducts that equal a ton. that's where you need to have the system designed.
#7 you don't need equal # of supplies and returns. you can have one large return so long as it is sized correctly. this is typical in most homes.
hope this info helps. with most jobs getting the design and installation done right is more important than the equipment you buy.
good luck

[dtx]

thanks for the info...

also, if, and I say IF b/c I have a feeling after reading posts for several days everyone will say Manual J calc, you had to use a rule of thumb approach....

is it better to go by square footage or how many vents you have?

[comfort comando]

Originally posted by dtx
thanks for the info...

also, if, and I say IF b/c I have a feeling after reading posts for several days everyone will say Manual J calc, you had to use a rule of thumb approach....

is it better to go by square footage or how many vents you have?


#6 there is no specific amount of supply ducts that equal a ton. that's where you need to have the system designed.

some guys go by ether or. the best way is load calculation.
with practice i can manualy do most homes in 15-20 min. i cant believe all estimators don't do it. of course it seperates the hacks and the rest of us.

[hvacstudentsteve]

DTX:

1. A BTU or British Thermal Unit is the amount of heat energy needed to raise one pound of water one degree in temperature.
2. A heat gain calculation tells you the amount of heat that will leak into the house during the cooling season, and thus the size of equipment you will need to keep the house cool. It is done with the manual J which is a series of calculations taking into account the square footage, number of occupants, outside design temperature and a bunch of other factors. The manual J is published by the heating and air conditioning contractors association. A heat load can also be done with computer software. In your case, just know that anyone who tells you they can size your system by "rule of thumb" is either a hack or a liar.
3. I have heard that heat pumps are inefficient under 35 degrees outdoor temp, but generally they are very good in some areas of the country and useless in others. I live in Georgia which is part of the "heat pump belt." My best friend lives in Pittsburgh and he couldn't use a heat pump.
4. A ton is the amount of heat energy needed to melt a one ton block of ice in 24 hours. One ton= 12,000 btu per hour or 288,000 btu per day. Air conditioners and furnaces are rated in btu per hour. In this context it refers to the amount of heat a machine can remove from your house in the cooling season or add to your house in the heating season.
5. What comfort commando said.
6. Ditto.
7. While we are on the subject of ducts, requirements vary from house to house. Again, a reputable contractor will use a formula to calculate duct sizes. Beware of a contractor who evades or tries to minimize these kinds of questions. It is your house, these machines are not cheap, and you have every right to make the most informed decision possible.
If you want some more information, you could try Wikipedia. They have an ecellent article on refrigeration.

[hvacstudentsteve]

oops.. in question 4 I should have said that furnaces are rated in btu/hour while air conditioners are rated in tons. Sorry.

[smokin68]

Originally posted by dtx
Hello: trying to make sure I have some basic terms and concepts down before trying to get a new system installed...please let me know if there is an existing post with hvac basics....

1) BTUs -- is this just a metric of how much air can be blown? i thought it only had to do with the amount of heat a gas furnance could put out

Like was stated,a British Thermal unit. The more units, the more heat.


2) what is the difference between a "heat-gain" calculation and a manual J or D calculation?

Heat gain is for sizing your A/C

Heat loss is for sizing your Heat

Manual D is for sizing the duct system once A/C and Furnace size has been established.




3) at what outdoor temperature are heat-pumps not energy efficient?


They're always efficient to very low temps, even in the negatives( -20).

The prob is they lose their capacity as the outdoor temp drops. If your home requires 60k btu of heat at 10 degrees outside, and your heatpump only puts out 30K btu at that temp, it needs help(back-up heat source). It's still efficient.

4) when I say a 3-ton AC, does the tonage only speak to the cooling or is that the amount of the air the system (heating & cooling) can move?

Each system is rated differently, and will have a rating for cooling and heating.


5) it seems SEER ratings range from 13-19 efficiency, what's the range for a heat-pump?

It varies. it could be 10- 24 from what I'm told.



6) how many 6" supplies make a ton? how many 8" supplies make a ton?

Depends on the manual D design....


7) do you have to have as many returns as supplies? ie- if you have 10 6" supplies do you need 10 6" returns (or their equivilent)?



No. You do have to have as much return capability as the supplies output or you'll starve the evaporator of airflow.


This could be one big return or multiple.

[tecman]

The answers to #3 are generally not quite accurate. Modern heat pumps can deliver 2 watts of heat for every watt of electricity at 0 deg F. This is double the heat out vs. power in, or 200% efficient. The problem is that the capacity (number of BTUs delivered) drops as it gets colder, and eventually there are not enough BTUs to maintain the heat loss in the house (called the balance point). A source of supplemental heat is then used, which loweres the OVERALL efficiency.

For a modern unit here in NE PA, if properly sized, the temperature at which supplemental heat is required can be down well in the teens.

paul

[smngmu]

I seem to remember hearing or reading something about this somewhere.YES!! MY certification class.Now if I could just find a web site to ask a doctor how to do a liver transplant at home.

[the dangling wrangler]

Good One

[smokin68]

I didn't think the questions were out of line. Google would have provided the answers...

[dtx]

thanks for the great replies....

and for some of the latter replies, I am not trying to do this myself, just would like to have some education before making a huge financial investment and to be able to help differentiate between a good contractor and a hack...my apologizes if these questions frustrate your superior intellect

got to have some idea of what's going on or it'd be like a stock broker selling someone's grandma a hot .com internet stock to replace her fixed income portfolio