for a regular house HVAC system (Fu 1

[317069]

for a regular house HVAC system (Furnace and AC unit), is the external static pressure the same in both heating and cooling modes? in my understanding the answer is No.
I mean can we say the furnace fan is 1600 cfm @ 0.5" for heating and 1600 cfm @0.5" for cooling.

 

[willard3]

If the cooling coil is always in the airstream and the fan runs at the same speed, the pressure drop will be the same.

 

[DrRTU]

Less in heating. 1. Most heating CFM required is less thus duct is oversized in heat. 2. Wet coil has a slightly higher DP thus higher in cooling.

 

[317069]

What about 1600 cfm @0.5" in heating and 1600 cfm @0.5" in cooling for the same furnace and same house, is it right to say that?,
And as you know when we calculate fan pressure we use index circuit ( the circuit having the highest pressure drop), is this circuit the same in heating and cooling mode? ( most house supply outlets have different cfm in heating than in cooling, if we say the outlet number 9 (100 cfm for heating and 75 cfm for cooling) is the index outlet, is it the same in winter and summer.
Wet coil usually have about 0.3 inWG pressure drop for coil only.

 

[berkshire]

Normally in heat mode you need less CFM , so it is not right to say the same fan speed and pressure for the same house . For heating the fan is normally run at a lower speed, this way you get the lower CFM when heating. You have the numbers reversed.
B.E.

You are judged not by what you know, but by what you can do.

 

[DrRTU]

You must run the loads, your program should give you heating CFM and cooling CFM. You should do it room by room. Orientation, glazing and wall construction will push the numbers. A house in Alaska may peak in heating as opposed to a house in Arizona. The system selection will also drive the CFM. A heatpump with low LAT comes in at the same CFM in zone 5A where I am at. Use acca manual to design the duct and static regain on the non critical runs. Rules of thumb are great for budgets but if applied to a design, you may end up with a E&O claim. Loads and duct design are required by most codes.

 

[317069]

I agree, we must run room by room load and duct design, but my question is about principals, I am saying that when heating cfms different than cooling cfms, then the index circuit in heating mode is not the same one in cooling mode, is it right?

 

[DrRTU]

You may be over analyzing a simple design. Look for the max flow into a room for cool or heat. Use the block load to design the fan system. Design the duct based upon critical run at max flow. Chose a fan cooling speed and a fan heating speed. Design floor plan to max room CFM. Balance to plan CFM and be done. You will notice from the loads, the cooling and heating CFMs will NOT be a direct ratio for every room thus a fan speed change will NOT satisfy all rooms. Look at an outside washroom in a heating driven climate, cooling and heating may be equal. South glazing with very little ext. shading will have high cooling cfm but if rotated to N exp., the cooling CFM will drop but have the same heating CFM

The grilles get adjusted by the HO so don’t sweat that. The only way to get the system to match the calculated is with 100% room by room zoning and then this will not fix all cases.

 

[dbill74]

Going back to the OP, what is changing between heating and cooling modes?
Looks like a single unit to do both modes; assume ductwork is not changing, no VAV boxes in ductwork I assume (not unless it is very big house).

Unless you want to analyze things to the n[sup]th[/sup] degree the pressure losses of 1600 CFM through a given duct system will be the same regardless of whether it is in heating or cooling mode. Just look at your ductulator, it's not like we have different ductulators for different air temperatures.

This discussion seams to have gotten hung up on CFM requirements between heating and cooling and which is greater. And when one runs load calculations, I agree, there will be differences, what I will contend is for a residential unit it does not matter which mode requires more. I say this because it is extremely rare to find a residential unit that changes the fan speed based on the heating/cooling mode. What is important is that the unit is able to provide the maximum required CFM through the ductwork.

 

[317069]

Dr. RTU, I agree with you but as I mentioned before I am asking about the theory not a specific design.

dbill74, my concern is about using the same cfm at the same pressure loss for heating and cooling for a regular small house with simple furnace, AC unit, and simple duct design, is it right in theory to say this furnace fan give 1600 cfm @ 0.5" in heating and 1600 cfm @0.5" in cooling.?

 

[DrRTU]

Re: Theory – Fan curve vs. system curve. Why are you so concerned about heating vs cooling at the same cfm and same dp? You will also have a density shift in heating. Don’t worry about dp. If you need a universal statement use - cooling = 1600 cfm at .5” and heating = 1600 cfm.

 

[MintJulep]

can we say the furnace fan is 1600 cfm @ 0.5" for heating and 1600 cfm @0.5" for cooling

No, it's not right.

If you are "rating" a piece of equipment you are (or should be) rating it in accordance with an established standard. Without citing the standard no one will ever know the conditions used to establish the rating, and the numbers are effectively meaningless.

 

[317069]

DrRTU, I am not worry about heating vs cooling, my concern is about if the heating dp equal to cooling dp.?

 

[willard3]

The density and viscosity of the air will change a very little bit from heating to cooling, but I think the error is small enough to be ignored in a residential system.

 

[dbill74]

is it right in theory to say this furnace fan give 1600 cfm @ 0.5" in heating and 1600 cfm @0.5" in cooling.?

Because you stated "in theory" the answer is NO. Go look up the properties of moist air at different states (temperature, RH and pressure), if know anything about fluid mechanics that search result should be enough.

But as Willard said, the difference is small enough that "in practice" the answer is YES.

 

[Compositepro]

If the temperature difference between heating and cooling is 100F the density of air will change by about 20%.

 

[RandomUserName]

Less in heating. 1. Most heating CFM required is less thus duct is oversized in heat. 2. Wet coil has a slightly higher DP thus higher in cooling.

Correct. On an evaporator wet coils (in cooling) have much higher pressure drops than dry coils (in heating).

As an example: I am currently looking at the cut sheet for a basic 4 ton nominal goodman residential unitary split DC evaporator (CAPF4860). At 1600 CFM the coil has a dry pressure drop of 0.220" w.c.; when the coil is wet with condensation that number jumps to 0.355" w.c.

 

[317069]

dbill74, when we say 1600 cfm @.5", does 0.5 " here includes cooling coil, heat exchanger, filter, duct system, and grilles or just duct system and grilles. please note that 1600 cfm and 0.5" are just examples to explain my question. I am not asking in regard to a certain design, I am asking in general.

 

[MintJulep]

In general you have gotten specific answers from multiple knowledgeable members here.

They all agree.

What else do you expect?

 

[matthewjohnston]

Agree with you RandomUserName, I have also read in an article that an evaporator wet coils (in cooling) will have much higher pressure drops than dry coils (in heating).