[handyman923]

My condo in central Oregon is currently all electric (stove, furnace, water heater). We are planning a remodel in the fall and were going to replace all of the above with similar electric (except use a heat pump with electric backup). However, I recently found out that natural gas is available in our condo building, so we are thinking of switching our appliances to natural gas. I have a few questions regarding the heat/ac system.
The furnace is currently in the crawl space and does not have any ductwork upstairs. We are going to move the furnace to an upstairs closet adjacent to the existing water heater and run new ductwork. I performed a Manual J analysis and came up with a heating load of 44k BTUH and a cooling load of 22k BTUH. The contractor I had out gave me estimate for a heat pump system and sized a 2-ton heat pump with 15kW electric backup. I am still waiting to her back from him regarding changing over to a gas furnace and conventional AC. In the mean time:
1) Would the AC size remain 2 ton with a conventional AC?
2) Would the CFM for the AC remain 800 CFM?
3) Would the furnace be set so that it delivers 800 CFM as well? It looks like we would need a 60k BTU furnace.
4) Is there any issue locating a cold air return over the door to the furnace closet? It will be a condensing furnace so the combustion air will come from outside.

Thanks!

 

[BayouRunner]

1 yes, 2 yes, 3 yes. Sorry I don't quite understand #4 question

 

[Bitters]

I agree with Bayou. #4 yes that would work. I wouldn't oversize the furnace by 20k btu.

 

[handyman923]

Thanks for the replies...

The contractor I talked with uses Carrier which looks like it comes in 40k BTU or 60k BTU. Are there other ones that come in 50k?

 

[user_12345a]

lennox comes in 44, 66 and 90k.

many are 40-60-80 etc.

they used to be commonly 50-75-100 etc.

Check out rheem because they tend to offer furnaces in many sizes, 10 to 15 000 btu increments.

You can get a 60k 2-stage and not suffer from the problems of oversizing provided it's controlled by a 2-stage thermostat.

Runs a 40k most of the time, when u need the extra capacity it's there.

Key is the 2-stage stat so it can cycle from high back down to low and only switch to high on demand. otherwise it will time out after 10 minutes or so and finish the cycle quickly.

U only want it cycling to high as needed, when bringing the temp up and a couple of really cold nights each year.

60 isn't terribly out of line. 80 would be.

 

[user_12345a]

3) Would the furnace be set so that it delivers 800 CFM as well? It looks like we would need a 60k BTU furnace.

The 60k will be 900 to 1000 on high, 600 to 700 on low if 2-stage. Today's furnaces move a lot of air to get the efficiency rating and not crack the heat exchanger.

Do check the load calculation inputs because an error can throw off the btu requirements -> for example over-estimating air leakage.

 

[handyman923]

I took a look at the air infiltration information on my Manual J software. Previously I had selected "Average Tightness" with 0.46 ACH for summer and 0.93 ACH in winter. If I change it to "Excellent Tightness" with 0.2 ACH for summer and 0.36 ACH. This reduced the total heating load to 33k BTUH. This seems like a huge variable.

The condo was built in 1977, so I doubt they did much for air sealing. The floor is constructed with 2x6 T&G decking and the roof is constructed with rigid insulation over 2x6 T&G decking. The walls are 2x4 with paper faced backing.

Do think my "average tightness" seems appropriate for the construction?

How close should one size a furnace to the design load? Lennox has 45k BTU furnace. At 96% efficient, it would be just below my original design load of 45k BTU.

Thanks!

 

[yuri]

I would not worry about +/- 5 k in either direction. I can heat my Uncle's 1978 900 sq ft poorly insulated house with a 45K Lennox G51 and am currently doing so. We get a LOT colder than you so I would not worry. I was amazed it works well but the next size up 60K was too big and the load calc said it would work. I have also seen it in old leaky 1950's houses of the same size with stone foundations.

As long as the gas water heater has it's own room and combustion air supply it should be OK. You cannot have them in the same room and suck air out with a open return vent.

If the ductwork is large enough you may get away with a lower than 800 cfm. The installer would need to check the freon pressures on a 70 deg F day and lower than fan speed to the point where the coil temp is safe and not going to freeze. It gives you better humidity removal and is quieter. If he does not check it properly and lowers the speed then damage can occur.

I would use average tightness. I have a very tight home but it has plastic vapor barrier on the walls and ceiling before the insulation and they caulked all the seams before sticking it on with staples. Plus I have 3 pane windows. Doubt you are in the same category and that is what they probably consider tight or well sealed.

 

[handyman923]

I took a look at the blower speed data for the Lennox 45k BTU


High: 1215 cfm @ 0.5"
Med-High: 1090 cfm @ 0.5"
Med: 980 cfm @ 0.5"
Med-Low: 705 cfm @ 0.5"
Low: 635 cfm @ 0.5"


Am I correct in assuming these are the blower speed options for "High" of the 2nd stage?


How many cfm do you recommend I design the duct work for?


Thanks!

 

[yuri]

There are specs for heat and cool.

The 1200 is max and for high speed cooling for a 3 ton AC. 1 ton = 400 cfm. Need 400 cfm/ton on average. However if your ducts are big enough you can go as small as 350 cfm/ton if setup properly.

I don't do duct sizing but it can never be too big IMO. Unless you got some old coal furnace huge old ducts.

I would use a 16x25 return and get a 4 or 5" filter box and use a Merv 8 pleated filter as they have less pressure drop/resistance.

 

[user_12345a]

I took a look at the air infiltration information on my Manual J software. Previously I had selected "Average Tightness" with 0.46 ACH for summer and 0.93 ACH in winter. If I change it to "Excellent Tightness" with 0.2 ACH for summer and 0.36 ACH. This reduced the total heating load to 33k BTUH. This seems like a huge variable.

Be careful with software. Definitions of average, excellent greatly vary.

The only way to be relatively accurate is to have a blower door test done, calculate ach natural (cfm / n factor) and manually input it.

Even with ach natural there's a huge range and it's impossible to be accurate within 10% or less, more like +/- 30%.

How many cfm do you recommend I design the duct work for?

Don't you have ductwork?

That's a job for a designer - the cfm would be based on whichever mode needs the most airflow and the branch lines as sized to deliver the correct cfm to each room based on heat loss/gain calcs.

I suspect that there is such a think as too big relative to airflow -> there has to be sufficient pressure in the duct to push air out to distant rooms, not just trickle out of near by vents.

A 45k furnace would probably need 700 cfm to 900 cfm.

Depends on the rated temperature rise range and what's desired.

Gets plugged into this formula...

airflow = BTU output / (1.08 x temp difference./delta-t)

 

[supers05]

#4.
If you mean no return ductwork and just a grill then everything in the room including the hot water heater would have to be sealed combustion. (intake pipes, completely glued) Even then some manuals don't allow for it. A short piece of return ductwork to the vent by the door would be just fine though.

My 900sq.fthouse is 60ish years old, detatched and is now heated by a 40kbtu furnace. (previously it was a 36kbtu heat pump) We have a similar climate.

Cheers!

 

[handyman923]

I do have some existing ductwork. However, it just feeds the downstairs. We are moving the furnace to an upstairs closet, so I will need new ducts to feed the upstairs and to tie into the existing ducts downstairs. I have started doing a Manual D duct design so I can get some preliminary sizes.

I looked at the Lennox 45k specs and see that it has a temperature rise of 35 to 65 deg. Using the above equation, this corresponds to a cfm range of 612 to 1137 cfm.

Does the HVAC tech set the Delta T to use? Or does he set blower speed? (Low, med-low, medium, etc) Also, what ESP should I use? 0.5"? How does the furnace know what ESP I used?

Thanks so much!

 

[user_12345a]

When the furnace is set up the tech is "supposed" to check the temperature rise and adjust the fan speed if needed. The fuel input is also supposed to be verified otherwise the temp rise means little.

40 to 50f is good. You'll be at least at 800 cfm especially if u get the version with the 3 ton drive. With a 2-ton ac, the cooling fan speed will have to be reduced, a lot of installers may leave it on high.

What static you get isn't set in stone, what happens on paper is different than what happens in the real world. 0.5" is what the manufacturers design their stuff around.

get a good designer.

 

[handyman923]

Thanks for all of your help! I think I am starting to understand how this works...

Couple more questions:

1) Using the blower data in Post #9 above, if he sets the furnace blower to medium, I should get somewhere around 980 cfm (depending on the actual esp). An using the heat equation Delta T = 43000/(1.08x980), he should get approximately 40 deg temp rise. Is this correct?
2) For cooling, it looks like I can use Medium at 980 cfm or Medium-Low at 705 cfm. With a 2-ton AC, I would have 490 cfm/ton or 352 cfm/ton. Is it okay using the 352 cfm/ton?
3) What determines the ESP you get? Is it a sum of all of the losses you get (i.e. supply duct, return duct, coil, filter, grilles)?

 

[user_12345a]

3) What determines the ESP you get? Is it a sum of all of the losses you get (i.e. supply duct, return duct, coil, filter, grilles)?

Yes, although the losses vary as airflow changes.

For example if u look at the specs of an evaporator coil, it will have a table which lists the pressure drops at different flow rates.

A function of the fan and all the components.

The static pressure drops as the fan speed is reduced, increase as the fan speed is increased so the difference in cfm between taps isn't that significant, maybe 100 cfm or so.

Relationship between fan power and airflow isn't linear, it's probably squared or cubed.

or something like that, it's over my head. just know there are fan laws used to calculate this stuff and esp isn't fixed and the airflow or esp has to be field checked once the unit is up and running.

The furnace will probably come set from the factory for low or medium low in heating mode and high in cooling mode. Odds are with properly sized ducts the factory setting for heat should be fine -> good installer would verify it anyhow. Cooling speed would have to be reduced for the 2 ton a/c

 

[user_12345a]

High: 1215 cfm @ 0.5"
Med-High: 1090 cfm @ 0.5"
Med: 980 cfm @ 0.5"
Med-Low: 705 cfm @ 0.5"
Low: 635 cfm @ 0.5"

I don't know which furnace you're looking at - looks like it may have an ecm motor.


If it has an x13 ecm motor it will compensate a bit for esp at each speed so the differences between taps will be greater.

The x13 is constant torque, meaning the speeds aren't fixed, only the amount of force being applied to the blower wheel is.

Gives a greater airflow range, the 3 ton psc would move 700-800 cfm on low at 0.5"

 

[handyman923]

I was looking at a Lennox 45k btu with an ecm motor.

With a design load of 43k BTU, do you see a major issue using a 40k BTU furnace (95%)? It seems like I have a lot more manufacturer choices (and thus contractors) at the 40k BTU level (rather than 45k).

We will probably be installing a 75% efficient 25k BTU direct vent fireplace as well. I figure that would provide supplemental heat since we are a bit short.

 

[user_12345a]

can't make the call online.

the load calc could be off in either direction by 10% or more.

depends on how fine you want to cut it. You could get a 60k 2-stage for the extra capacity if needed.