Temperature Rise

[sgbroimp]

This regards a CT home with a 15 year old 92% (propane fuel) Furnace. At an outdoor temperature of 20 degrees, is there a rough rule of thumb as to how long it should take to raise the indoor temperature from 65 to 69 degrees if the furnace is sized, ducted and firing correctly? (House is 1982, but with all windows/sliders replaced with Low-e in last 3 years.)

[pecmsg]

At Design conditions it should run continuously. Design conditions for your area is around 5*f

[beenthere]

No set rule of thumb.

[sgbroimp]

Interesting. I am going to have a temp rise check done and compared to spec. Is it fair to say that before doing so air filter should be clean/new and any rooms that are shut down (heat off, registers closed) or even set back (say to 60 degrees) should be opened up before the measurement taken? Also fair to say that with said rooms closed/set back that the temp rise still needs to be comfortably within limits and well south of the max?

[BNME8EZ]

If you are going to have someone check the temp rise on your system you will want to start out as is since this is the way you apparently want to run it. If the temp rise does not fall into manufacturer specs then make changes till it does. If you open everything up, do the test and set everything back the way it was after they leave you have nothing because it was tested at different conditions than you want to run in.

I do not recommend closing vents/rooms except maybe in a basement but only in special circumstances. I won't get into all the issues that arise here, just needless to say it generally causes more issues than it fixes, which are numerous and varied.

[sgbroimp]

Fair point. My only reason to open everything up and have contractor do a second reading is to see how close it comes to what the original installer intended. We only have our bedroom shut off as we like to sleep in a cold room but some rooms are set back to 60 so will get them set higher per your suggestion.

[BNME8EZ]

Fair point. My only reason to open everything up and have contractor do a second reading is to see how close it comes to what the original installer intended. We only have our bedroom shut off as we like to sleep in a cold room but some rooms are set back to 60 so will get them set higher per your suggestion.

Is this system zoned? If so how many zones?

[sgbroimp]

Single zone. Contemporary house was originally all electric, then converted to LP/Forced air in 1990. Current (second) furnace (92% HC Conquest 90, 75K btu input)15 years old gets regular maintenance and works wonderfully. Huge difference made by replacing all glass, especially in LR/DR gabled area (22' ceiling with virtually all glass wall southwest side) where all large custom glass panes replaced with tempered 1" Low-E with 1/2" airfill/laminated outer sheet.

[sgbroimp]

The numbers came out like this with no difference with all registers full open or a couple not so (my usual operating practice). These were 129 over 66 for a rise of 63 degrees. Strangely there is no spec sticker or plate on the furnace doors inside or out, but looking at the spec sheet it looks like my unit is either a 75-E3A (45-75 rise) or 75-E4A (40-70 rise), so I guess I am in decent shape overall.

[sgbroimp]

Or should the tech step the fan speed up one level to see if it can hit the center of the temp rise range (sweet spot)?

[pacnw]

The numbers came out like this with no difference with all registers full open or a couple not so (my usual operating practice). These were 129 over 66 for a rise of 63 degrees. Strangely there is no spec sticker or plate on the furnace doors inside or out, but looking at the spec sheet it looks like my unit is either a 75-E3A (45-75 rise) or 75-E4A (40-70 rise), so I guess I am in decent shape overall.

Someplace there is one.
Inside the cabinet.
In the blower compartment.

[sgbroimp]

Will have another look. Agree, must be somewhere. In the meantime I had a look at my run/off times. 30 degrees outside last night (design is 9 for my location I think, but almost never see that.) Anyway what I saw surprised me: 10 minutes firing/ 14 minutes off. Then I asked myself why such a small run/hour (less than 50%) and I had mu Eureka moment. When the house was converted from all electric, the contractor did a very careful calculation, going up just a bit for the huge 23 x 25' room with 18' gable, long wall 60% glass. He installed a 75K Lennox G16 (1990) which ran for 12 years then started to misbehave. He moved to FL so another contractor came and installed the same size furnace, no Journal J calculations (mistake). Guess what? The Lennox was an 80% so output of 60K, the replacement is 92% so 69K which is a big difference. Then we replaced all that glass, temp drops 2-3 degrees less at night setback. OK, I will wait until it gets colder, but I suspect I am overfired. Next replacement will get full Journal J calculation but I am guessing a 60K 96% Furnace will do the job. We are heating around 2100 sq. ft on 3 levels including the large gabled area of almost 600 sq. ft.

[sgbroimp]

Someplace there is one.
Inside the cabinet.
In the blower compartment.

Well, of course you were right. Inside right wall of burner compartment. It is a 75E4A with a range of 40-70 and limit of 190, so I guess I am at the upper end at 63. That plus the firing time at 30 degrees (about 40%) maybe point to some oversize that could be "fixed" when the unit is replaced, but doing the Journal J and seeing if a 60K machine might fit? Also is there an argument for a 2 stage (non-modulating) unit or not worth the extra $ in this situation?

[pecmsg]

Now your aware the unit was oversized to begin with and with the changes that have been made to the envelope its even more oversized. Operating at the upper end of the limit is a duct issue so that needs to be looked at.

Is there A/C on this furnace? Consider a heat pump!

Personally id go oil before LP but that's my preference.

[sgbroimp]

Thank you. Getting the furnace size right on the next round (this is 15 year old unit, so I am not likely keep much longer) would take care of the duct issue, correct? Yes, not enough duct for the furnace push now, but going to a 60K btu unit (just as estimate subject to actual calc.) would probably cure the duct matter right?

[pecmsg]

Thats a big assumption pending a Manual "D" calculation.

The new furnace will push slightly less CFM's so yes the temp rise "Should" go down! There are other things that can affect temperature rise so it does need to be looked at!

[sgbroimp]

Got it. On my square footage calc's they are going to have to add something for the 600 ft sq of gabled area as on a cube basis it acts more like 900 sq ft probably. Still, if it turns out we are heating, say, 2500 sq ft with a 60 K unit, that works out to only 24 btu/sq.ft and pretty low for my area (CT shore) I would think. We probably benefit for 1/4 of our total footage being open space but changing out all that glass was something the we noticed the very same night/day as it was done in winter. Much obliged for your time and thinking, sir!

[pecmsg]

There is no Sq Ft calculations.

Manual "J" for Heat Load / Loss
Manual "D" for duct size
Manual "S" for equipment selection

[pecmsg]

Depending on the ducts Heat Pumps with Hot Water coils may be an option.

Where it Ct?

[sgbroimp]

OK, will certainly consider that technology. Am in Clinton, between NH and Old Saybrook. We sail over to Shelter sometimes.