EcoRenovator  

Go Back   EcoRenovator > Improvements > Conservation
Advanced Search
 


Blog 60+ Home Energy Saving Tips Recent Posts Search Today's Posts Mark Forums Read


Reply
 
Thread Tools Display Modes
Old 11-09-13, 03:36 PM   #11
Quest
Apprentice EcoRenovator
 
Join Date: Feb 2011
Location: Canada
Posts: 109
Thanks: 13
Thanked 13 Times in 11 Posts
Default

@OP:

Why don't you post the make/model of your 2-stage furnace for discussion?

Some 2-stage variable motor type furnace must mate with their custom thermostat (*typically already came free as part of the package during installation*) like my parent's Trane XV95 will set the blower to low until more heat is being called for... I think it has 3 or 4 steps to high (blast) but unless your house is experiencing extreme cold outside , otherwise: I only came across their (parents) furnace on high blast during the coldest winter days or nites.

Q.

Quest is offline   Reply With Quote
Old 11-10-13, 12:10 AM   #12
MN Renovator
Less usage=Cheaper bills
 
MN Renovator's Avatar
 
Join Date: Nov 2010
Location: Minneapolis, MN
Posts: 939
Thanks: 41
Thanked 116 Times in 90 Posts
Default

The OP's post and most of the thread was from January. The OP posted that he has a Trane TUD080R936K, which is an 80% 80k BTUhr 2 stage furnace.

"The fact you have a 25k loss with a 57k furnace installed is just more evidence of how much furnaces are routinely oversized. Most 2stage units you can simply unhook the 2nd stage and never miss it."

See that's the problem, the 2nd stage is more efficient so unhooking it does a disservice to the burner efficiency. ..but if the 2nd stage is running very short cycles because of the gross oversize, the burner inefficiency might even out a bit with the losses that come while everything is warming up(heat exchanger, ducts, and the 'warm up' period before the blower kicks in, and all of the losses that come after the furnace shuts off and the ducts are cooling off). This is less of an issue if the ducts are located in the living areas, more of an issue if the ducts are in an unconditioned area such as an attic, and somewhat of a loss if they are heating an unoccupied basement that isn't actively heated. I don't know the OPs situation but I think the majority of people have ducts that run through either a basement, or worse, an attic so the loss of heat from the supply ducts with short cycles can add up. I know specifically for my house it takes about 10 minutes of blower time before the ducts stabilize into their delta T, which in the mean time I could extrapolate the temperature difference from solid state efficiency and find out how much a cycle 'costs'.

I think the best thing to do is really find out your true load, not through a load calculation but through collecting data of how long the current heating system runs on the coldest winter night with the math done to the output rating of the furnace(best to have data for at least two IMHO) when there is no sun between midnight and sun up. Extrapolate the result to the design heating day if its colder than that specific winter night temperature and you should find yourself within a decent idea of what size your furnace should be. In my case a 40k single stage would be perfectly fine, it is still oversized for the load but it is also the smallest unit I can buy and for a 2100 sq ft house, it still takes awhile to heat that much air volume so IMHO it is plenty acceptable and I don't feel the need of going to a single stage and running it on 1st stage all the time, its not worth the burner efficiency loss since my cycles wouldn't be short, with a 40k furnace in my house, the cycles would be about 15 minutes or more every time the furnace kicks on and longer if there is actually any real load imposed on the house.

I also use setbacks and feel comfortable that 40k could handle my setbacks just fine, if I locked out 2nd stage the ability to use setbacks would be reduced, even on a mild day. A 40k 96.2k furnace would raise my house up 8 degrees per hour, then subtract the load on the house. To me that's fine, if I had setback at 55 degrees and an hour before I get home I have it start returning from setback, I get home at about 63 or 64 degrees and it is plenty comforable at that temperature while the furance is blowing the hot air around and after about 30 mins of being home it would be comfortable enough for it to turn off but it will continue to run for a little bit longer and when it finally shuts off I'll be nice and comfy.

Last edited by MN Renovator; 11-10-13 at 12:16 AM..
MN Renovator is offline   Reply With Quote
Old 11-10-13, 05:13 AM   #13
Quest
Apprentice EcoRenovator
 
Join Date: Feb 2011
Location: Canada
Posts: 109
Thanks: 13
Thanked 13 Times in 11 Posts
Default

Sorry MN, I didn't follow the postings all the way back to January.

Q.
Quest is offline   Reply With Quote
Old 11-10-13, 07:39 AM   #14
Servicetech
Apprentice EcoRenovator
 
Servicetech's Avatar
 
Join Date: Nov 2013
Location: Moore Oklahoma
Posts: 267
Thanks: 108
Thanked 23 Times in 21 Posts
Default

My point was if the furnace is even 50% oversized there would never be enough heat loss for 2nd stage to kick in. The only thing 2nd stage would be used for is faster recovery from setback. Most furnaces are in fact at LEAST 50% oversized. I have a 44k (40k output) furnace (as yous stated, smallest you can buy) that replaced my 88k 80%. Even the 44k is too big, what were they thinking when they originally installed the 88k?

Calculating load based on cycle times on the coldest winter night is great if you live in the house. When you downsize cycle losses decrease, you could actually go a bit smaller than the cycle test indicates. I personally have yet to see a furnace that wasn't at least 50% oversized, much less undersized...

Servicetech is offline   Reply With Quote
Reply


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 09:16 AM.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
Ad Management by RedTyger
Inactive Reminders By Icora Web Design