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Author Topic: furnance heat exchanger  (Read 2458 times)

heat550

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furnance heat exchanger
« on: June 28, 2017, 02:43:31 AM »

Has anyone installed a heat ex changer in a furnance where the a coil goes .. I just had a ponder at furnace
in my second house .. It be pretty easy to put a heat exchanger in there its Lp gas furnace.  Has a A coil in it now
but the cooling systems not hooked up ( window air conditioners can do great job of cooling)  .
  Maybe a coil and heat exchanger fit in there that be pretty sweet but it be alot for the fan to push thru .
any advise be great . Its only a 1344 sqft house no basement . Boiler already hooked to fan in a closet
sounds crud . but its tightly fit to top of closet very custom fit 12x18 exchanger output 70,000 btus . living room can be 80f at any outside temp.
but another in the furnace would give whole house a more even heat going thru duct work . but can furnace
Handel the heatmor boiler temps 160f air .

Heat550
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E Yoder

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Re: furnance heat exchanger
« Reply #1 on: June 28, 2017, 05:13:33 AM »

The air temp is not going to be 160.. maybe 120-130 I would think?

If you size the hot water coil properly the air restriction shouldn't be a problem. I usually aim for 80 sq in of finned area per ton (400 cfm) of cooling.
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aarmga

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Re: furnance heat exchanger
« Reply #2 on: July 01, 2017, 02:21:23 PM »

I hardly get 120 from mine with 180* water.  I'm not positive what the temp is in the furnace plenum but the closest port sees 90-95* from the vent while the farthest sees about 85-90*. With just the furnace running on gas the air temp coming from the vents is 75-80* quite the difference!
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shepherd boy

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Re: furnance heat exchanger
« Reply #3 on: July 01, 2017, 06:02:30 PM »

I did a unit like that about 12 years ago and it's worked great, but you should get air temps over 110*. I have seen as high as 129*.    90 to 95* seems really cool.  Most blower motors have thermal protection at about 160* and you normally don't trip that. However if the coil stays hot all the time and the fan comes on and off to control your heat you could have the potential to trip your thermal if the air in the cabinet reaches close to water temp when the fan is not running.
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aarmga

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Re: furnance heat exchanger
« Reply #4 on: July 03, 2017, 10:51:34 AM »

Hmm works fine for me.  About the same as the father in law as well. 
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mlappin

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Re: furnance heat exchanger
« Reply #5 on: July 03, 2017, 01:34:24 PM »

I get a minimum of 130 degree air from the register directly above the furnace, has a whole 3 foot of duct on it, temperature drops quick the further away you get from the furnace. If some of the duct was going thru unheated/unused space I’d insulate it, but most travels thru the basement so if it loses a little heat no big deal.
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aarmga

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Re: furnance heat exchanger
« Reply #6 on: July 04, 2017, 06:05:47 PM »

I'm sure you are correct. The closest run for me is 10 feet from the furnace plenum. 95 degrees is what my little heat gun reads at the register.  The furthest one is probably 25 feet going upstairs to the master bathroom.  I read about 90 degrees from that one.  Thing is, my furnace heat is only near 80 degrees at the register opening.  Now you really have me curious as to what the temperature is at the Plenum.  This way I could see what kind of btu I am really loosing.  None of the ducting is insulated, I'm sure I could do it quite easily.  Do you have a certain way that works good for you Mr. Lappin?

I run the wood stove at a high limit of 180 degrees, with water temp around 170-180 at the HX in the furnace.  My little temp gauge is right at the interchange from pex to copper.  My furnace is a tad bit undersized for the house.  When it gets into the negative temps it has to run non stop to hold 68.  I've always used wood here, which used to be in a fireplace.  I decided since we are staying in this home for a while to upgrade and move the stove outside instead of upgrading the furnace.  The furnace is somewhat new, 2011 and is very efficient for similar models. Just a little undersized which works fine because we only use it durning the off season.
« Last Edit: July 04, 2017, 06:16:22 PM by aarmga »
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mlappin

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Re: furnance heat exchanger
« Reply #7 on: July 04, 2017, 06:40:00 PM »

I use a pyrometer and just set the probe on the register then let it sit a minute or two.

How is your furnace HX plumbed? My hot water enters downstream of the air flow, or in other words, the air exiting the HX sees the hottest water, kinda like the cross flow in a plate exchanger.

Long as your happy with how it heats your setup is fine, the longer the fan runs the more even the temperature in every room. Ours is a 200+ plus year old farmhouse, has very few registers. Far as number of registers only have five downstairs and one upstairs that splits between two rooms, this is for a 2800 plus sq ft house.

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E Yoder

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Re: furnance heat exchanger
« Reply #8 on: July 04, 2017, 07:49:04 PM »

Yes, counterflowing the water through the hot air HX can definitely help air temps.
 Also spreading the air out through the whole coil can really help, if the coil is too close to the blower it can be flung through one end of the coil. A louver can help but make sure it's in solid or you'll have a lot of noise.

 I've known guys to double stack coils too and jack the air temps up 10-20 more degrees, but you'd need to watch the airflow if you have A/C to avoid freeze-ups.
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