Forced-air heat exchanger sizing

[coldfeet]

I have purchased a Vedolux 37 (143k BTU's) and 500 gallons of insulated closed storage. It's all the storage I can fit.

I have a 65k BTU Trane XE 80 furnace jetted for propane and an 1,872 square foot house built in 1999. The heat-loss calculations come in at 60k BTU.

During the very cold times when we did run the propane furnace (EXPENSIVE) it would run about every 10 or 12 minutes.

Here's where it gets interesting... I've had three different 'vendors' size the furnace exchanger with different opinions and I don't want to make a mistake here and here's the confusing part- my plenum goes from 19 3/4 x 19 3/4 and then angles out to 24 x 19 3/4 so a bigger exchanger could be installed. The first 'tech' only measured the smaller 19 3/4 x 19 3/4 for an exchanger, the next one measured the larger area above and said "put the biggest exchanger you can get in there...", and the next person said to take the BTU's of the furnace and double that and get an exchanger that size.

I can fit a 224k BTU exchanger in the larger area. I know that's 4x the BTU's of the furnace output. My logic tells me the furnace will just run a shorter period because the air is hotter so the thermostat will just shut off sooner (assuming the storage is 185 degrees). As the storage cools down the furnace will run at a more normal pace because the air won't be so hot.

I just want to do the right thing the first time so if anyone has any suggestions or real-world experience that would be much appreciated.

[Holmes]

 Welcome to the FF coldfeet  You can not get 224,000 btus out of your ducting or your blower. Doubling the size of the heat exchanger should do every thing you described with lower water temps. the  Vedolux looks like an interesting boiler let us know how it works.  Holmes

[coldfeet]

Thanks for the help and advice Holmes!

I have another question... to stratify or to not stratify... that is the question. All I can spare (spacewise) is a 500 gallon tank. One person says to run the pump on the main loop only when heat is called for so the tank can stratify, another person (doing this for 30 years) says no way run the main loop constantly with the pump on so as to mix the tank.

I have spent hours online and the general concensus I find (from scientific studies no less) is that stratification yeilds more BTU's in all tests.

Does anyone else have their 5 cents on this subject?

Thanks!

[Holmes]

 Thats a good one.  ;D   I would have agreed with running the pump all the time but if studies show that stratification gives more btu storage that is the way to go, and you will get the savings of not running the circulator 24 hours a day.  Holmes

[coldfeet]

But there is a downside too... let's say the furnace sits for 20 minutes and there's also no call for hot water. My run is about 55' and I will insulate the pex well everywhere. But still the water sitting in 55' feet of pipe for 20 minutes will cool down significantly. So... when the furnace does call for heat that water will have to run for some time to heat up the pipe and get hot water to the exchanger in the furnace.

I guess there's simply no perfect way to do this! You just have to do what seems best.

[Holmes]

 When the pump comes on the cooled water will be replaced very quickly. It should not create any delays in you getting heat from your furnace.   Holmes

[Bart May]

I know exactly what you mean. Those heat exchangers rating are not at all accurate in their btu ratings. My 143kBTU oil furnace puts out 125 degree air at the exhaust duct. A 19"x21" air exchanger in the same duct work connected to a OWB with 185 degree water only produced 100 degree air at the duct and the blower ran continuously. I finally stacked two heat exchangers on top of each other in the plenum to get heat up to the equivalent 125 degrees. Works great and the air is very hot just like the oil furance!

[coldfeet]

Wow, that scares me! My furnace has the condensor in it for the AC. I'm scared to death to put in just ONE heat exchanger let alone TWO. I would think it would block the air flow from so much resistance going through two of them?

[coldfeet]

Also, (and I don't know how experienced you are- I am a babe-in-the-woods on this stuff) have you verified the gallons per minute are flowing through the exchager at the rate the exchanger is rated? If for some reason your piping or pumps are not matched properly and the flow rate through the exchanger is not up to par that would be the symptom...

Oh, and I just bought 'Modern Hydronics Heating- 3rd Edition" for $160.00. In about 30 days I will be a seasoned pro- , yeah right! I can't believe all the variables that are involved in sizing and setting one of these systems up. It takes an incredible amount of math, etc. to properly install one of these systems with no issue.

[Holmes]

coldfeet  That is a great book.  You are right about the pump sizing and flow rate.  Your pump should not be very big by pump standards, the biggest number will be the heat exchangers "head".  The piping head will not be that much if you run 1" pex.  Now coldfeet if you want to get warmfeet read up on radiant heating.Holmes

[coldfeet]

If you mean radiant floor heating I know. But I missed the boat on that when I built my house 11 years ago and thought nothing of the heating system. Forced-air fueled by propane- when it was $1.19 a gallon. Now it's $2.89 a gallon.

Regardless, I can't have radiant floor heating now house is already built. :-(