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[CountryBoyJohn]

Ok, I know that 1" pex carries approximately twice the BTU than 3/4".  However, I'm not quite into a full understanding of how that affects your system's performance.  I have a cousin who is installing a Hardy H4 for a 1200 sft house with decent insulation and using 3/4" pex.  He may eventually run a separate line to his garage and heat it as well, maybe 500 sft.  Today, he hinted that he may route his return line from the house to the garage and run a heat exchanger on that same 3/4" line.  I've tried to tell him that 1" is the way to go, but the guy he got his stove from has installed dozens of Hardy's with 3/4" and has has no problems.

My understanding is this:  his return temps are going to be very low.  I'm thinking sub-100 degrees.  Am I right?  What else is wrong about running small supply lines?  Will he be ok if he is running his domestic water out to the stove like most Hardy's?

[AirForcePOL]

I ran 3/4 to my house and it does just fine.  I wish I would have ran 1" when I didn't but what's done is done.  However, I only heat the house.  No DHW or anything else.  I am adding my garage which is 1600 sq ft and I ran 1" for that.  If I were him I would use two seperate loops and use 1" for both and be done with it.

[Scott7m]

Its all about flow and efficiency, its possible for him to move enough water through 3/4 but his pump size would have to be much larger and also costlier to operate.  However, im sure since the dealer skimps on pipe hell also skimp on pump size, your right if he does skimp on the pump thst returns will be cold.

I know the hardy folks well and they dont care if water returns are 80, as long as house is warm.   The stainless does give them some protection over condensation in firebox but it may contribute to there stress corrosion problems and such

[hondaracer2oo4]

I was very up to speed on this subject when I did my install 3 years ago but I am a little rusty now. What I do need to know to get started is what heat exchanger he is putting in, size, btu rating, friction loss of the heat exchanger if the manufacturer gives it. Also need the length of pex to be run. From my reading I learned that there is an ideal temperature for returning water to the boiler. That number is 20 degrees and called the delta t. To give an example I too have an h4. My heat exchanger is 24x24 water to air and is rated at 200k btus. I have a 100 foot run from the boiler to the house, so 200 total. In order for me to get my delta t to around 20 degrees I had to run two 1 inch lines each way with a bell an gosset nrf36 pump. I get a 22 degree drop return. The reason why you want to return 20 degree drops is for boiler efficiency. It is very difficult for a boiler to take drops in temp that are higher than that.

[LittleJohn]

Not sure if I missed it or just didn't read it; another major difference between 3/4" and 1" is head loss.  Some applications where house is a long distacne from OWB, pipe is up-sized to lessen head loss and reduce load on pump.

[Wood Nutt]

Use basic math to help explain it to him.  Since you are dealing with a circle (pipe), the cross section of the pipe is calculated by multiplying the radius of the pipe squared times pi (3.14).  So if you double the size of the pipe, you bascially get 4 times the amount of cross section, which you can equate that to almost 4 times the flow capability as well. 

Some quick number crunching, a 3/4 inch pipe has a cross section of .44 sq inches and a 1 inch pipe's is .79 (a 1/2 inch pipe is .20 or .2x4=.8 thus 4 times the area by doubling the pipe size from 1/2 to 1 inch).  You would only get slightly more flow from TWO 3/4 inch pipes than only ONE 1 inch pipe. 

You also need to factor in some friction of the water flowing thru the pipe.  The smaller the pipe, the greater the loss in flow from the water's friction with the sides of the pipe, this is an even bigger factor the longer your pipe run is.  If you start adding a bunch of elbows, the flow reduction really gets bad, they can really knock down flow especially if you start with a minimal pipe size.   Ask someone from your volunteer fire department about this and how much more water they can flow as the hose size goes up.  Same concept here.

So, not only do you get a much greater flow capability with the larger pipe, you get less frictional loss of the flow and thus you can deliver many more BTUs by only increasing the size of the pipe by 1/4 of an inch.  The increase in cost between the pipe sizes is a small price if the 3/4 is already marginal and does not work out and he has to dig everything up again.

I considered 1-1/4" when I did my install due to my long main run (225 feet one way), but the price of that size pipe jumped considerably over 1 inch pipe.  I ended up using 1 inch Pex-AL-Pex because its nominal inside diameter was larger than 1 inch standard PEX so I gained a little bit by doing that.  I got it for less than $1/foot.  My pump has never been above medium speed even during the coldest periods here.

[juddspaintballs]

I went with 1-1/4" when I did my install a few years ago.  I never even considered 3/4".  I also setup my system with a much larger manifold with outlets that I didn't need at the time because I may need them in the future.  It's better to oversize the piping a bit in case he adds on to what he wants to heat in the future (addition, bigger garage, previously unheated basement, sidewalk, hot tub, etc.).  As previously stated, if he goes with larger pipe the pump can be sized smaller and will work less as well as the return temperatures will be more stable.  Smaller pumps are cheaper than bigger pumps too.