Tennman said:
Heaterman, Your last post made me feel somewhat better about my 1 1/4" piping system. Per the Taco TD10 data sheet I knew I'd be exceeding the TD10 maximum 1 1/4" recommended flow rate of 11.2 gpm. With my computed head loss of a little more than 22 feet and assuming my 210kbtu boiler could consistently put out about 170kbtu I computed a required flow rate of 17 gpm (20F deltaT). Those two numbers flow rate and head put me right on the Taco 0013 curve. I had not read of anyone here using more than 1 1/4" PEX so I presumed others were routinely exceeding the recommended flow rate on 1 1/4" PEX. The existing 115kbtu propane furnace was adequate to keep our downstairs adequately warm during our short heating season, so I didn't attempt to do a heat loss. Since I'm about to dig up my PEX to foam insulate, from your experience am I way over pushing the flow rate in that 1 1/4"? Until recently I don't recall ever hearing discussions about 1 1/2" PEX.
The TD10 document implies the gpm limits are to keep flow noise at acceptable limits. But they also say keep flow velocity between 2-4 fps. At 17 gpm and 1 1/4" I'm at 6.25 fps! Now I'm wondering if I need to add a 1" line to my existing 1 1/4". Geez what a learning experience related to the underground part of this system.
Reading between the lines of what info you have posted here I wouldn't get to concerned about it. 1-1/4" will give you plenty of flow to heat your house. Charging your storage with no other load on the system will be the only time you actually need the 17gpm. ......depending of course on how the piping is configured.
As to the availability of 1-1/2 pex, I would say the the majority of our systems (mostly Garn based) go in with that size tube when the entire load is served via a single underground line, so it's out there if you need it.
Remember also that you can reduce your flow requirements by over sizing your heat emitters. As an example, I just visited a job that was done in 2002 where we did the inside piping and HX's after the customer had installed the underground himself. The building is a big old farm house with 5 bedrooms and 2 full baths and the heat load was/is very high per sq ft of floor space. The guy had run a pair of 3/4" pex lines from the OWB to the house which for all intents and purposes is severely undersized. He didn't want to dig it back up and go with bigger tube so we went with plan B. We used one pair of lines for heating and the other for DHW. I ordered him a custom heat exchanger for his furnace, 24"x24" with a four row coil configuration, to help extract heat from the water. I would say we are lucky if the flow rate hits 5GPM on that system and it should be in the neighborhood of 10 under normal design parameters. Suffice to say the darn thing heats and heats well. We got a little over 30* drop by using that ginormous HX in the plenum and when you increase the temp drop the flow requirement of course, is reduced. More than one way to skin a cat
