Pressurized Storage Questions: Temperatures/Mixing?

[49chevyman]

Maple - yes the big expansion tank is mounted right beside the boiler in the barn. Its about 18" x 48" tall, and has a bladder to separate the air/water. I read the pressure on the air side using the Schrader valve. I will check it next time I burn when it is hot to see what the pressure changes to.

How do I know what the lowest system pressure is? I guess it would be at the inlet side of the boiler circulator, but I have no way to measure the water pressure there. Be real nice to have pressure taps on either side of that circulator to tell what is going on in regards to head and flow.

Again - I inherited this system with the house less than a year ago. These are very unusual around here (northeast TN), so the only help I get is from this forum and you guys are awesome.

Jim - the idea of putting two circs in series may just work. Garn design manual states 6 to feet per second maximum velocity. For 1", this puts me at 11 to 14 or so gpm. Pressure loss charts are almost all at 60F, but I found one that shows the pressure drop is less as water temperature increases. Should I calculate the pressure drop for the return and supply lines differently since one runs about 160F and the other is around 110F? Also - is this the delta T that you are referencing? Thanks for the TACO curves.

 

[Bob Rohr]

Best to use the tables for the actual piping material, here is an Uponor table for 1" HePex.
4 is the conservative number, I'd say 5 fps max under continuous use.
Your tube size is the block in the road.

 

[49chevyman]

Bob - I made a system curve for the 250 feet of 1" pex using the head loss for each run at its average temperature 140F for the supply to storage and 100F for the return from storage plus my fitting losses. Plotting that against the curve for a Taco 0013, it looks like I can get about 9.0 gpm, which would put the velocity at close to the 5 fps you recommend as a max. I think I am going to try using a Taco 0013 in place of my B&G NRF-22. Flow through the system should close to double what I have now. Still may be short on being able to move all the heat from the boiler, but it should help quite a bit in reducing idling.

I will have to consider digging up my underground piping next summer. I do not like digging! Some of it is under the barn, which will be really fun.

 

[maple1]

Maple - yes the big expansion tank is mounted right beside the boiler in the barn. Its about 18" x 48" tall, and has a bladder to separate the air/water. I read the pressure on the air side using the Schrader valve. I will check it next time I burn when it is hot to see what the pressure changes to.

How do I know what the lowest system pressure is? I guess it would be at the inlet side of the boiler circulator, but I have no way to measure the water pressure there. Be real nice to have pressure taps on either side of that circulator to tell what is going on in regards to head and flow.

Once the expansion tank starts accepting expansion water, the air pressure will be the same as the system pressure. So checking air pressure when system won't tell you anything. You should only have to check/verify/adjust the air pressure in the bladder tank once - that is, before you expose it to the system pressure, to check precharge pressure.

Lowest system pressure, is system pressure when your system is at coldest typical temperature (eg. when storage is depleted). To try to determine what that should be you also need to factor in the height difference in your system, top to bottom. Example: if the height is 20 feet (close to what a system in a two story house with boiler in basement would be), that alone translates to 8psi. (20x0.4). You don't want negative pressure anywhere at any time, so add 2-4 psi to make sure you have positive pressure all the time at the top, and your cold system pressure should be 10-12psi at the bottom. Then you have to consider where your system pressure gauges are with respect to that height. If they're at the bottom, then you want them to read that pressure. If they're 6 feet off the bottom, they can read 2.5psi less. (6x0.4). That ballparks your system pressure. Then to determine what your precharge on your bladder tank, you also need to consider where it is tied into the system. If at the bottom, it should be the same or maybe a pound more than system at the bottom (10-12 psi). If tied in at the top, that would be 2-4psi. Mine had 18psi in it when I took it out of the box - way off what it needed to be for my system. Do all this when static & not pumping if concerned about running circs affecting pressure readings. Also would suggest having multiple pressure gauges around the system, they can be inaccurate. I think I have 7 scattered around mine.

 

[49chevyman]

Guys - I realized that I never updated you on what I did as a result of all of your input. I swapped the boiler pump with a Taco 0013. Made all the difference in the world. All of last season, I ran one burn in the evenings, with a couple loads of wood. Never really reached the 190 idle temperature at all, and was able to nicely heat up my storage. Not planning on replacing the 250' of 1" PEX now. Going to work on a new nozzle cover and have just upgraded my controls with RIB relays.
Also have a good two years of dry wood put up. That just makes me smile.