Pipe sizes for hw expansion

Brad Barbeau

Now that I've got my system running really well, I'm ready to expand into another part of the house. I've got the rads (CI, four of them) already and they were sized to the heat loss (28,415 BTU). There is the potential to go up to 50K in this part in the future if I decide to heat some other parts of the house when they are renovated - we're undecided right now.



Anyway, getting pipes to this area is going to be a bit tricky because of tight spaces. I don't think I will be able to get 1" lines in - 3/4" should be okay I think. Will supplying this area with 3/4" be okay? I could divide it into two zones and run two sets of 3/4" I suppose but that complicates matters when getting into the room.



Thanks for your suggestions,

Brad

Mark Eatherton

Pipe size....

is dictated by load and designed differential in temperature.



Heat loss calculation will tell you the load, and you can conservatively guess at a 20 degree differential. Once you know that information, you will know how big the pipes need be.



3/4" is usually limited to around 4 gpm, which at a 20 degree delta T would equate to around 40K btuH. A 30 degree DT with the same flow rate will deliver 60 KbtuH.



Over sizing the pipes can lead to air elimination/air noise issues. Undersizing can create noise issues and physical erosion of the pipe.



It's a science, tempered with art...



ME

Brad Barbeau

Delta T

Thanks Mark,

A 20 degree delta T is typical isn't it?  The outdoor reset is set so that it will run almost constantly except when the solar gains increase the indoor temperature in the late afternoon.  But I have noticed that most of the time the delta T between the supply and return, according to the boiler at least, is around 10.  I can't really remember but I believe it was greater on cold days.  If the flow is slower that will increase the delta T won't it?

Brad

Mark Eatherton

A common situation.

The 20 degree DT is a "design" principal. It makes the math easy for finding how many GPM you should be flowing (theoretically) but I have yet to figure out why, other than the easy math.



Having a lower DT is not detrimental to system operation. It simply indicates that the flow through the emitters is higher than anticipated, or (more likely) that the load is only half of what you thought it should be.



I have seen MANY systems at design condition that only had a DT of 7 to 10 degrees F. A common situation.



Slowing the flow will result in a greater DT, but not necessarily a greater energy delivery, and a slower flow can create air binding issues in certain system designs.



Which moves more energy, a flow rate of 100 GPM at a 1 degree DT, or a 1 GPM flow at 100 degree DT...?



I'll save you some time. They both deliver the same amount of energy :-)



Theoretical heat loss calculations DO NOT take into consideration the "real time" things that influence actual heat demands, like solar gains, internal gains and the flywheel mass affect. In reality, it has been my experience, that even in systems that I did an accurate heat loss, at design condition, the real time heat loss is about 1/2 of what the theoretical loss should be.



With that said, uncontrolled infiltration is the one factor that can really throw a wrench into the works. To avoid any issues associated with infiltration on existing systems, maybe you should consider hiring someone to perform a blower door test. This way, you can locate and plug the big leaks, and fine tune your load calc's to reality, instead of pulling an "air change per hour" factor out of thin air (pun intended).



Hopefully I've not confused you with reality. It is what it is.



3/4" will probably work just fine.



ME

Brad Barbeau

Thanks

That was really helpful, thanks Mark.



We've had the blower door test done with limited success. The house is stone and there are so many places in a stone wall that air could be coming in according to the guy who did it. It actually wasn't too bad, I can't recall the exact figure right now though.



3/4" it is then!