1" or 1.25 -- Pipe/ Spirovent

TAG

Wondering what's the common pipe size used for an under 100kBtu mod con boiler -- primarily and secondary?

The last big project I did with hydronic -- needing a new boiler/ layout -- still used a CI Buderus NG. This was about 10 years ago. After some short threaded pipe -- I went to 1.25 CU pipe for the primary with 1" CU take off to each of the manifolds. I also used 1" to the indirect from the primary loop. The Spirovent being 1.25 -- set up as in "pumping away"

Looking at some of the pictures -- think I see some 1" piping and Spirovent. Nothing larger.

My plan was to use 1.25 for the primary ....to and from the LLH -- and 1.25 out and in from the LLH to the zones -- each zone would be a 1" take off. 1" take off from primary to indirect.

All the CI Buderus were around 115K BTU input boilers. This new project is a small Viessmann 67k

hot_rod

The boiler manual generally tell you the flow rate they require. Is the 100K the input number?

EBEBRATT-Ed

@TAG
The pipe gets sized by the flow rate of water that you need to move. 1" will handle 8 gpm if your above that use 1.25".

The flow rate is based on the boiler output and the temperature drop used for the system.

TAG

My post is really asking as a "student" ... trying to learn. Sometimes looking at charts .. does not give the full picture. Real world experiences.

It seems that with a 30 delta 1" CU would be fine with taking 100k BTU with some room to spare. Almost 7gpm is needed (if my thinking/ calculations are correct)

The now discontinued CI Buderus boilers I always used were in the 112-119k BTU range -- input at 80%. I was told by the engineer I hired on the first project to use 1.25 ..so that's what I used. I just copied the same setup going forward when I did additional projects.

This project is going to use the smallest Viessmann wall type 200-W B2HA 19 -- 67k . I'm not sure what I'm going to use for my outbuilding .... but another boiler is going to be used.

Think the Viessmann boiler uses 3/4 connections -- the Viessmann LLH is 1.25. Boiler wants 6gpm through the boiler. We don't have the indirect .. but I think the Viessmann uses 1" for the indirect coil (Buderus SS tanks do)

As I said above -- my thought was to use 1.25 CU and a 1.25 Spirovent. I can't see how it does any harm .. but it looks like I could use 1" ... and save a small amount of $$.

I remember from fluids class -- Laminar flow can do strange things to calculation at the upper limits

hot_rod

1” is adequate if 6 -7 gpm is the flow required.

EBEBRATT-Ed

Yes and don't ever consider the size connections on the unit. Size the pipe for the flow rater needed

TAG

I get that the pipe is sized to the flow .... I guess people use the larger 1.25 because it gives some wiggle room.

hot_rod

Wiggle room for what?
Possibly if the system was to be expanded some day? Really no good reason to oversized piping and components for what you have described.

TAG

Well there must be some reason?

-- the vast majority seem to use 1.25 .... when "it seems" 1" Cu would work for many of these wall boilers. I'm looking at pictures here.....

That's why I'm asking .... I'm curious.

I also don't see how using the larger pipe would cause any harm and maybe it does affect flow ......

hot_rod

You could push 1" copper to 10 gpm or even 12 gpm and still be below the 5 FPS threshold commonly accepted in hydronic work. Around 6 FPS you start to hear the velocity noise.

The main reason not to oversize is low flow velocity. Around 2 FPS the air removal becomes more of a challenge, the low flow does not move the air bubbles along with the fluid and back to the air separator. The small micro bubbles tend to cling to the wall of the tube. I have built clear tube demo boards to demonstrate how velocity plays into air removal.

And if the tube was used for heat transfer, fin tube for example, the low flow velocity could go laminar and heat transfer plunges when the flow goes straight.

Cost difference aside, there is usually a tube size close to what the job requires, that is why we have so many sizes available

hot_rod

https://www-engineeringtoolbox-com.cdn.ampproject.org/ii/w1200/s/www.engineeringtoolbox.com/docs/documents/930/pressure-loss-copper-pipe-2.png