So I just bought a Navient 210 NCB Combi boiler, and my contractor is here installing it. When I checked on him, he was getting ready to install the primary loop, but he was laying out the Tees at 12" apart. We checked the Navien application drawings, and they disagree with themselves.

page 6 of the application drawings (printed in 2013) calls for the tees 6 inch min, and 12 inch max apart. However, the install manual page 29 calls for them "maximum of 4 pipe diameters but no greater than 12" apart"

I called Navien support (they mostly blew me off for not having a contractors license), and called them on it. After some confusion over the discrepancy, they basically told me that with 1" pipe I should still go with 6" because anything less would cause "laminar flow", but everything I can find online says laminar flow is relevant to the distance between the tees and any other fittings, but not between the tees themselves.

My contractor, and another contractor I talked too both swear that if the tees are "too close" that the boiler will short cycle.

Has anyone ever heard of a minimum distance between tees? Thanks, Tejay

Your issue has nothing to do with laminar versus turbulent flow. Your issue is about maintaining hydraulic separation so the primary pump does not interact with the secondary pump(s). The degree to which pumps interact with each other depends on the head loss of the piping path that they share in common (i.e., their "common piping"). This is the reason for keeping this tee section as short as possible...to minimize this head loss and the pump interaction. You can read all about this in John Siegenthaler's "Modern Hydronic Heating".

With regard to laminar versus turbulent flow...you want turbulent flow because that maximizes the heat transfer between the hydronic fluid and the PEX wall. Turbulent flow keeps the hydronic fluid well mixed and prevent creating a colder layer near the PEX wall that would inhibit good heat transfer. To achieve turbulent flow, you need to achieve a specific minimum flow rate for the given diameter of PEX. You can read all about this in John's book too. Anyhow, I hope this helps...good luck!

Combi boiler are not for the novice.

The spacing is less than 5 pipe diameters and the Navien will not short-cycle if the pumps are sized properly.

If your installer doesn't have the fundamentals who will you have service this high-tech gas appliance?

Combi boiler sare not for the novice.

The spacing is less than 5 pipe diameters and the Navien will not short-cycle if the pumps are sized properly.

If your installer doesn't have the fundamentals who will you have service this high-tech gas appliance?

It's not just my contractor, the MANUFACTURER is saying 6" minimum. If it were just my contractor, I'd tell him he's nuts, but it's hard to argue with the application drawings from the guys who make the appliance. I came here to verify what I've seen everywhere else, 4 pipe diam MAX, never more than 12". Everything seems to agree with that, and at least you two are also agreeing. I figure a few thousand installs between you, so that leaves me with pretty high confidence. Thanks for the input. Any idea's why the manufacturer's literature would have it so wrong?

Yes, the fact Badger and I are in agreement should indeed give you great confidence, LOL!

Just to add to what Badger stated, all boiler installations (and especially gas fueled boilers) really need to be accomplished by a person who knows how to do so properly.

Short cycling is also not the issue here. Short cycling can occur if the control system was not installed correctly or if it subsequently fails while in service. However, most short cycling occurs simply because the heat capacity of the heat source exceeds the heat capacity of the hydronic fluid in the zone(s) calling for heat. You may want to read our instructions about buffer tank design to learn more about this:

Borst Buffer Tank Design Software

In this particular application, the spacing is being used to create some pressure differential such that there is sufficient flow through the boiler. It's not "closely spaced Ts" - make it between 6" and 12" or it may short cycle due to lack of flow.

I beg to differ.

The Navien 210 NCB combi-boiler has its own circulator assuring minimum flow. The folks at Navien are simply new at this and will catch on with time. Matching the limited space heating output with the DHW loads is the challenge when using any "tank-less" WH as a combi. We do it with great care borne from long and trying experience.

Good point, although the induced flow may serve some purpose (fault detection?) with no significant downside.

Other than waste already parasitic electricity and in some systems cost noise or in the most extreme cases cause erosion of fittings.