I've looked and looked around on the net trying to research this, but have had no luck whatsoever.  We installed a new Navien high efficiency combi boiler to replace our old 1958 conventional cast iron beast.  The installer had talked about that particular boiler not being large enough to handle adding on when we finish the basement.  They set the supply water temp to 170F when I've read about the temp reset sensor that sounds like it really maximizes the efficiency.  Our home still has the old 1 pipe "monoflow" system, and I'm wondering how much less (if any) efficient this is vs. installing a separate return water line.

We install Navien combi boilers in Minneapolis and warn people of the limited output of this particular combi boiler. It is more like a comb-water heater with full output of DHW and limited output for space heating. Perfect for certain applications with certain design criteria. The distribution piping is less critical than say, heat load, radiation, zoning, design water temperature...

I've done a number of monoflow systems and i don't see why you cannot use the Navien boiler. I assume the boiler is using the outdoor sensor but it will not be efficient if it is always asked to stay at that high a temp. That said, the most important thing is to have a primary/secondary boiler loop and use your old pump for the monoflow system. The flow is more important than temp depending on the number of rads on the system.

We sell a lot of navian and pump panel designed for them, the are a good product,
However..as Morgan points out the r ratings are less on the heat out put than the DHW side,
in other words, a 199,000 BTU unit provides 199,000 BTU for the DHW side but on the residential heating side the units seem to top out at about 100,000-120,000 BTU depending on application.
Still a lot of capacity but clearly not the 199,000 implied by the overall BTU rating.
Navian requires primary secondary loop.
As to the old 1 inch pipe with monoflow T's, sounds like the path of least resistance is to use what is in place if it worked in the past.
I would sugest perhaps 2 zones if the building will work this way,
Zone pump 1 the old area
Zone pump 2 the new basement area
You then could do the home run system on the new area should you wish.
Dan

The output of the Navien, much lik other combi-boilers is also limited by delivered water temperature, e.g. the hotter the water the lower the boiler-side output.

The mono-flow system--now all but extinct--would qualify as a primary loop and any boiler being in the loop would not necessarily require it's own pump. If a pump is added to an existing monoflow system it must be sized appropriately with the secondary boiler pump regulated for optimum heat transfer and electricity draw. Not a DIY project.

I would definitely not consider the monoflow system on this type of boiler without the primary /secondary piping and its own pump. The monoflow is not rocket science but it needs a minimum flowrate to draw properly from basement rads. Maybe not DIY but try to find a heating contractor who knows enough not to mess up the system. They are not easy to find and he should ask for references to make sure.

P/S piping is not religion it is more; an almost-idiot-proof piping method to ensure proper flow through a low-mass boiler. If the mono-flow system is sized properly for the load the pressure drop is likely small and the if the boiler is sized accordingly with proper circulator a second pump would just burn more electricity.

I just spent a couple of thousand dollars for a nice folks in St.Paul that trusted an "experienced professional" to install their Lochinvar Kniight. It never had a chance, with a guy that didn't know his elbow from his thumb and a never-opened installation manual. We would have done it with one pump (which Lochinvar allows when appropriate) instead of four and it would have worked for the last 5 years without the dozen useless service calls, changing parts and bad-mouthing condensing boilers. I wish I could upload a picture but they only allow a stick drawing here.

There is a way to add pictures but it is not a direct route. I've got to remember how but I think you need to go to an advanced posting option (duh). Do you have the attachment pane below the response box?

I agree that proper design will have the flow rate fall within the boilers best zone and on new installs that is easier to do but on retrofits it is sometimes a bit harder to do. I am running my cast iron rads and my floor heating on the internal boiler pump and it is well within the proper zone.

That said, we don't know what the heat load is for this building or the proper flow rate. It is more foolproof to have the P/S pumping and it may be necessary in this case.

Alternatly,
Think primary secondary loops as I sugested,
primary being just boiler.
secondary loops
zone 1 (pump 1) Monoflow system left in place. I assume it currently works and instal was paid for long ago. Monoflows while old in design are prety functional and have sub zone abilities using thermostatic heads.
zone 2 (pump 2) use the home run maifold and pex if thats easier,
Unsure about flow in a standard residential ap,try thinking about using a B&G ecocirc auto, you can set flow based on Proportional Pressure Control, now you have the option to test and adjust based on preformance. The flow and head on these B&G units are fairly substantial for a residential radiator loop GPM can be dialed up or down right on the pump.The B&G ecocir has a bit more flow than the new Taco Bumble bee or Grundfos Alpha and dials down to a crawl. All of these pumps though create interesting options for pump flow and electrical/ fuel efficency and cost about the same.
Naturaly one would look at a pump curve chart to check the numbers and be sure flow is adiquate.
Dan