Both Rinnai & Triangle Tube have good products & reputations, (IIRC, Rinnai is the worlds largest gas fired appliance vendor) but what models are we talking about here?

And the answers to NRT.Rob's questions?

Are there any tanks (even tiny ones, internal to the product) involved with either/both?

The definition of short cycle depends on many things, but in a low-mass burner like a tankless or a modulating-condensing boiler it means any burn where less than 5-10 gallons runs through the heat exchanger. In most heating systems that's not a very long time, but it's a handful of minutes, not 150 seconds.  I doubt the Rinnai would ever short cycle on a call for heat unless it's set up incorrectly so that heating is always at it's highest output.  The minimum-modulated fire, the pumping rate, and the amount of water in the system determines how short the shortest heating cycles are, whether it's a boiler or a tankless.

During small hot-water draws << 5 gallons (say, a coupla quarts for hand washing, or a quick-rinse in the kitchen sink) the efficiency is quite low- under 50%(!).  Some units are designed with small internal tanks to pre-determine a minimum-burn (as well as avoiding the "cold water sandwich" effect of  delayed ignition in a tankless), which increases the short draw efficiency significantly, at the expense of a small standby loss.

Heating loads in Seattle are relatively small, and even baseboards can usually deliver the heat at low enough temps to take advantage of condensing burners.  But if it's costing $2000 more to go from ~83% standard-efficiency tankless to 93% efficiency condensing boiler and you're only burning ~500 therms/year the payback is longer than the anticipated lifetime of the equipment. (At the current ~$1/therm delivered retail I'm paying in New England that would still be only $50-60/year, which even on a simple-payback scenario is over 30 years , and in a present value financial analysis the payback never.)  But if costs are about the same, higher efficiency is allways better....but...

...the efficiency of the system is only as good as the system designer, and the RELIABILITY of the equipment is only as good as those who maintain it.  (And they'll both need maintenance at least every 2-4 years to keep them up to snuff.) There are plenty of ways to screw up either- get references.

I know the Seattle Rainer valley project well. My company was trying to direct a better approach when this project was in design phase. We did supply the Highpoint group. (500 + units).
For Rob and Dana,
System desciption;
40 & 50 gallon water heater
1 bronze pump always on
multiple turbonic wall air handlers switched by a line volt thermostats 1 per air handler.
Kitec pipe, failing in some applications.
Key problem was water heaters would not keep up, not enough for domestic demand.
Pipe failure also was an issue on some residences.
Add the fact that water heaters are in a always ready at 145 mode. waste fuel.
This is an example of not spending the money to do it right. These were cheep to install systems, but with a bit more system dollars applied in the right place they would not have these issues. There are a lot of these systems in the Puget sound area. Open loop with water heater heat plant and 1 bronze pump on a timer. Developers like the cost on the install side because it is low, my view it is a Guaranteed failure in time.
I am surprised Rinnai is part of the solution,
In my opinion they should get away from the open loop. This is a part of the design that bit them in the first place.
That said Open loop is the only way a Rinnai is approved as heat source.
another big issue that may be a problem is gas BTU capacity at the gas valve.
There probably is not the 180,000 BTU capacity there for the Rinnai. The design of housing authority buildings are fairly specific, the plumbing contractors would not oversize a gas line x 4 because they are nice.
sundbergtoday READ check available BTU at gas valve before you decide.
There are 2,000 plus houses in this development with the same system design.
If it was my option I would lean toward the boiler option.
Use that outdoor reset.....
Good luck,
Dan

Talk about ways of screwing it up! (YIKES! What a disaster!).

The smaller Rinnai units are 150KBTU in (not 180K) and would keep up with 1-bathroom units in Seattle just fine, but 150K in is still likely to exceed the gas distribution plumbing's capacity if it was all set up for 35-60KBTU tank burners. The tiny TriangleTube Solo 60 might be able to drop in without replumbing all the gas lines but then you'd need an indirect tank to handle the DHW loads- 60K un-buffered just won't cut it for showers or tub-filling in mid-late winter.

Be sure the bids include gas line capacity upgrades &/or indirect tanks as-necessary. (They might try to use the existing tank as a buffer for the system and set up the tankless or boiler to maintain it at some lower modulated fire than full-on to stay within the gas distribution capacity, but that would seem ripe for disaster- as kludgy as the original or worse.)

Without system details on the proposed replacements, it's hard to advise. Are they sticking with the original air handlers? Installing baseboard or...??? (Surely it's not going to be radiant floors!)

We probably all agree that open loop here is a bad idea. They probably had to keep it at 145F & constantly running to avoid Legionella hazard, and that would continue to be the case. Last thing you need is 145F water circulating in your air handlers all summer leaking unwanted heat into the space burning fuel just to keep keep the lines sterile enough for drinking.

yep,
This is a good example. The Seattle Housing Authority is a good builder. This is a beautiful addition to Seattle, removed a large section of worn public housing and replaced with contemporary buildings, parks, walk areas.These are 2-4 bedroom Town houses that sell in the $350,000 plus range. The units are finished and served by in the wall mounted fan units (Turbonics). No other option to change out. There are 2,000 town houses like this, this is only a small portion of the repair coming.
I sugested 75-85,000 BTU water heater and X Block at one time. This may fit the gas availability and venting requirement (existing) and isolate the system, contol run time better. I am guessing there is not the upper end BTU capacity to cover the Rinnai. 1/2 inch pipe has a limit. stretch it 90 feet and what do you have? Not much.... I don't have a gas pipe sizing book handy.


This is a good one to watch.

Here is a link to the Rinnai manual, it will provide flow information based on model and BTU,
http://www.blueridgecompany.com/documents/RinnaiManual.pdf

Dan

(These numbers are in KBTU/hr., max ratings.)

You can't even run a typical 35K tank at 90 feet- let's hope they at least have 3/4" gas lines!

But even if they did, north of 150K that Rinnai would have issues regulating the pressure a mere 25' out. You really need 1" to go even a short distance with a decent sized tankless.   (You may be able to cheat the odds with half inch at 5' though. :-) )

Even the TT Solo 60 would crap out past 30' if half inch is all they have, but it would be fine out to 125' with 3/4".  (This may be a deciding factor if it's 3/4 and one could avoid having to replace it all.)

Dana,
Good resource on the chart.
So you see when you have a large commercial plumbing company on build out, they size the gas to work with the appliance specified and prove to the city inspectors that there is capacity. To oversize x 3 or 4 the required pipe on 2,000 unit (house) project is pouring money on materials and union labor down the proverbial drain.
These companies do not do that. There is likely not much more supplied at the pipe than what would feed the water heater specified, I am guessing but maybe 80,000 BTU on the out side averaging all units, this is a margin of error of %50.
We supply one of the plumbing companies on the Seattle Highpoint project boilers and Radiators. When we provide for there projects it is all about the BTU and the cost of goods. No waste.
Think they ran 1 inch, 60 feet = 159,000 btu for a 50,000 BTU water heater ???? Doubt it.
like I said, best choice is a X block and a 50-75,000 btu water heater, in the 50 gallon range.
At least they could employ a flow switch to shut out the heating during domestic demand or something like that to minimize problems with capacity.
But who knows,
Dan