Navien Cascade System, New Home

TanklessDan

1st time post here, have found information on this forum very helpful in the past.



I have now had two Navien tankless units installed in two separate homes and have absolutely loved them. Upfront the issue was the clueless installer rather than the unit itself. Navien phone techs are great and once these things were dialed in the way they should have been, smooth sailing.



In the process of constructing a new home and looking at changing up the way I have done things in the past. What I am out to do is provide potable hot water and radiant heat (over subfloor, under gypcrete) with one system. Conventional plumbers I have found push boilers and indirect tanks. I am interested in trying Navien's latest combi unit in their cascade setup.



I have all sorts of questions but right now these come to mind:



1. Does anyone here have experience with the Navien Combi units? Good / Bad? Was it used in the same application I'm proposing?



2. I realize I'm probably going to get some "get a boiler" comments, but with all advice I've seen online - no one ever explains why. Please elaborate if this is your take.



3. I spoke with a rep from tanklessguys.com yesterday who advised three combi units strapped together for my application. I have a hard time taking advice from people with a stake in what I purchase. Any advice as to determine how many units I would need would be appreciated.



4. With this type of setup is there a good way to connect up to an air handler to heat a section of the house with forced air that I don't want to pipe in?



Thanks in advance!



Heating / Plumbing Details:



-Infloor heating area of 7045sqft

-Basement area to be heated w/ forced air 1683sqft

-Garage area to be heated w/ forced air 909sqft **Only want heated to 55F-60F**

-16 Sinks

-6 Shower heads (all of which have separate hand showers)

-7 Family members

TanklessDan

One more question, Sizing

Thought of one more thing to ask!



On my construction blue prints it calls for a heating system that provides 174, 560 btu/hr.



However, I've received two estimates from contractors that quoted units providing significantly less than that, under 100,000 btu/hr.



Is the output requirement different between furnaces and boilers?

TanklessDan

One more question, Sizing

Thought of one more thing to ask!



On my construction blue prints it calls for a heating system that provides 174, 560 btu/hr.



However, I've received two estimates from contractors that quoted units providing significantly less than that, under 100,000 btu/hr.



Is the output requirement different between furnaces and boilers?

Jean-David Beyer

Blueprints?

Where did the guy who drew the blueprints get that number?

Have any design aspects of the building been changed (improved) that have reduced the heat load since then (e.g., insulation, windows, etc.)?

TanklessDan

Heat loss calc

His formula was the sqft of the 3 levels x 20



Nothing has changed from those plans.

Jean-David Beyer

I am not a pro, but

"His formula was the sqft of the 3 levels x 20" sounds rather lazy. Did the two contractors do an actual heat loss calculation on your premises? If so, I would be more inclined to go with their calculataions. If not, I would drop both of them and get a contractor who will do a correct heat loss calculation.

Unknown

no

unless 1 unit is 50% eff & the other is 100% something aint right

TanklessDan

Heat Loss

Yes, I agree the x20 approach is lazy - BUT my understanding is that's a rough estimate.



Both contractors quoted units in the 70-80k btu range. I just have a hard time believing this house will require less than half of the 'ballpark' estimate AND be able to supply my hot water. Neither stated they had done a heat loss calc. Will probably try and use the guides online to do one myself within the next few days.

Jean-David Beyer

Heat Loss

My house is two zones. One zone is 448 square feet and the other is 745 square feet; so by that rule I should need 23,860 BTU/hour. I calculated it with the old Slant/Fin program and get 6,473 BTU/hour upstairs when it is 0F outside, and 22,761 BTU/hr downstairs; total 29,234 BTU/hour. Here in New Jersey I do not remember it ever going down to 0F, though it does go below 10 F for an hour or so a few days a year. Now the smallest boiler I could get puts out 71,000 BTU/hour running flat out, and it will modulate down to 14,200 BTU/hr when it is warm out. So for me, I would be better off with a smaller boiler, say half the size. It is my view that with a mod-con, you want the smallest boiler you can get bigger than your heat loss, so you can take full advantage of the modulation range of your boiler. I do not know if I could get by with a 23,860 BTU/hr output boiler or not; I think it might work, though it would be close. Most of the time, only one zone runs, so it just might work.



I do not think, as a non-contractor, it is a good idea to order a boiler based on just a rough estimate. Too small and it will be cold sometimes. Too large and it will cost too much to operate, and will do more short-cycling than it needs to.

CMadatMe

No No No

I'm going to sayand I may be wrong that you are doing hydro air for the forced air zones. Does the Navien offer 0-10V so that you can work two heating curves plus that huge domestic load. A condensing boiler can. Why the huge investment on multiple units when a single unit can do the job. Navien is limited on DHW gpm which is forcing you to go to multiple units. Labor cost to do what you want will surely exceed the cost of a single condensing boiler with an indirect.

Plumdog_2

Taking the low road

Sounds like a recipe for disaster. You are already convinced that you want to use these tankless heaters for space heating, and want confirmation. You won't get it from me. I've seen the results and nobody is happy in the end. And that 20 btu per sqaure ft. might be ballpark for the whole structure, it will be way off in some areas. Each room is different (a lot different). You need Room by Room heat loss to be able to size or configure your emmiters. And be sure to factor in the cost of having your tankless heaters serviced A LOT! They don't like being run at minimum firing rates for long periods. And don't let a Tankless salesman or Pex on Line type guy help you design your system. It's difficult to resell a house with fatal flaws (like a home-made radiant system that doesn't perform).