Reminder: I'm building in Western Montana, heating a 2900 sq ft house, ICF to the eaves, and a 550 sqft garage with hydronic radiant tube in 4" polished slabs throughout. Man J reveals 39,500 btuh, and as we discussed previously the ManJ is often 20% high. But that's what the ManJ says. Dana recommended the Westinghouse/HTP UFT 80 Mod/Con 10:1 boiler. There are other options out there, and I'm sure a lot of preferences based on the experience you guys have. So here is today's question: The HTP UFT 80 boiler will throttle down to 8k btuh, there is a 5:1 Lochinvar boiler that will also throttle down to 8k. Given that my system will often be calling for 8-10k btu, and only occasionally ramping up to cover the rated 39,500. How does one go about choosing? I want efficiency, and ease of ownership. Initial cost of the system is a consideration. While make/model recommendation are welcome, what I'm really after is the thought and logic behind how the decision gets made. Thanks! Scott

The tiny 5:1 turn down Lochinvar is still on the order of $1000 or more expensive than the UFT/ WBRUNG-080W up front, and Lochinvar demands that it be configured primary/secondary rather than pumped direct (they void the warranty if pumped direct), which doubles the number of pumps, and roughly doubles the pumping power use. The UFT/WRUNG is specifically designed to be pumped direct in almost all systems, to simplify the system design and cut down on system component costs. (It also is plumbed with separate port and internal controls for an indirect HW heater, which would have to be external with the Lochinvar.) Add it all up and the total initial cost is much higher, and the lifecycle cost of ownership is likely to be higher with the Lochinvar, even if it lasted twice as long.

Also, having a higher max-output means the recovery times on an indirect hot water heater will be substantially shorter too.

If Navien has better support than HTP in your area, the NHB-80 is also a 10:1 turn down stainless HX unit at a comparable price point. The NHB-55 has the same low-fire output as the -80, but only ramps up to 55K-in.

http://gadgetsgo.com/Navien-NHB-80-hot-water-boiler-gas-heaters.html?gclid=CPXC-_rgscsCFdgQgQodfDcCIQ

http://gadgetsgo.com/Navien-NHB-55-series-hot-water-gas-furnace-heating-systems.html

The gold-standard installation for a Navien is also primary/secondary, but you'd be able to pump direct if you did the math correctly.

Dana, Thanks! Talk to me about the relative efficiency(or more likely, expected efficiency, since these things aren't you tested down at their minimum output) of an 80k vs a 55k boiler running one or 2 circuits at 8-10k btu.

When you say direct, you are talking about a closed system that pumps directly from the boiler through the radiant PEX?

I'll take a look at the Navien. I've looked at the UFT and read everything out there. I like the concept. Thanks for helping with my learning curve. It's near vertical!

Scott

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Pumped direct means that the flow through the boiler is the same as the flow through the radiation. With primary/secondary configurations the boiler flow is determined by one pump, the radiation flow by a different pump (or pumps). Interaction between the flows are minimized by a hydraulic separator (either closely spaced tees, or a specially made component.) The original reasons for doing this with mod-cons of yore had to do with the relatively high pumping head/impedance of the heat exchangers, but many newer mod-cons work just fine in most systems without hydraulic separation to guarantee meeting minimum flow requirements through the boiler without some monster sized pump. (An even older reason for doing it with cast iron boilers had to do with keeping entering water temperatures at the boiler above the condensing range when dealing with high water volume radiation.)

http://www.pmengineer.com/ext/resources/SHT/Home/Images/PME_0907_Feat2Fig10Lg.jpg

The comparatively low pumping head of fire tube heat exchanger designs makes hydraulic separation unnecessary, even counter productive in many )(or most) systems, since it raises the entering water temperature at the boiler. The same pump that is driving the radiation flows will almost always provide sufficient flow through the boiler to keep it in an efficient heat-exchange range. It's possible to design a system where that couldn't work, but you'd have to get creative, but then, the more idiot proof you make something the more creative the idiots become! :-)

In general the peak efficiency of recent mod cons has been somewhere in the lower quarter of their firing range, but that has been evolving to ever lower fractions with every new heat exchanger generation. The raw combustion efficiency is a function of both firing rate and entering water temperature on the boiler.

http://www.mnshi.umn.edu/images/boilergraph-1.jpg

When the firing rates become too low there isn't much turbulence on the fire side of the heat exchanger resulting in laminar flows and lower heat transfer efficiency, and the stack temps go up (=efficiency begins to fall). I suspect the reason they didn't run the NHB-55 any lower than 8K is exactly that problem. A couple or three generations of mod-con HX designs ago that knee in the curve was happening at about 1/3 of the max firing rate, and most mod-cons had no more than a 3:1 or 4:1 turn down ratio.

I doubt very much that a mid-mass fire-tube design would fall off an efficiency-cliff at 10% of maximum fire, even if it might not be the absolute peak. A 95% figure is quite credible, maybe even on the low side for slab radiant applications, where the return water temps are reliably quite low, lower than AFUE test conditions, with raw combustion efficiency higher than standard AFUE test conditions.

Gosh I feel good today. Mind if I use that in my book Dana?

Dupe. This thing is shooting doubles today...

Might want to clean up the grammar first- today seems to be overly redundant run-on sentences with no end in sight with stray characters and random punctuation for me...

Brilliance has it's own burdens. Worth the trouble I think.

Dana, Can't thank you enough for explaining this in this way. It gives me confidence in how the systems work, and to make an equipment decision. Neither the HTP, nor the Navien have real support in Western Montana. But, I can purchase the HTP from trusted sources, and get advice and parts quickly. I'm paying for the engineering to get the right PEX layout and the right pump and piping. My questions here are an effort to learn how the system works, in layman's terms, and be able to implement what the engineer suggests.

I'm sure there will be a round 3, so thanks for the expertise available here.

Scott