Switching to tankless combi-boiler

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Livingston county, NY
Greetings! My fiance and I are new homeowners as of November. Our home came with a Buderus G115.3 boiler system that did an excellent job of keeping us warm this winter. Unfortunately the boiler and water heater in the basement take up so much room, we are switching to a tank-less combi-boiler for our heating and DHW needs.

Initially we thought it would be as easy as calling our trusted HVAC guy and setting up the installation. After contacting 12 local HVAC installers and not receiving a single call back, let alone an appointment, we realized we were completely on our own. This was no matter as I am a master automotive tech and looking forward to learning, I decided to order the Navien NCB250-150H combi-boiler from my local distributor.

The more I started to read about properly piping a system, the more I realized I may need some guidance in designing the new system. I'm pretty confident I'm on the right track but I'm hoping some fine folks here can point me in the right direction.

Please forgive my poor drawing skills, but I have mapped out our current boiler setup in addition to a proposed setup for the new unit. Pictures are attached. Basically the current system has one pump with the boiler in series and 3 zone valves that control the baseboard radiators upstairs. From what I have read, it is better to control the zones with individual circulation pumps and check valves, and to pipe the boiler in parallel to the primary piping.

On a side note: I will be using Sharkbite fittings to tie into existing copper. I am no good with a torch and solder and figured that Sharkbite would be a safe bet (other than additional cost). They are certified for use in hydronic heating systems according to their website. Can pex be used instead of copper to tie into existing piping? The boiler is moving to a different part of the basement so some additional piping will be required.


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Peace valley missouri
Hope you didn't buy that navin boiler yet. Buderus looks like it's 85k btu's navin is 150k btu's way over sized. Any combi boiler is 95% efficient with return water temp 130*f or below. Above 130* f 86% efficient. Now for some homework for dhw. What is the coldest incoming water temp, do you have a spa type shower - total gpm, do run more than one shower at a time, bath tub, clothes washer, dish washer. Need to know total hot water usage your going to be using at one time. Heres a install manual page 34 for piping diagram. Just scroll down to manuals.
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In the trades
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As a general rule combi-boilers are WAY oversized on the heating end for most homes. Since you have a winter's worth of experience with the Buderus, use the meter reading dates and heating degree-day data from a nearby weatherstation to MEASURE the design heating load using the G95 as the measuring instrument, as detailed here.

The radiation type & amounts zone by zone are also important when specifying a mod-con boiler (combi or otherwise.) The modulation ranges are not infinite, and they can short cycle when the radiation can't emit the full minimum-fire output at condensing water temperatures. See this for a more detailed explanation .

Also, there are internal exhaust leakage issues of concern with any Navien product that uses water tube heat exchangers. If you install , make sure they bubble test the seam between the burn chamber and heat exchanger (just below the dot marking "Fully modulating metal fiber premix burner"), and from the inside of the cabinet, the rubber gasket below the vent collar (the component under the dot marking " 2" venting up to 65'... " )

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