Replace 12 year old Navien Combi CH(?) with Navien NCB-240/130H or something else?

HollyDay

We moved into our home one year ago. For heating, cooling, hot water:

1) the home has an early gen geothermal system for heating and cooling (uses oil in the underground tubes and no one will touch our system so when it goes, it cannot be repaired)

2) 12 year old Navien combi hot water and heating (used as back up to geothermal if needed). The Navien CH model (don't know the exact model) is showing that there's a leak in the heat exchange unit with visible buildup on the outside. The flame rod needs to be replaced along with the expansion tank. We were told today that the heat exchange unit may not be able to be replaced (none in stock with distributors) and would be very expensive to replace. It's unclear if the unit, with the flame rod and expansion tank parts replaced, will keep going for several months, a year, or more with the existing heat exchange unit.

Our choices are:

1. replace the flame rod and expansion tank for not a huge $ amount and keep an eye on the unit with goal to replace the unit when it looks like it is ready to quit on us
2. replace the Navien with NCB-240/130H - the recommended unit by local plumbing company, which is an authorized Navien repair company - a huge $ investment
3. Do something else. We are previous city dwellers with zero experience with heating systems etc.

We are a family of five with teenage girls and hot water is a constant need. Our Navien is in a 4" high crawl space under the house that also houses the three large condenser units for the geothermal system. I do not know if we have space for a large water tank. The former owners must have had a water tank because the Navien tankless unit was installed in 2012 and the house was built 5 years before that. We live in the Northeast, not sure if our water is especially hard.

We would like input on what our next steps could be. Is it wise to repair the unit we have for now with new flame rod and expansion tank and let it go the course and how long we might have? Any thing else we should be doing to maintain the current unit or questions to ask our maintenance company? We like having a back up to the geothermal system with the Navien combi providing heat as well as hot water given our geothermal could fail given no repairs at this point could be done and we'd need to go to a new heating/cooling system like an air to air heat pump.

thanks for your input!

SuperTech

Anything would be better than another Navien combi in my experience. I find them to be the most problematic and lowest quality unit available. If you plan on staying in the home for more than a year or two I suggest you look for better options.

I prefer to see a boiler with an indirect water heater rather than a combi

hot_rod

any way to get the mechanical equip out of the crawl space?

Hot_water_fan

I think a system using a tank provides better performance in all situations except space constrained installs. There are plenty of fine tank options that should work if they’ll fit. That’ll cover the DHW and backup heating and a A2A heat pump can cover the geothermal when it breaks.

Jamie Hall

The very first thing to do is to determine the actual heating load of the house — a Manual J calculation. You really don't know what size boiler you need until you do that. The existing boiler may be right… may not be, too.

Then you need to compare that load with your actual hot water demand. What is your peak hot water demand, in gpm. One of the real problems with combis — whether Navien or any other brand — is a mismatch between the required domestic hot water capacity and the heating capacity. If space is available a separate mod/con and on-demand water heater may well make much better sense.

From your description of the space available you probably do not have enough space for an indirect — or any conventional — water heater.

You are very wise to have a backup for your heat pump system. It WILL break down, completely without warning.

EBEBRATT-Ed

Is there even such a thing as geothermal with oil used for heat transfer? Maybe ethylene glycol?

hot_rod

methanol and ethanol are geo fluids also

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Larry Weingarten

Hi, I agree about going with a tank for domestic hot water as they can be simple, with little to go wrong. I've added exterior sheds to homes when there simply wasn't any other place to put a tank. It's good for equipment to live where it will be easy to maintain. 😊

Yours, Larry