Navien NCB-H manual has wrong info for flushing heat exchanger?

Chris Alonzo

My Navien NCB-H just turned 1-year-old this weekend and it gave me a warning that it is time to service it.

I have never flushed it before, so I opened the User Manual and I believe the manual is wrong. It is telling me to connect the hoses to the Heating Supply and Return ports, instead of the Cold and Hot DHW ports.

There is no other mention of flushing any other ports.

Am I wrong?

Jamie Hall

Your domestic water side effectively gets flushed every time you use hot water. While it may scale up in time -- a slightly different problem -- it doesn't accumulate crud. The heating side is just recirculating the same old water around and around -- and the heat exchanger is the place it will accumulate.

Chris Alonzo

I appreciate the quick response.

We are trying to get rid of scale, right? The same old water recirculating has a limited amount of minerals to cause scale. But fresh water constantly flowing through has the potential to keep adding new minerals to cause scale and accumulate. Everything I have read on the internet so far has pointed to flushing the DHW side. For example, this YouTube video.

https://youtube.com/watch?v=Z1YQKjet80A

KC_Jones

I thought the combi boilers had a diverter valve to switch from space heating to DHW. So, isn't it just the single heat exchanger that is doing both, so there is only one thing to flush.

GGross

The diverter valves diverts the space heating fluid from flowing through the system to flow through a flat plate heat exchanger. You cannot have domestic water run through a heating system unless every component of that system is low lead/ potable water approved (and even then who would shower in boiler water?!). Generally you DO NOT want to flush out a boiler space heating heat exchanger. there are exceptions to this for sure, but in a normal system that has good fluid to begin with, and no leaks, you will never need to flush the water side of the space heating heat exchanger, it will only add more minerals to the system, and require you to purge air again. You DO WANT to flush the flat plate DHW heat exchanger periodically, these will absolutely scale up over time if you have even relatively hard water, they won't scale up as fast as a standard tankless water as they see lower temperatures, but they will scale up, they can become slightly plugged, and that will reduce flow, lower heat transfer and lead to unhappy shower goers. Generally there will be an inlet filter on the domestic cold side, this needs to be cleaned, you can flush through the flat plate heat exchanger, or if you are exceptionally handy the whole critter can be removed and soaked. Check to see if there is a section for flushing the DHW heat exchanger in there.

EDIT: If I had to guess why they show this in your manual, that model navien uses a standard tankless heater style primary heat exchanger, there is a possibility that it may plug up and need to be flushed, or they were just lazy and copy/pasted the instructions from their tankless unit to this manual.

psb75

Chris Alonzo: your questions are very relevant. It is true that mineralization will form on the heated, potable water side of a heat exchanger because of the mineral-rich, new supply of water. When I flush a Rinnai on demand DHW unit, the white vinegar always goes into the potable water isolation valve-flushing ports--red and blue. It is difficult to discern the nature of the Navien heat exchanger. I don't like their documentation. Hard to sort out.

dko

While they do not give you a diagram in the User Manual, they do have a tidbit in the Boiler Manual for the DHW heat exchanger.

psb75

There you go! Thanks dko.

Chris Alonzo

Thanks for all the comments!

My plan is to flush out DHW side yearly.

But since this is a new system, I will also flush the Heating side this one time to remove any scale. After that it should be good as long as the system doesn't have to be drained and refilled.