Changing out my hydronic heat source

csnellin

My 20 year old wood fired water heater recent failed and I would like to replace it with an on-demand condensing boiler like a Navien. The pex in my slab-on-grade is not oxygen barrier type. This would be a closed system with no DHW. Can I use this type of heat source? Obviously I have used boiler chemicals with my old system so is there a way to flush out the tubes sufficiently to use an open system with DHW circulation? Any suggestions will be appreciated.

Jamie Hall

I'm really not keen on open systems... the temperatures in the slab are much too friendly to a variety of unfortunate bacteria. A good mon/con boiler, though suited to the heat demand of the building, should work well -- provided you keep the water in the closed system properly treated to control corrosion. Do not use an on-demand water heater; they aren't meant for the purpose.

Zman

My concerns are two fold. Jamie's bacteria concern is one. Introducing whatever trace chemicals are left in the old system is the other. There are plenty of combi boilers in the Navien price range that would serve you well.

hot_rod

I doubt you would be able to scrub all those past chemicals out of the piping and components. It is tough to even get the small out.
Run a load calc and find a Combi to fit. Stay far away from open systems

csnellin

Thank you for your comments. Do any of you recommend a sediment trap on the cold water return?

ethicalpaul

Could someone give a quick definition of open vs cold systems as used in this discussion?

Zman

csnellin said:

Thank you for your comments. Do any of you recommend a sediment trap on the cold water return?

Assuming a closed system, a sediment trap is a good idea on any boiler replacement, especially with non barrier tubing.

Zman

ethicalpaul said:

Could someone give a quick definition of open vs cold systems as used in this discussion?

@ethicalpaul
An open system in this case would be DHW and heat in the same pipe with the 2 sides open to each other. A closed system would be the traditional closed loop heat setup.

ethicalpaul

So DHW would be in contact with the boiler interior and the radiant tubing?

Jamie Hall

In closed systems are those with an indirect for the domestic hot water. The indirect is heated by a coil, in which a circulator circulates hot water from the boiler. An aquastat on the indirect directs that circulator to turn on and one on the boiler or feed line directs the boiler to turn on if the boiler water isn't hot enough.

Then the radiant or baseboard or air handlers or whatever are also fed by their own circulators or zone valves, and mixing valves as needed, also by the boiler water.

DHW is never anywhere near the boiler or the radiant tubing.

It is also possible to use a domestic hot water coil in the boiler; again the DHW is separated from the boiler water. This is not, in most cases, as satisfactory, as the boiler must run hotter to get anything like enough, hot enough DHW.

Open systems, on the other hand, the domestic hot water and the water in the boiler and the water in the radiant piping are all one and the same, and interconnected. Since the boiler water and radiant tubing water should be treated at least for pH and anti-oxidant control, and the DHW, obviously, should not be treated, there's a problem here...