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Converting from Combi-boiler to hot water tank.

RobertsBrown Member Posts: 7
edited December 2019 in Radiant Heating
I have now successfully switched from a leaking Navien Combi-boiler to a simple gas hot water tank. The conversion was really scary, but as it turns out it was no big deal. If you have/had a boiler system and you want to swap it with a tanked system, here are the tips I wish I had received:

The boiler switches on and off due to demand. This means it responds to a thermostat that is measuring room temperature. The tanked system does not do this, and the thermostat is not used. Control of water temperature is 100% at the tank.
You will be adjusting flow at the metering blocks/valves to the zones to be heated. rate of flow will either move hot water quickly through your system, adding less heat from more volume, and increasing room temperature, or it will slow the flow allowing more heat to be release from less volume, equalling less heat added to the room.
The recirculating pump will never stop running. Be sure you have a good purge of air, and that you have constant flow. You can check the heat of the pump by putting your hand on it...it should be equal to but not more than the incoming hot water. If it's hot, something is wrong and you need to check for air in the system.
Incoming cold water comes in on the 'hot' side of the tank. It seems wrong, but once the system is purged and circulating cold water only enters the system when the system pressure drops below the regulator setting. it's only adding make-up water.

It's not hard. Don't psych yourself out. I did, and it took my over a year to make up my mind to try the conversion. It took less than one day, and it was a cinch.

You can do it!


    HVACNUT Member Posts: 5,750
    Is the tank used for domestic hot water as well? Open loop? Heat exchanger?
  • JakeCK
    JakeCK Member Posts: 1,356
    And this is why I eventually want to move out into the sticks my self... So there is at least a mile between me and the other guys knucklehead system.
  • GroundUp
    GroundUp Member Posts: 1,866
    Well....... apparently it's not that easy because you have it pretty butchered up. The tank will maintain its own temp but you still need to control the pump and/or zone valve(s) with thermostats- the circ should not be running all the time. Cold water should never enter the hot side of the tank, for any reason whatsoever- you have it piped backwards. You should never need makeup water for any reason whatsoever unless you have a leak, but I'm betting your expansion tank is either shot or way too small and when the water cools it adds makeup water due to contraction and then when heated back up it pops the relief valve and continues this vicious cycle.