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In floor Hydronic

hot_rod
hot_rod Member Posts: 22,108
Author: Glenn Sossin (gsossin@tashsales.com)
Subject: Takagi - Radiant not a good idea

Like Andrew said, the Takagi unit is not a boiler, it's not rated as a boiler.

The problem is, these units were designed as instaneous water heaters. As such, they expect to see inlet pressures of 40 psi or more at the heat exchanger. This pressure is used to push the water through a very narrow heat exchanger during the heating process with a very high delta T 60 -80 degrees or more. This is so the heat energy the burner is creating can be absorbed by the water flowing through the heat exchanger. The lowest firing rate for one of this units is just under 20,000 btu's. If one or two of your bathrooms call for heat, that might typically be 3,500 - 7,000 btu's. Short cycling is a likely result. Because of the narrow opening in the heat exchanger, you will need to use a high head circulator to generate enough head pressure to push the water through the unit to generate sufficient pressure and flow rate to turn it on.

Depending on the model, they are designed for a flowrate of several gpm - and a minimum operating pressure of 15psi. They typically require a minimum flow rate of .75 gpm for the burners to ignite. You have to be sure there is enough flow rate and heat load for the burner when it's firing. Radiant systems are usually characterized as low flow, low btu loads - the exact opposite of what these products were designed for.

The typical radiant heat system is designed around a 20 degree delta t and may have flow rates that are fractions of a gallon. If a small flow rate and low delta T system design are pushed though one of these units, it may cycle excessively and have an increased potential of going off on high limit - shortening product life. When that happens, you have no heat. Consider using antifreeze.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream

Comments

  • D_4
    D_4 Member Posts: 2
    Boiler vs. On Demand

    Can I use one on demand hot water heater to supply my house (3 bathrooms) and my in floor hot water? If o how many GPM do I need to look for? The in floor tube is in 3 zones each just over 300 sq feet.
  • D_4
    D_4 Member Posts: 2


    Thanks, I guess I'll have to research a boiler for the in floor. Can I use the boiler for my domestic water too or do I need to have two separate systems. I was really hoping to "never" run out of hot water with one of those efficient on demand systems.

    D
  • hot_rod
    hot_rod Member Posts: 22,108
    Check out the Laars Mascot

    it is a small mod con boiler with a instantanous heat exchanger for DHW built inside the case. have one at my inlaws home for radiant and DHW. Works great, easily provides 2 GPM maybe more depending on incoming water temperature.

    Here are a couple shots of it being installed and replacing an old cast iron with P/S and an indirect.

    hot rod
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
This discussion has been closed.