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General question: do you need a tankless coil to connect FHW boiler to indirect DHW?

BTUBTU Posts: 14Member
I only ask because sometimes I see existing FHW w/indirect systems where the tankless coil is piped to the heat exchanger in the indirect DHW tank... and other times I just see an extra heat supply zone that's piped into the indirect tank.

In what cases would you want a boiler w/tankless coil piped into indirect tank, vs when you can just pipe another heat supply zone into the indirect tank?

How about when your existing FHW boiler has tankless coil piped to indirect - must the new boiler have a tankless as well? Sometimes cheaper to get a new FHW boiler without the tankless coil; also can get better AFUE on some models without a tankless coil.

This is a very general question. Any and all responses from professionals and experts will be very much appreciated!


  • newagedawnnewagedawn Posts: 562Member
    to answer, never, thats the way its supposed to be done always
    indirects are supposed to be a separate zone off your boiler always, not run through a tankless coil,(inexperienced techs and those stuck in the way we always did it mode will pipe this way) buts its redundant, plus you get better make up with indirect piped as separate zone altho the cost is significantly more , but the saving in the life of the indirect pays for itself , food for thought
    "The bitter taste of a poor install lasts far longer than the JOY of the lowest price"
  • delta Tdelta T Posts: 801Member
    Well if you pipe a tankless coil in a boiler to the coil on an idwh you would need a separate expansion tank, relief valve and water feed just for the loop between the tank and the tankless coil.....
  • PhilDavidPhilDavid Posts: 63Member
    Some indirects don’t have an internal coil. They heat the water using an external source like a plate exchanger or tankless coil. I’ve recently come across a Therma-flow indirect that was used for DHW and a zone of radiant. The indirect had no internal coil and the DHW was heated using the tankless coil in the steam boiler.
    @delta T is absolutely correct
    delta T said:

    Well if you pipe a tankless coil in a boiler to the coil on an idwh you would need a separate expansion tank, relief valve and water feed just for the loop between the tank and the tankless coil.....

  • HVACNUTHVACNUT Posts: 2,873Member
    With the tankless coil application, are you sure you're not looking at an aqua booster? Typical in looks to an electric water heater, the thermostat energizes an inline bronze or stainless pump yo circulate between the tank and the coil.
    I've never seen an indirect piped through a tankless. It wouldn't keep up with demand.
  • delta Tdelta T Posts: 801Member
    @newagedawn what do you disagree with?
  • Danny ScullyDanny Scully Posts: 1,220Member
    edited April 2018
    Are ou sure you’re not looking at a storage tank @BTU?
  • Big Ed_4Big Ed_4 Posts: 1,199Member
    I thinking the same , Let's make this clear....

    If the hot water heater is pumped through an existent hot water coil submerged in a boiler , it's called an aqua booster . The tank is just a storage tank . The storage water is pumped through the coil until the tank is built up to the desired temperature ... The heat exchanger is in the boiler ...

    An indirect is a storage tank with the heat exchanger is in the storage tank itself and it's piped like a heating zone ....

    The aqua booster is cheaper ,but normally much slower to recover . Normally the coil in the boiler is only 1/2" which would be the supply to the tank.. +- 28k per hour. Where the indirect supply pipes are larger and can deliver the full BTU's output of the boiler with large enough supply pipes ...
    I have enough experience to know , that I dont know it all
  • Steve MinnichSteve Minnich Posts: 2,516Member
    Indirect water heaters are, in effect, heat exchangers. Cold potable water meets hot boiler water via a coil within the tank or a tank within the tank.
    PHC News Columnist
    Minnich Hydronic Consulting & Design, LLC
  • New England SteamWorksNew England SteamWorks Posts: 1,278Member
    I like to use tankless coils when possible on indirect installs if the boiler is steam. Best not to use the boiler water if it can be avoided.

    New England SteamWorks
    Service, Installation, & Restoration of Steam Heating Systems
  • BTUBTU Posts: 14Member
    Thanks all, insightful comments. Turns out I been confused between the indirect and booster DHW tanks. Good point about the steam boiler water issue as well @New England SteamWorks
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