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Thread: Tankless running a fan coil?

  1. #1
    DIY Junior Member sirrox's Avatar
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    Default Tankless running a fan coil?

    Hi All, first post...

    We're building a new home. It's a large home 3 storey home with 4-1/2 baths. We live in northern Canada where the water supply from the street can be close to freezing.

    I would like to run tankless propane hotwater for domestic supply. However I would like to use this heat source as my primary heating to supply a hi-velocity fan coil unit. My inital research tells me I would be running two units like the Rheem 95 series linked with the EZ-Link.

    Questions:
    1. Is the required rise the temperature differential at the tap (ie. 32F to 105F = 73F) or the temperature the tankless heater is set to (ie. 32F to 140F = 108F)?

    2. Is supplying a fancoil from a tankless a good idea? If so should I run a heat exchanger or open loop to the fan coil?

    3. How does the water get returned to the fresh supply prior to the tankless heaters for reheating? Would a valve on the fancoil open on call for heat and the water would move from tankless -> fancoil -> fresh supply line -> tankless on pressure alone or would it need to be pumped?

    4. Considering the heat requirement of this return water to be reheated should be minimal on call for heat alone, should the unit I choose be able to fire at low or multi-stage BTU rate to meet this requirement?

    Couple points...

    We can run 4 months a year without the call for heat, this would be an efficient time for the home on this system.

    However the house will be properly designed so that the fancoil will provide the required BTU's for the home to maintain 68F on a -43.6F day. I will need to supply the fan coil at 140F to obtain the required BTUs. If I supply the fancoil in the 130F range, we would be borderline on the deisng requirements for BTU's. We will be using the largest fancoil this company uses.

    Thanks!

  2. #2
    In the trades Dana's Avatar
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    In order:

    1: The temperature rise for the heater will be from it's inlet-temp to output temp. The internal controls are designed to regulate the output temp. If your incoming water is 32F and it's programmed for 140F, that's what you get. (Up to it's max firing range, anyway). If you have a tempering valve or mixing valve on the branch to the domestic hot water to keep it at 105F, the flow throught the tankless will be proportionally lower. The delta-T in space heating mode will be much lower, since the return water from the coil will be above room-temp.

    2: The safe bet is to go with a heat exchanger and keep the heating loop isolated. (Required by code in many areas.) But if the total volume of the heating loop is low, and is purged at least daily your risk of stagnant-water issues are low. If the fan coil and heating loop are plumbed in such a way that the incoming water MUST pass through the fan coil ahead of the water heater, the risks are essentially nil.

    3: The fan coil loop would be driven by a properly specifed hydronic pump to handle the head of both the coil and the heater (or heat exchanger.) All pumps for potable water would need to be bronze, to tolerate the corrosiveness of fresh, oxygenated water. Iron pumps fail quickly when used with constantly replenished fresh water.

    4: All tankless heaters modulate the flame based on input water temp and flow rate to deliver output water at the setpoint temperature. In space heating mode there's no input of 32F water, and the water entering the tankless will be 90-100F or more (depending on flow rate.)

    This level of heating design isn't always a great idea to do as a DIY based on web-forum input, but the general approach CAN be made to work. I understand that in Whitehorse the heating system design talent pool isn't as big as in Vancouver or Montreal, but SURELY there must be somebody willing to take this on!?!

    Also, why fan coil rather than hydronic baseboards, convecting radiators, or radiant floor or something? Hot air systems use more electrical power to distribute the heat, drive outdoor air infiltration, and are less adaptable to micro-zoning for better balance, etc.

  3. #3
    Scotsman mcconnellplumbing's Avatar
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    Some of the manufacturers do not warranty their tankless water heaters if used in this fashion. They note that water constantly circulating through the units to go to a heated loop or hot water circulating loop creates more hard water deposits than normal in/out flow. They will require a separate water heater to maintain the circulating water temperature. New water added to the system goes through the tankless heater only. Navien has models with a circulating pump built in to the unit and will warranty the unit with a circulating hot water loop. It has a buffer tank and will cycle on and off to maintain water temperature in the loop. Be advised that the constantly circulating hot water through the tankless can cause a build up of hard water deposits in the heat exchanger. Most tankless units only carry the warranty for labor for one year. Replacing the heat exchanger can take 2-3 hours depending on the level of experience and how mechanical or dumb you are.

  4. #4
    In the trades Dana's Avatar
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    Quote Originally Posted by mcconnellplumbing View Post
    Some of the manufacturers do not warranty their tankless water heaters if used in this fashion. They note that water constantly circulating through the units to go to a heated loop or hot water circulating loop creates more hard water deposits than normal in/out flow. They will require a separate water heater to maintain the circulating water temperature. New water added to the system goes through the tankless heater only. Navien has models with a circulating pump built in to the unit and will warranty the unit with a circulating hot water loop. It has a buffer tank and will cycle on and off to maintain water temperature in the loop. Be advised that the constantly circulating hot water through the tankless can cause a build up of hard water deposits in the heat exchanger. Most tankless units only carry the warranty for labor for one year. Replacing the heat exchanger can take 2-3 hours depending on the level of experience and how mechanical or dumb you are.
    Really? This is the first mention I've read of such a thing- I wonder what the source of the minerals would be in a closed loop of heating system water??? It would seem that recirc loops of FRESH water might accrue deposits faster than a unit used in a space-heating only application (or one that used a separate heat exhanger to isolate the fresh water from the heater loop. Or is that what you meant by "...separate water heater to maintain the circulating water temperature." ??

    "Open systems" that use potable water in the space heating loops seem like an inherently bad idea on several levels, but that doesn't seem to stop people. With a hydronic coil the volumes are low and if plumbed correctly the health issues can be much easier to mitigate than with other types of radiation, but it would't address the liming issues on the heat exchanger. (The combi I'm running at my place is fully isolated, and the HX in the tankless never sees fresh potable in the heat exchanger.)

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    For the cost of a couple of tankless units, a 'real' boiler and indirect tank might end up being simpler and more efficient. You owe it to yourself to run those numbers, too.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    In the trades Dana's Avatar
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    Yabbut, they probably don' t have the space heating load high enough to make a mod-con worthwhile on such a tiny place in such a warm climate, eh? ;-)

    I wouldn't dream of running a tankless based combi to heat any sized home in Whitehorse myself, but that's not the question he asked. I also wouldn't want to heat it with ducted air and a fan coil in new construction (but that can make sense in retrofits with pre-existing heating ducts.)

  7. #7
    Retired Defense Industry Engineer jadnashua's Avatar
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    Don't know, but Yukon Territories can get REALLY cold. Canadian codes require really tight houses, so hard to say what's really needed. With near freezing incoming water, I'd be leary about heating the house and the potable water, too.

    And, you'd be getting something designed for 100% duty cycle, too. Most tankless systems aren't designed for that.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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