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Question DetailsAsked on 1/28/2016

Can a tankless gas water heater supply enough for a hot-water fan-coil heat exchanger? Would a tank model work OK?

How might a tankless gas unit perform as the source of hot water for a fan coil-type heat exchanger? Electric rates skyrocketed in Central Ohio last year, and I need to go back to gas heating. Am hoping to utilize the existing air handler (to save $) by inserting a hot water coil and deactivate the 15KW coil "back-up." Would using an existing (de-energized) tank unit help raise the incoming "cold" water to the tankless unit?

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4 Answers


If you have gas available I would replace the existing fan coil with a gas fired fan coil unit.

If you have air conditioning now how old is the unit? You might need to replace the entire system for effiency if you have a ten year old unit. You will save on the electrical use.

A heat pump will furnish adequate heat above 40 degrees and is more efficient than electric heat and uses an electric heat strip only for backup in very cold weather. This suggestion depends on your winter temps.

The scenarios you mentioned are iffy but may work. You need a reliable professional to analyze your needs and design the best system for you.

Source: Bob Ross

Answered 4 years ago by PoppyRoss


In addition to what PoppyRoss said - you would have to get a heat-exchanger heat source or continuous duty rated unit - many tankless heaters are not designed to run frequently for short periods of time like that, and unless it is oversized for the purpose could also ending up having a heavier duty cycle (percentage of the time firing) than it is rated for in real cold weather.

Wish I had a picture of one almost new wall-mounted tankless unit I saw last winter - was melted down to a pool of plastic and a few warped metal components on the (fortunately fire-wise) garage concrete slab after being asked to fire continuously during a quite cold period to supply a baseboard hot water heating system.

Certainly whatever coil and source you use would have to be sized and rated for the heating load they are being asked to supply - and the coil would have to be sized to deliver that heat at the design airflow temperature and flow velocity across the coil to provide the BTU's you need - not something to off-the-cuff.

Note that using a tankless unit for that, if it is also supplying your domestic water uses, will also result in some conflicts when the two are both in demand. Not a real problem for the heating side except will mean longer heat-up time when hot water is also being used for shower, tub or washer - but when it is serving the heating coil your available hot water for the domestic uses may be severlky hampered, especially if the coil is significantly closer to the heater. It would be possible to rig a demand valve to minimize that effect by cutting back on the heat to the coil when hot water is being used in the domestic pipes, but adds one more thing to fail in the heaitng system.

The question on the "de-energized" tank unit I presume to mean you have an unused conventional water heater - those can sometimes be compatible with a tankless unit, though many are not designed to receive hot water supply so depends on the unit. However, if you are doing that, why not just stay with the supply tank unit forthe domestic water, and a separate dedicated gas-fired hot water coil for the heater - one that is designed for that use.

What you are talking about is almost like a central consolidated hot water system used in many newer hydronic (hot water) heated homes - where a boiler heats water to about 200 degrees, which is stored in a large storage tank (commonly 80-100 gallons), and used at that temperature for hydronic heating, and diluted down to 130 degrees or so for domestic water.

Also - bear in mind, unless you go with a fairly big coil, domestic water temperature (130 degree or so) water is going to make for a slow warm-up. Remember an electric coil is probably running at around 900-1000 degrees - quite a bit higher than your 130 degree water, so a LOT of air is going to have to pass over it to get the same heat transfer, or it is going to have to be a large coil, to transfer enough heat to get anything like a rapid heating response or to keep up on a real cold day. And you are going to be running the fan unit a LOT more to move the same amount of heat, which is an energy loss that decreases the efficiency of your unit - much more so if it is getting outside makeup air fed to it also.

One last thought - the heat transfer from the electric coil is basically 100% efficient (counted from your meter that you pay on the basis of, to the heated air), whereas gas heating of water will be at most about 90-95% efficient with normal equipment, and possibly a lot less if your unit is in an area outside of the "conditioned space" of the house.

Bottom line as PoppyRoss said - better off to go with a unit specifically designed for that purpose, which if water-based would normally run at about 200-200 degrees.

Answered 4 years ago by LCD


LCD and Poppy Ross - - thank you both for your thoughts. Poppy, I have a 14 SEER heat pump that's 7 years old. Plenty of life left in it, but still very pricey for heating, so would use for cooling only. LCD, I think I'll abandon the dual-purpose tankless water heater idea and check into boilers. Not much demand is put on my electric tank anyway, so could manage OK with it.

Now, think about this: If cold water is run through the water coil, could it lower the temp. of air before it hits the A-coil condenser such that it would in effect have cool it less that normal supply air? Thanks, guys.

Source: Steve, Ohio

Answered 4 years ago by Lustron


On the issue of running cold water through a coil to pre-cool the air before it gets to the evaporator - basically you are talking what is done in large commercial buildings, and also in geothermal systems. Bottom line yes it can work, but likely not readily so in your case because:

1) unless you have a geothermal well to recool the water after it passes through the coil, in a closed loop system it will quickly be warmed up and become ineffective.

2) unless you have a way to recool it, the only way to keep running cold water through the coil would be to waste the water - make it a single pass system like power plants along rivers commonly do, for instance. Of course, in many areas such wastage of the water would also be illegal - certainly in probably all locales if using a public water supply as the source as opposed to a personal well. Recycling it though the drinking water well is not a good idea - can start biologic growth and iron/manganese bacterial buildup in the well from the added warmth, plus your "cold" water supply to the house would be warmer because of it - not only changing your cold water feed to the house to warm, and likely adding bacterial and mineral content in the water (affects taste, color, and in-pipe buildup unless you have a water conditioning system), but also as the ground around the well gradually heats up the circulating water temp would go up as in 1), reducing the benefit. A lot of geothermal wells have this same set of problems, which is why buried loop systems are generally now used as the heat sink/source for geothermal heat pump systems.

3) this would add a second coil in the air handler system that could grow mold as moisture condenses on it, and would need a drip pan and drain under it to catch and carry away the condensate

4) the primary A/C evaporator would, in the summer, be having cooler air passing over it, which would likely do two things:

a) if the system pressure (hence temperature) drop over the evaporator was not adjusted, could result in freezing up of the coil, because it is designed for a certain minimum air temp passing over it, which you might be below

b) because the incoming air passing over the evaporator would be cooler, hence have less moisture carry capacity, not only would your evaporator tend to remove more moisture (and hence commonly accumulate mold faster), but the ducting between the two coils might show condensation and start rusting out quickly.

Basically, I applaud your attempt to be energy efficient and reduce cooling costs, but even factory designed and tested systems commonly have these types of problems - homemade systems raraly come out anywhere near satisfactory.

Answered 4 years ago by LCD

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