DHW and radiant floor

tjflounder

Can I combine a tankless DHW unit for a 200 sq ft radiant floor through a heat exchanger or does it need to be separate systems?

Geosman

Very interesting question. The answer may depend on if your tankless DHW unit supports recirculation. Not all tankelss units will have a turndown ratio that can support operation with warmer water returning to the boiler that may be only 10 to 20 degrees below its supply temperature.

Separation using a heat exchanger and recirculation pump between the tankless unit and the heat exchanger is an absolute yes. The temperature provided to the radiant floor will be set based on your DHW delivery temperature minus the temperature drop across the heat exchange so I might anticipate the temp for your radiant floor will be perhaps 10 degrees less and 110 is not too hot for most all radiant floor applications provided they are in the slab.

If your radiant is immediatly under the floor surface you may want to consider using a TACO X Block, (The TACO mixing block is a heat exchanger with primary and secondary VS pumps to modulate higher to lower temperature between a water heater or boiler and a radiant floor). (Depending on the pressure drop across your tankless unit and the pumping head of the X Block you may also need to add a booster pump to help overcome resistance of the tankless water heater heat exchanger for effective operation of the X block).

If you are using aluminum plates under a sub floor the temp may need to be much higher (perhaps in the 120 to 140 range), and may exceed the temperature setting on the tankless unit unless you are just attempting to take the chill off the floor. I have found that with a conventional water heater and using an X Block that I needed to increase the water heater temperature to 140 to produce adequate warmth using extruded aluminum underfloor plates on 8" centers, and conducting the warmth through 3/4" subfloor 1/2" backer board and 1/2" floor tile along with an X Block for separation between domestic water. I have several very happy customers who delight in their warm bathroom floors using this method.

If drawing DHW simultaneious with floor heat your gpm of available DHW will be reduced by the BTU that are contributed to the radiant floor. In theory such a system is workable. You may also want to look at the BTU that can be released via the radiant area as it compares to the minimum fireing rate on the tankless unit. If the BTU on the radiant side is less than the minimum fireing rate for the tankless unit you may expect the system to be constantly short cycling. Frequent short cycling will or may require more frequent replacement of the ignition electrodes and for backflushing of the DHW heat exchanger to remove scale deposits. It is all a matter of cycling and total operation hours vs your water quality.

Additionally, small low head circulators intended for DHW recirculation will not work with most tankless water heaters. Those that I am familiar with require pumps with a pumping cutoff head closer to 30 feet to overcome the resistance of the passages in the hot water heat exchanger and water heater controls.

hot_rod

200 sq ft? Just get a small electric water heater. 2.5 or 6 gallon works well. A simple, single pump, 120V system

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Kaos

The issue is min fire on most tankless units is around 10000BTU. That 200sqft is simply too small for it. Even if the tankless unit supports recirc, it will cycle itself to death trying to run that low.

The way I've gotten this to work is with a buffer tank downstream of the tankless unit. This has the added benefit of eliminating startup delays and cold water sandwich issues on the domestic hot water supply. Doesn't have to be anything big 2 to 6 gallon is more than enough.

The simplest way to plumb this is a beefier recirc pump from either the end of your domestic hot water line or after the buffer tank hooked up to the aqua stat on the tank. Your feed to the floor heat plate HX is now taken from the buffer tank instead of the tankless. The recirc pump will run only when the heat in the tank gets used up which reduces cycling. The tankless unit will still automatically fire for any domestic hot water use, so generally outside of heating season, the recirc rarely runs.

Make sure your tankless unit is rated for combi heat, not all are.