Small system not quite dialed in

dempsey

Hello and thanks in advance for any assistance given. I have the basics of the above system installed minus the mixing valve. Takagi tankless 180k btu max, 15k btu min. The output of the 10-plate heat exchanger is used to heat a single room, many windows but only 12x12. Staple up pex around 200 feet. 5000 btu load at most. Domestic hot water part works fine. The recirculation loop that feeds the heat exchanger is very short. All piping is 3/4' copper. Call for heat turns on both pumps. Grundfos 3 speed cast iron pump for radiant, Taco 006 bronze for the recirc loop. According to the heater the loop's flow is 1.8 gpm. What happens is that the heater gets the water to temp (set at 135) very quickly and when the return water reaches around 125 the burners shut off and wait until the return water drops to 115 then light again. I would like to eliminate this short cycling, of course. But just as importantly, the heat in the room is not great. I have it set to 68 and it will keep up until it is below 35 out side but it struggles beyond that. The water in the heating loop seems to be around 115, which I think is a little low. The floor (ceramic tile over plywood) while the system is running is around 80 degrees, which doesn't seem far off. I should mention that the floor part of this system was installed 20 years ago and was run off of a 30 gallon tank heater for all of that time. That system never had a problem keeping the room at 68 degrees until the outside temp went below 0. I'm wondering if a small buffer tank would solve my problem. And if I go with a buffer tank do I need to find a way to shut off the flow back into the heater when the tank reaches temperature and just circulate the tank water in the loop? How would this be done? Finally, in case anyone asks, the old water heater died and so did my old 50 gallon for dhw, so i went with this. I'm aware that a tankless is not a boiler, but for this small and simple system I thought it fit. Thanks to all.

EdTheHeaterMan

Another case of using a DHW heater for space heating. I wonder why something designed for another purpose is not working the way you want it to?

1. The tankless heater is not the same animal as the old tank water heater that worked for 30 years
2. The tankless heater is designed to heat cold incoming water to 120°F for taking a shower. That would be a ∆T of 60°F or more.
3. The BTU output of a tankless is much more that the BTU requirements, so it is oversized. what happens with an oversized boiler in a small load condition.... It short cycles.
4. My advise is to get a buffer tank or perhaps a separate tank water heater for that small load. it worked for 30 years, if it ain't broke....