Tankless water heaters for house heat...condensing or non-condensing.

mmoreland

In installing tankless heaters for radiant floor heat, I have observed that the condensing feature does not function well when the incoming water is too warm to enable condensation in the flue gasses. For example, in a recent installation, the incoming water is 95 deg., and the outgoing is 125. In cases where both DHW and floor heat are provided by the same heater, condensation does occur when water is being heated for domestic use, but in that case, the incoming water is cold enough to cause condensation. Would it make more sense to employ a non-condensing, modulating heater for radiant floor heat where that is all it is being asked to do even though they are less efficient and more difficult to vent? I suspect the efficiency is reduced if condensation doesn't occur. Any thoughts on this topic?

pecmsg

Radiant runs well below 140* so a condensing boiler is recommended. 

Unless there’s mixing valves 125* is too high. 

Hot_water_fan

No, you should run the floors at a cooler temp. 125 is too hot. Usually, a leaving temp 10 degrees higher than return temp is typical - a gap wider leads to cold spots. Often, you can get by with an average temp of 80-90.

Domestic hot water heating often will have higher BTU needs so if you’re watching it more condensation will happen but the % would be the same.

hot_rod

It depends on how the tube is installed . Tube below a wood floor with heavy flooring may need 120- 140 in some cases. Staple up rubber usually needs high SWT for example

concrete slabs may work fine with 90 SWT, how is your tube installed, what is the floor covering?

Is this a combined system domestic and radiant  from a tankless?

Jamie Hall

At some risk of stting the obvious... you should be using a mod/coin boiler designed for heating, not a water heater designed for domestic use. Wrong tool for the job.

Hot_water_fan

I think few people understand there’s a difference between a tankless boiler and a tankless domestic water heater and use “tankless” interchangeably. 

Jamie Hall

you're probably right, @Hot_water_fan

hot_rod

Hot_water_fan said:

I think few people understand there’s a difference between a tankless boiler and a tankless domestic water heater and use “tankless” interchangeably. 

What is a tankless boiler? A coil type HX like Sermeta?

When I hear tankless and radiant, my mind always go to a combined system nightmare The common online radiant company designs.

GroundUp

Never use a tankless water heater for radiant heating. They are designed to heat domestic water, nothing else. If the supply temp is under 130*, any boiler will condense regardless of return temp.

mmoreland

I guess I'd like to read a real reason why a tankless water heater is not an appropriate heat source for house heat in some instances. If they're intended, for example, to be used in laundromats where banked together they may run at full bore for hours on end, in a mild climate such as ours in mid California, running a few to several hours per day at a very low modulated rate (1.1 gpm) seems quite feasible. At least, it has worked for me for years, and through experiment and much failure I've worked out a method for enabling a single heater to heat both the floor and domestic by introducing a flow switch and a time delay relay that prevents the pressure bounce during domestic use from erratically triggering the heating cycle.

Those issues, however, weren't my question. My understanding of the term "condensing" in tankless heater jargon is that there is a secondary heat exchanger that preheats the incoming water by extracting any remaining heat in the flue gasses after they have passed over the main heat exchanger. I wondered if it might reduce the utility of the secondary exchanger if the water entering the heater was already an an elevated temperature such as 95 degrees. My thinking was if the secondary exchanger is rendered ineffective or less effective because of the hot incoming water, might it not be more appropriate to use modulating/non-condensing heaters in such a situation because they are cheaper and have fewer parts. I don't know how hot the flue gasses are that come off the primary heat exchanger, but I'll try consulting some of the manufacturers to find that out.

Regarding set temperatures, I have, thanks to the suggestion made above, reduced the maximum heat point for my heater to 120 degrees. Since it's designed to serve both space heat and domestic hot, there is a separate, external heat exchanger for the floor loop, and at 120 D, the water comes out of the heat exchanger at 95 D which in our situation allows the floor to heat up pretty quickly. I'm thankful for that suggestion. mm

mattmia2

Tankless water heaters don't have the right controls for domestic heating. Trying to use one appliance for both without a heat exchanger for the dhw poses risks of bacterial growth in the domestic heating system and mixing that bacteria in to the dhw. A mod con or combi boiler is the right appliance for this application and is as efficient as a tankless water heater if not more when applied with the small delta t of a domestic heating system.

hot_rod

Tankless water heaters are designed to quickly heat the flow rates across a wide delta tee. One pass. Incoming water at 55F or lower to 120 or higher instantly. So a small, high flowrate heat exchanger.

A heating boiler works quite the opposite, adding a low delta tee over multiple passes. Different safety controls, different relief valve settings.

And that pesky ASME rating plate that most boilers sport.