Again, it is usually best to continue working directly with your designer/installer/supplier(s) until your hydronic radiant floor heating system is fully operational and operating well. Are you not getting the support you need?

Outdoor reset would be nice for optimal comfort and effeciency, no doubt. In this garage-case, I'll let the thermostat (air) call for heat. The system might have to play catch-up of the outdoor temps plummett, but that is okay (again, a garage).

Here is my simplistic view. Let me know where this logic (or the reality of this) breaks down:

- Keep water temp output set point as low as possible. I think this is around 104F on the Tagaki. One goal with this is to not short cycle the unit and attempt to have it operate at a lower BTU, for a longer period of time.
- Keeping the setpoint low in theory should allow the unit to maximize GPM/flow (i.e. not modulate down the flow because it cannot heat it fast enough) and not add extra/dynamic head beyond the static head "built-in" by the heat exchanger.
- Adjust circulator GPM/flow to bring in delta T to where the unit can accomplish what I need it to without having to increase the output temp set point drastically high. This is the part that I feel will be "tricky". If too slow, delta T will be high and unit will modulate up the burner (maybe not a big deal?). If too fast, risk of pushing the limit of the circulator pump (if not properly sized). It seems like if the user could "disable" the flow/GPM modulation, that would make integration of a tankless for this use a little easier.

In general, it seems like the industry an manufactures could fill this void. Purpose-built boilers are available, yes, but it seems someone could come up with a tankless that allowed the user to control the burner/flow modulation to better suit a small heat load situation like a small garage.

You are trying to use a "water heater" for "space heating"; it is always challenging to misapply one "purpose built" appliance for another.

Invest in a properly sized ModCon.

Dana is right as usual. Both options proffered by the Master of Math are functional and practical.

What would a properly sized ModCon be? My loads are so low, even the smallest boiler is overkill. If by properly sized, strictly the rated BTU output (not considering any modulation/turn down ratios).

The Dunkirk Helix 50 runs down to 10mbtuh input.

The HTP UFT comes in an 80m model with a 10-1 turn-down giving you 8mbuth.

Neither of these boilers will short cycle in any condition due to the mass of your slab. I size condensing boilers according to the block load and the smallest zone. To do this you have to consider the the minimum and maximum outputs.

Experience matters most.

Heat source minimum cycle time is a function of the hydronic fluid volume in the system, the system load, the heat source minimum output, and the heat source high/low temperature settings. If you know these quantities, you can determine if the minimum cycle time required by your specific heat source will be satisfied without a buffer tank, or determine what size buffer tank is required to satisfy the minimum cycle time required by your heat source.

Borst Buffer Tank Design Software

Or I will do it for you, for a fee :-).

If you think you need a buffer tank for a single zone <10mbtuh then you are beyond hope, but here is the formula:

V=T X (Q heat input - Q min. heat load)
Tank temp. rise x 500 – Q min. heat load

V = buffer tank volume
T = desired heat source minimum on cycle
Q heat source = heat source output btu/h
Q min. heat load = heat output to minimum load

Tank temperature rise in degrees Fahrenheit


Or you can buy Siegenthaler's software, which features a very useful buffer tank simulator I use often, though I don't use buffer tanks unless I am fixing someone else's mistakes. Heat pumps excepted of course.

https://www.hydronicpros.com/downloads/index.php?id=21

To not short-cycle a Takagi soluion would take keeping the flow rates low for a higher delta-T on the radiation, and set the output temp to something where it will always deliver at least it's min-fire output. You may be able to do that with a smart pump using a delta-T control, but you'd probably find conditions where the Takagi's minimum flow necessary for firing up aren't met. It's do-able, even if not perfect.

I'd agree with Morgan that you'd have to set it up pretty badly to actually short cycle a mod-con that has a min-firing rate of 10K or lower, but it's not idiot-proof (as creative idiots everywhere prove on a daily basis. :-) )

Our Borst Buffer Tank Software, like all of our hydronic radiant heating software, is based on the excellent engineering detailed in “Modern Hydronic Heating” authored by John Siegenthaler PE, which is likely the best book on this subject. All of our hydronic radiant heating software has been carefully validated using Siegenthaler’s software. All of our software is available for use on our DIY website without needing to pay any fee.

As explained in the instructions for our Borst Buffer Tank Software, “when low thermal mass heat sources are combined with zoned hydronic radiant distribution systems, it is possible for the heat source to short cycle when only a couple of these zones call for heat. This occurs because the rate of heat produced by the heat source is much greater than the rate of heat used/released by these zones. Low volume thermal mass heat sources in combination with low volume hydronic circuit and distribution systems cannot absorb this excess heat rate without experiencing a rapid rise in temperature. This causes the heat source to reach its high temperature limit very quickly resulting in very short on times and short cycling. While electric heating elements may tolerate short cycling, gas valves, oil burners, ignition systems, and compressors will have a significantly reduced life expectancy.”

The competence of the HVAC community is certainly variable. Basic math and science is unfortunately not often in their repertoire. Frankly, I wouldn’t trust some to even accomplish a simple furnace replacement much less a hydronic radiant heating system design/installation. As explained in the instructions for our Borst Existing Building Energy Usage Analysis software:

“When replacing an old inefficient furnace with a new efficient furnace, you need to be cognizant about not allowing the rated efficiency improvement to cause over-sizing the new furnace. This is easier to get wrong than you might think. For example, if your old furnace is rated for 50,000 BTU per hour and 60% Average Fuel Utilization Efficiency (AFUE), it is currently providing 30,000 BTU per hour of heat to the building. If catalog engineering is the extent of your HVAC knowledge and you happen to replace it with a new furnace rated for 55,000 BTU per hour and 95% AFUE because the next smaller size is less than 50,000 BTU per hour, this new furnace will provide 52,250 BTU per hour of heat to the building. This is almost 75% more heat than the original furnace provided and this over-sizing will most likely cause short cycling resulting in reduced efficiency and reduced furnace life. Assuming 30,000 BTU per hour is truly the maximum design heat load required for this building, you would be much better served by installing a new 95 AFUE furnace have a rating of 31,500 BTU per hour or perhaps even less. This smaller furnace will cost less to install, will cost less to operate, will have a longer life, and will provide the required heat.”