Radiant heat plumbing design

A thankless is not the correct appliance for the job. It wants to heat a small gpm flow across a wide temperature rise, say 50° incoming water to 120°

A boiler would heat a high gpm over a smaller temperature rise, maybe 100- 115 for radiant for example.

As such the HX is designed differently, controls are different.

Possibly you can force it to work with a primary secondary piping, like this.

The manual should tell you how many gpm you need to pump through the heater to get full output. Some manuals show a pressure drop curve for flow rate sizing.

Somehow you will need to run up around 180 to get an indirect to perform to it rating, not all tankless go that high. If the one you have oly goes to 140F,the indirect performance will be low. Possibly the indirect manual shows various input temperatures?

Basically the boiler pumps into a loop, the loads pull off the loop. The heater may need a high head circ to get full output.

This would be a two temperature system, high for the indirect and staple up, a 120 degree from a 3 port mix valve for the radiant zones.

You need more info for pipe, pump and mix valve sizing. Don't fire the parts cannon until you do some math.

How many gpm to the indirect?

How many gpm for the staple up? What supply temperature

How many gpm for the radiant, what supply temperature for the radiant?

A small mod con boiler would have most of the controls and flow capability to pull this off.

One primary secondary piping option.

Straight up ADDING an indirect heater, to then feed a regular water heater will just cost you more fuel so definitely do not do that.

A lot of what you are saying here doesn't make sense…. if the slab can use a constant 180,000 btu/hr without ever stopping and never overheating the space you have either WAY more slab than you mentioned, or you have a huge leak (like massive leak, one for the history books), or you are just running fresh well water through the whole system and then dumping it somewhere (also kind of a leak).

Adding components isn't going to fix your system (well maybe you need some component, just not your drawing), can you take some photos of how the space heating is set up currently? and just to clarify your tankless space heating system is completely separate from your domestic water yes? if you have a fill line you have a pressure reducing valve and a backflow preventer correct? what is your heating system cold static fill pressure?

How long are those pex loops? How thick is the slab? What type of insulation under the slab, around the edges. Any heat load number for the areas?

The load, heat emitters, concrete slabs, dictate the operating condition of the heater. Cold slabs take a lot of energy and time to ramp up. Initial start up will take hours, maybe a day and near constant firing of the heater. Don't try and set back those slabs.

Is the indirect on a priority zone?

Looks like a recirc pump on the indirect. If so that recirc loop is another heating loop using fuel and tankless output.

12 loops at .5 gpm per loop, (typical for 1/2" pex) you need 6 gpm just for that manifold.

1" pex is good for around 7 gpm so the pex header at the pumps just covers that 12 loop manifold. Not much capacity for any other zones.

That pex header size needs to be able to handle the total gpm of all zones running, if that is what you want?

If you need the full 162,000 the boiler can put out, and if you can pump it to get that, then you want at least a 1-1/4" headed for 16 gpm. Does the building have a 162,000 load?.

Here is the pressure drop curve for the various sizes. 8 gpm through a 180 is 30 psi pressure drop. That will take a sizable circulator to get the heat out, which is why you need a primary loop with a dedicated heater pump, probably a Grundfos 26-99. And you still may not get the full heater 162,000 output.

Is that bubble wrap you put under the slab?

If that is the case, there is your problem.

I would check what is actually under the slab. Check if you have an area where you can break a the concrete without risking any pipe damage to verify. Around me code min for heated slabs is R10 rigid, there is a reasonable ROI for even more.

The efficiency difference between a properly set up fuel burner is about 10%, so you are not likely going to save much operating cost by re-plumbing.

If you are missing insulation under the slab, the best solution is to abandon the heat and convert to baseboard or panel rads. Without insulation, depending on soil moisture, you are loosing somewhere around 50% of the heat to the soil.

Provided your burner has enough BTUs you can heat any slab and get it nice and toasty. Look at some of the snow melt system out there.

The problem is if the slab is not thermally isolated from the dirt, lot of the BTUs the boiler is putting out is going into the soil not for heating the house. That is real heat loss that increases your fuel use.

Modifying plumbing won't help with that.

If there is really only a radiant barrier bellow the slab, your heat will never be efficient no matter what you do. You need to figure out what is actually under your slab and if there is no rigid, your best bet is to abandon it if you don't want to keep throwing more fuel cost at it.

Yeah, the bubble wrap is a scam.

Still air is about R-4 per inch, anything that relies on trapped air for insulation is going to be about that. Most materials are lower R-value than air so the combined assembly is slightly less.

Some closed-cell foams use a gas other than air to fill the space and are able to get higher R-values. Polyiso foam and closed-cell spray foam are both R-6 per inch. But as the foams age the gas leaks out and the R-value drops.

The foil is BS yes. if thats all you have for insulation there is your answer, you don't have any insulation and the heat from your slab is going to transfer more easily to the coldest area, which is down to the ground.

Would be curious what a load calc for your house looked like before construction began? That way we know how much over your load you are using. it's hard to tell from afar, some people use a ton of fuel. I figure you are using about 2 times (maybe even more than that) the btu/hr my house would be at design day conditions if that 200 gallons number is accurate . i have a 2500 sqft house in northern michigan

some highlights from the linked study

"Via email, Dr. Straube informed us that his results were showing Ultra CBF rFOIL with an R-value of approximately R-2 and that, because preliminary results were not looking favorable, Covertech ended this study prematurely before more accurate results could be determined." Manufacturer ended the independent test because the product does not perform well

"The insulating value of this type of product varies depending on what other insulation it is combined with and where it is installed on the slab. At best, its insulating value was only slightly better than two layers of bubble wrap without the reflective foil layer, or a maximum possible R-value of 1.5 to 2.0. This maximum is unlikely to be achieved in an under-slab application and impossible if the air gaps are not maintained." Max R1.5-2 if installed perfectly

" The least effective application of a bubble-foil-bubble insulation from an energy standpoint is an installation under the full slab area on top of R-10insulation. In this case, heat loss from the slab may be increased by as much as10% due to the thermal bridge effect of the aluminum foil, as illustrated in Figure 2. In other words, the R-value of the slab insulation system would be decreased by the addition of the bubble foil-bubble insulation" The product will actually decrease the effectiveness of regular R-10 insulation by 10%

I agree with @Kaos that I don't think upgrading the physical plumbing of the system will decrease fuel usage.

You mentioned 6- 200' loops in the garage. If the tube is 12" on center, the garage is about 1200 sq ft? Even with a high heat load that 180 boiler is plenty to keep that space warm.

You just will being using more fuel to get the job done, with miimal insulation.

You have the boiler power to do more that the garage if that is what you want to accomplish?

"Right now I've had just the water recirculating thru the system with the heater off and it's been maintaining the garage at 59 degrees. there's still usefulness in the system. It's pulling heat I assume from the ground/basement and putting it into the garage."

I really don't see how this is useful.

"The garage is only concrete floor heated and it's not negotiable it has to be heated. radiant heat let's my tooling and cnc mills not rust because of condensation."

The only kind of heating that won't keep humidity down would be an unvented gas burner, which I wouldn't recommend either.

It's going to be cheaper to put in a different heating system than replace that slab.

Was there any sort of heat load calculation and design done for the job.

If so that spells out what needs to be installed as far insulation, tube lengths and spacing, pump sizes, etc, etc. An example below.

Also from this Uponor design manual their slab on grade method.\

i would start with a call to the building department in your town. Ask the plan check people or the mechanical inspectors what is required for a radiant installation.

In my area they need an official manual J load calc sheet. In some areas they have an insulation requirement. Also ask about an energy code that would cover under slab insulation.

With this info you meet with the builder for his thoughts on the system installation and cost of operation.

I would give him the opportunity to make suggestions for the fix.

Read through this manual to learn the terminology and proper design and installation steps.

https://blog.twinsprings.com/wp-content/uploads/2013/05/Uponor-Radiant-Heating-and-Cooling-Design-Assistance-Manual-Part-1.pdf

Let's make an assumption that you do not get relief from the builder and are on your own.

The logical solution is to abandon use of the slab heat in the garage, and install some sort of heat pump there. The price per BTU will be much lower than trucked in propane when you consider the COP of a good heat pump. It would pay for itself quickly.

A couple of split minis could service that space. You have the power there, you have mills and CNC machines to keep warm.

1 ton units are $1000 plus install.