Help choosing a boiler for high mass and low mass radiant heating.

A few questions:

1. There is 1100 feet on 8 circuits (2 are  kickplate heaters), of 1/2” Uponor  PEX A.  Installed on approximately 8 inch centers below the 3/4” Advantec sub-floor, covered by 3/8 inch laminate flooring. Is the PEX attached to the underside of the flooring with Uponor’s aluminum track,  between the joists?
2. Are all 1100 Sq Ft of floor using the 8" space tubing? (or only the part that is not heated by the kickplate heaters)
3. What percentage or what square feet if the 1100 is radiant and what percentage or square feed of 1100 has the Kickplate heaters?
4. What size are the Kickplate heaters?
5. How are the Kickplate heaters connected to the rest of the system?

I also want to know if what you are calling a Kick plate heater, also known as toe-kick or kickspace heater, are compact heating units designed to fit into the 3½-inch cavity beneath kitchen cabinets, bathroom vanities, or stair risers.

If that is the case, then you have three different heating-water temperature requirements to deal with. The kickplate heaters are very comfortable operating with 150°F to 190°F water, while the in-slab radiant and the under-floor radiant will normally operate at substantially lower temperatures.

The basement slab may occasionally require supply-water temperatures above 110°F under design conditions, depending on the heat loss and slab construction.

I would be particularly careful with the radiant tubing under the laminate flooring. The required water temperature needs to be calculated based on the heat loss, tubing spacing, aluminum transfer plates, subfloor thickness, and the R-value of the finished flooring. More importantly, the floor-surface temperature must remain within the laminate flooring manufacturer's allowable temperature limit.

So you really don't have one water temperature that is necessarily appropriate for all three types of emitters. The kickplate heaters, slab radiant, and under-floor radiant may each need their own temperature control or mixing strategy.

Once you have determined the basic strategy for the system, almost any boiler can give you the results you are looking for. A cast-iron 80% AFUE boiler with proper return-water temperature control will work, as will a modulating-condensing boiler.

The boiler selection is not the big question here. In my opinion, the system design is much more important.

1. Start with the load calculation
2. Select the temperature control design for each of the heat emitters
   1. Kick Space Temperature
   2. Slab floor Temperature control
   3. Under Floor/ staple-up Temperature control system
3. select the boiler that is the correct size for the system.

I kind of like the Taco iSeries-R valves for this job. You can use the boiler’s outdoor-reset function to control the highest-temperature KickSpace loops.

Then you can use an iSeries valve with its outdoor-reset control to essentially “reset the reset.” I think this is a great idea, and I hope this technology does not go away anytime soon.

The idea is that the lower-temperature zones take the water temperature provided by the boiler and mix return water with it, adding or mixing in hotter boiler water as needed to produce the lower supply-water temperature required by that zone.

The control uses its own outdoor sensor to automatically lower the water temperature supplied to the related zone as the outdoor temperature rises, while increasing it as the outdoor temperature falls. This allows each lower-temperature zone to receive only the water temperature needed to maintain the desired comfort level.

In this above example there are 4 different temperatures. The highest temperatures come directly off of the hottest boiler water and can return to the supply with closely spaced tees or back to the return, both are acceptable depending on your design. The second and third zones temperatures are using a mixing valve with the iSeries-R actuator to obtain the desired temperature. the last zone is using the lowest temperature boiler water from the system and that is using an injection design to add just enough hot water to the loop(s) to get the desired floor temperature.

Those valves are not inexpensive, but they eliminate the need for additional or separate valves and ODR controls to operate those valves. @hot_rod has more experience designing these systems and may have a different idea. But I think this might be your lowest cost design for what you are trying to accomplish.

https://www.tacocomfort.com/wp-content/uploads/2019/08/PA04_iSeriesRValve_2015.pdf

Kick heaters are often used if the floor cannot cover the load. Of maybe it is a one, low temperature system and the kick adds some kick?

If it is the fan type they are a bit noisy, suck and blow pet hair. Use them only when necessary.

i'd like to see you do a two temperature system. With low loads and required temperatures within 20° of one another, here is a simple method.

Assume the basement is one zone.

The boiler is a 10-1 turndown so it ramps down to 8,000. Perfect. It is probably a two temperature boiler control, however one temperature at a time.

An option I have used several times is proportional reset. From Idronics 7

Run the boiler at the highest temperature required for the plates, lets say 130 on design day.

Run the boiler on ODR.

The low temperature zone is done with a manual 3 way mix valve. It tracks along with the boiler temperature.

So set it at 100° on a design day for the slab zone. As the boiler temperature ramps down, so does the mix temperature.

Here is an examples.

If you have a buffer, better yet, it give you some capacity and a hydraulic separation.

A small electric tank as a buffer.

Boiler S&R on left. High temperature teed off that S&R as a 2 pipe buffer. 3 way mix valve on top.

Boiler sensor in tank. The tank operates the boiler firing.

A control and motor could be added to the 3 way for more temperature control, also.

I also stuck a 4500 w element into it for 15, 300 back up (dual fuel) option. If the boiler fails, plug it into a 30A range receptacle