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

There are plenty of of great mod cons out there. Many share the same components, heat exchanger, gas valve, etc.

I like the Lochinvar Knight, based on many installations, rep and factory support, easy to service. It comes with a variable speed boiler circulator. I think the Knight comes with an additional sensor to use on the tank.

The 3 keys are:

Proper installation, piping and venting

Good water quality, buy a deionized cartridge to fill

Proper combustion setup, day one!

If you are handy a mod con can be cleaned and serviced in a few hours, watch the factory videos. You’ll need a combustion analyzer and some training. Both for start and after cleaning. I suspect all brands have online training webinars and videos.

How often they need service depends a lot on what they breathe Dust, pollen, bugs, etc. some can go years without needing service, others need yearly cleaning. It is site specific.

The pic is a 6 gallon Lochinvar tank. I chose it because it had top and side connections. Most small tanks do as they are under counter use.

It came with a 1500w element, 120v. It is a standard screw in element, you can get up to 5500w on a 240v 30a circuit 5500w = 17,270 btu/ hr.
There are also 6000w elements but they tend to be too long for the small tanks.

There are multiple ways to pipe a system like this, all have pros and cons. The manual 3 way is inexpensive and gives you ODR on both temperatures. It is limited to one zone however. Multiple loops are fine, on one zone. I used this system in two of my own places, so I know it works well.

Here is how I controlled kick space heaters on low radiant temperatures. This is adjustable down to 70f.
Heat output will be limited at low SWT. But for a mud room or bath air movement, anything above 110-115 still feels warm. Just swap out the factory one that snaps on the heater piping.

Simpler yet, Harveys Horseshoe

Boiler injects into u shape loop, high temp, and low temp via a 3 way thermostatic.

If you zone the kickspace they would be your lowest load, but still a good match for a boiler that turns down to 8,000 BTU/ hr

Good Bluetooth connected analyzer

The Taco iSeries-R valve is pricey. I believe it is a little overkill for your small system. My own radiant-floor system, and others that I have installed for customers, all used the “dumb” mixing valve, and I believe that the idea @hot_rod alluded to will serve you just fine for less cost.

That two temperature line shown in figure 4-9 ( and 4-10 if you happen to look up the article ) will be as accurate as you need it to be. It will just take some fine tuning/trial and error. The Valve actuator on the Taco valve will be less trial and error, but you will still need to make fine tuning adjustments for the first season and perhaps the second season.

I would not spend the $$$ on the valve unless I found the dumb mixing valve to be too problematic. Truth be told, you may not even need two temperatures with those two different floors. I used the same temperature for a slab floor and a staple up system in a home I lived in for several years and was happy with the wall thermostat being the only control for each zone, while the ODR from the boiler did the temperature adjusting based on the weather. I thought the staple up was lagging behind most of the time, but that was just me being over-sensitive to the workings of my design.

If I still lived there I may have tried the Taco iSeries-R to see if it made a difference, but alas, I never got the chance.

The Lochinvar Nobel is a price point mod con. Smaller HX, less control function. Still plenty of features for what you need. It would be a 110,000.

The 110 a little bit larger than what you need. The control has some nice features, you can limit the firing rate to say 68,000. It has an anti cycling function also, and gradient limiting. All these can be set up to limit excessive cycling.

You still want to have some sort of hydraulic separation.

This could be primary secondary piping, a hydraulic separators or a buffer tank.

The cost difference between a separator and a small buffer is not that great. If you want the electric back up, which could be tied to a PV grid someday, for some additional RE input, then use a small electric tank s a buffer

I wouldn't hook my boiler system to a softened water feed line. look into installing a glycol feeder instead, and fill it with a controlled boiler fluid like DI water or a premix ready to use glycol blend if you are using glycol