Low flow electric micro boiler plumbing?
 

Is it really necessary to plumb a hydronic floor system with an electric boiler that is 1 temp, 1 zone(with only three loops) with the primary loop/secondary loop design that I have seen said needed by some micro boiler manufacturers that needs two pumps. I want to invest in a good new ECM pump so only one pump saves. Boiler will be heating 500 sq' single slab each night so it should'nt be short cycling

Your question is so open ended that nobody wants to take a stab. However, I'm a glutton for punishment, so here we go.

First things first: economical versus reliable versus diy. A 500 square foot space isn't a massive one, but you can get a lot of money tied up in the system if you want to. Home vs commercial vs industrial gear. On the cheapest end, a sous vide immersion heater in a bucket, plumbed to a fountain pump on a thermostat could probably do you well. On the other, some pid controller and a cast iron industrial circulation pump and a steel box with plumbing fittings and pipe could cost a lot.

Please elaborate on your goals and the build level you want to employ. Digital fuzzy logic variability with bluetooth, vs one switch and internal "magic" operation.

Sorry for being short on details. I am ready to build controls and heat source for my in floor NO pex heating for our new 600 sq' super insulated/sealed living space. It has three closing spaced(6" at exterior walls to 12" interior) 200' loops, all one temp, one zone, one slab(each loop is valved for simple balancing). I plan to recharge floor each night with off peak. If needed.

My heat source for water(closed system) are limited at this time because I want to be on my local elec COOP's off peak program and they have requirements that won't allow me using an HE elec DHW heater as a source for the low demand hot water I will need(which would be my choice). I do not want to be tied to fossil fuel with a propane DHWH or have to deal with combustion in the tightly sealed space. So I am looking into a mini elec boiler. The Hydroshark brand that is displayed all plumbed at a local box store is plumbed with a primary loop that has a pump that circulates fluid thru boiler and to a close valve exchange to a secondary loop that has pump and controls that feed pex zones and loops. I have seen DIY systems plumbed with just a single pump/loop(but different brand boiler) which is much simpler/afforadle but is it advisable? Both ways single or double pump designs have all they other recommended components, shut offs, expected tank, fills, air trap etc. Are mini boiler brands different operationally?

Ok so from your response, you are planning to build bionic and above code requirements. Diy and separate (free/cheap) licensed inspector. Great. Now pick your price point and power sources.

Cheap from China ain't the same as industry standard or commercial bionic made by union labor. Not just the tankless or tiny tank heater, but all the parts that make it do it as well.

All we really need is your design heat load and flow rate and target dT with respect to the supply side of the system.

Have you done a heat loss calculation, if not you are groping?
A tankless water heater is a terrible choice, they are designed to put out short bursts of very hot water, inslab heating needs a steady stream of warm water. My own inslab system never heats the water past 110F even at 20 below, but spends most of the time idling along in the mid 80's or 90's.
Once you know how many BTU's you need, you can begin to design a system, you can work out your emitter spacing and your boiler size (BTU output).
I designed and built our system, spent a year studying everything I could find on Hydronics, and still did some things that I would like to take back. You need to study hard in order to get it right.
Best thing to do while studying is insulate under the slab with as much insulation as you can afford and insulate around the edges, become an insulation overachiever & your system will love you for it.

What do you consider superinsulated? I'm a little north of you in Wisconsin, so have similar heating needs. My house has 4" foam under the crawl space and footings and outside the foundation, R30 walls, R90 ceiling, and R3 windows. It has 13,000 BTUH heat loss at 70 F inside and -20 F outside. I consider it well insulated, but not superinsulated.

Do the heat loss calculation, then calculate what slab temperature you need to provide the heat, then calculate what combinations of water temperature, flow rate, and tubing length you need to get that slab temperature.

Sounds like you are pretty well insulated, but insulation is like money and love, you just cannot have too much of it.

Wrote that last night & was a little tired. To answer your primary secondary loop question, if you take your three loops and calculate the headloss, many boilers have a pump and if the pump is sufficient for your headloss, you do not need a second pump.
My boiler's pump is adequate to drive seven 250 ft loops of 1/2" pex so does not need a second. I bought a Rifeng manifold which came with flow control valves for balancing the loops, and just connected them up. Remember that you must have an expansion tank, and you must pump away from it.
IMO your heating system may cost less to just run it 24/7 than to use intermittent heat.
Your 13000 heat loss is full time, so if you only heat 1/2 time you need 26000+ from your boiler.
A thought is that you could use a hot water tank if it has sufficient btu's although my professional friends might shoot me for suggesting it.
Depending on the thickness of your slab, you you may run into problems of temperature over runs, when the sun warms the room and your floor is at full warm.
Lots to think over, there are many web pages to pore over.
Cheers.