I do a lot of radiant floor installations for the company I work for with high efficiency modulating, condensing boilers or high efficiency condensing, modulating,  boilers as a back up for a geothermal system. Sometimes we install electric boilers - we've installed a few Monitrons, Argos, and Thermolecs. The company I work for seems to be partial to thermolec as of late because they're relatively inexpensive and come with an outdoor reset. They also seem to have a good warranty and there's not too much that can go wrong with them.  

But now things are getting "personal." I might be buying my mom's house potentially, which is a small little rancher (about 1000 square feet), and I've thought about putting radiant floors in there by putting the O2 pex lines in the joist spaces which I've done in the field.  For various reason I've figured a small electric boiler would best suit the application since most even small condensing boilers don't modulate down low enough to really offer continuous outputs at the heat losses I would anticipate in the shoulder seasons, which for VAncouver BC, is a lot (we don't spend a lot of time at, or near our peak design temperatures for heat loss, so having good modulating is important). Currently there is a mid efficiency furnace, cold laminate and cork floors (in the winter anyways), and a gas fireplace. The nice thing is the gas fireplace doesn't need any power if we lose power, which happens at times. And I figure I could even slightly underdesign the radiant system a little bit (I have my radiant/hydronic designers ticket as well) as the fireplace could take up the slack in extreme cold temperatures. 

So I'm curious what electric boilers you like? I'm thinking in the 3 to 6 kilowatt range. I suspect the heatloss will be around that much give or take (haven't done a proper one but just based on previous experience).  Other then the boilers mentioned above, is there anything else? Anyone tried a Lion Electric boiler?

Electro Industries Inc.

Tell them Morgan sent you.

What does the house have for domestic hot water (which is probably bigger than your heat load)??

For ultralight loads like that some tankless hot water heaters (condensing or otherwise) can modulate down to ~2.5-4kw output. (Morgan hates the idea of using a tankless as a boiler though :-) ) Takagi T-K3 or T-H1, Navien CR180 are possible candidates. (Takagi warrantees their units for combi & space heating use. Navien does for many models- not sure if the CR180 is one of them, but it has <6kw output on the low end of it's range, and uses condensing heat exchangers.)

They're not rocket science to design around either, but bearing in mind that they only self-modulate for constant temperature output. To keep burn cycles long some buffering is advised. (The efficiency of a tankless will generally be near it's rated efficiency, as long as you pump 5 gallons or more through it per burn cycle- it's the short cycles that'll kill it's efficiency.) If you want to use outdoor reset, slave the tankless to a buffer tank, control the tank's setpoint with an outdoor reset, run the radiation off the tank. OTOH, leaving the setpoint of the tank at the desired design-day setpoint and controlling the flow to radiation with a PID thermostat achieves the same comfort level with only a slight hit in efficiency if your design-day water temps are under 130F (keeping the return-water under 125F under constant flow conditions.) In low heat load conditions the tank will stratify a bit, and return temps will be somewhat lower at the beginning of a burn cycle.

Lots of low-moderate heat load + DHW load combi configurations are possible. Davis Energy Group in California has done numerous combis with tankless water heaters & air handler coils in their "low energy home" demo & test houses, as have many others (eg. http://dsp-psd.pwgsc.gc.ca/collection_2007/cmhc-schl/nh18-22/NH18-22-106-108E.pdf ). Radiant temps can run even lower than air-handler coils, with lower gpm on the pumps too. DHW can be done either as a seperate zone w/ heat exchanger, or if you use a constant-setpoint buffer tank, make the tank a reverse-indirect. (In the latter, the tankless will modulate up as the DHW load increases, since the return water drops dramatically with sub-50F water entering the heat exchanger at the bottom of the tank, while the temp at the top of the tank stays pretty much constant.) If you still want to use outdoor reset in that configuration, go with a reset-controlled mixing valve (as opposed to NONE.)

Alternatively, if you have standard tank DHW heater you can run the radiation off IT with a heat-exchanger, controlling the radiation temp with a reset-controlled mixing valve (or none, using PID thermostats instead.)

But if you truly HATE Rube Goldberg machines & 1-man bands, go ahead with the electric boiler, by all means! The design will clearly be simpler, and the cost lower too. Not sure how your electric rates stack up against gas rates, but with a super-light load, who cares? Sounds like your goal is more around creature comfort than (up-front or operating) cost.

I forgot to mention- the tiniest of mod-cons have min-modulated outputs in the 3-6kw range (eg. Peerless Pinnacle P50 or PF50, Triangle Tube Solo 60. They too might need some buffering on the heating end, but running your DHW with an indirect will keep it's duty cycle & efficiency up too. (Morgan DOES like this approach. :-) )

There is currently a Bradford White 40 Gallon Lowboy electric which I replaced last year for my mom.

I think I'm with Morgan in that I'm not a fan of jimmy rigging things that are not designed solely for said purpose. Besides, a seperate, electric boiler, with a small pump, and all the proper components, I could build the thing with bloody nipples and fittings for the most part out of black Iron. I'd put a spirovent air seperator, a spirovent dirt seperator in the inlet/return for the boiler a Wilo Pump with isolating ball valve flanges, Wirsbo HePex, build the manifold with Dahl balancing quarter turn valves with the allen key balancing screw. The manifold would have around 4-5 loops. It'd be a nice simple, no nonsense system and the boiler would see fairly low temps it's entire life. Also limited investment cost. And yes it's more for comfort, haven't considered or cared for the long run in this situation as you already figured out. I just want a nice no nonsense electric boiler that is durable with a low input rate. Hell if they had a stainless steel electric I'd be all over it but they don't, at least I don't think they do?

If you're electric, why use a boiler at all? 1000 sq ft of cable or laminated radiant panels (like Calorique) has got to be cheaper and easier than buying a boiler and running tubing and spreader plates. I'm a supplier, so I'm biased. But still . . . have you costed this out with an electric radiant system?

Why use a boiler? Thems fightin' words! hehehee

Because he might want to change his mind. Noing going back once you bury the cable.

What about the Combomax?
http://www.smartenergyne.com/combomax_en.htm