Electric boilers are 100% efficient by AFUE standards, very reliable, compact, last a very long time with minimal care and can put out about 100K BTU for 30KW.

Energy costs aside, even a 24KW electric boiler which will have a net output of about 82,000 operates at just over 100 amps/240V. Are you willing and able to dedicate a 125a dble breaker and do you have the service to support that? most don't.

I think multiple boilers for large loads, controlled properly are the way to go if redundancy is the desire, however for most homes when a fully modulating boiler of required size is available, their is no efficiency to be gained in fact you lose because of additional mass, energy consumption and piping loss

Are you willing and able to dedicate a 125a dble breaker and do you have the service to support that? most don't.

No, I don't think, we can have more than a 20Amp line. But the backup's role is not to keep the house comfortable and the showers running it would only be to keep the pipes from freezing, while the failed gas boiler is being fixed, or the gas supply is being restored by the utility...

Will 20Amp be enough for that? I think multiple boilers for large loads, controlled properly are the way to go if redundancy is the desire

That's the question are we being "exuberant" by desiring such redundancy, or is the boiler completely failing mid-winter a valid concern?

20 amps?
A plug in 1500 watt (5,100 btu) draws about 12 amps which doesn't go to far heating your house

A plug in 1500 watt (5,100 btu) draws about 12 amps which doesn't go to far heating your house

Zl, I don't plan to heat the house (to the comfortable temperature of 70°F). I just need to keep the water in the pipes and the cast-iron fixtures above freezing (at 35-40°F) while the boiler is being repaired.

Would the 7000BTUH, that 20Amps at 110V afford, be enough for that?

Short answer: My personal experience is the Ultra is above average for reliability, availability in parts and service.others may have a different experience and that should not come as a shock for any brand of equipment.
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Long winded but inquisitive replyall things being equal, with that input/output you would be able to heat 32,000sf+/- of surface area constructed to Germanys Passivhaus standards, 13,500+/- sf of surface area constructed to something like Canadas R2000 standards or 5400+/- sf constructed surface area to typical building code standards for North American cold climates...(give or take a few hundred sf)...

So if youre building to the first two (Passivhaus & R2000) in a cold to moderate climate and in the presence of failure in utilities or equipment, it could take up to a week or more for pipes to freeze and by that time you could have drained the system or had it serviced.

Typical American home in cold to very cold climatesa couple of days and youre done (exception: radiant floor heating will buy you a few extra days).

So what are you building? A huge energy efficient home or a typical low efficiency North American one and how secure is your supply of power and fuel?

As usual it comes down to

1.how reliable is (in this case) the WM UG230 (and does two WM UG 105 change the concern over reliability) and,
2.how secure is your supply of natural gas/ oil/ and electricity and,
3.is the investment/risk in your comfort zone.

Have you and your contractor considered...

1.if a primary pump fails - having two boilers doesnt change anything, one would need redundancy on the pumps as well and,
2.if the temperature and zone control valves cannot be manually operated having redundant boilers and pumps doesnt change anything eitherand,
3.if the power grid collapses or gas compressor stations fails or oil trucks and service providers cant get out due to weather it doesnt matter what redundancy you have in your mechanical room - your homes thermal efficiency is the safety net.

I think you get my message redundancy or backup is not a single piece of equipment it has to be built into the system which includes the efficiency of the home, redundancy in critical components and if necessary anti-freeze in the heating system.

Im a big fan of building efficiency and equipment redundancy especially in cold to very cold climates where equipment reliability could be questionable, where service and parts is a crap shoot and where there is historical evidence of power and fuel failures for days or weeks at endothers wise its just another vanilla/chocolate/butterscotch design with a vanilla/chocolate/butterscotch approach to equipment.

230,000 Btu/hrnot chump change - so what is it - hugely efficient mansion built to Passivhaus or R2000 standards or sizeable American home built to minimum standards? Urban or suburban? History of failure?

35% up charge...maybe better off to improve the building efficiency and get the load down to a single smaller boiler instead of making the mechanical system more complicated - 35% - worth discussing yes - so do tell..

so what is it - hugely efficient mansion built to Passivhaus or R2000 standards or sizeable American home built to minimum standards?

The house is over 80 years old... But we've renovated it quite a bit the exterior siding is new (HardiPlank), the windows are new (3-plane OKNA), all new insulation (R-18), and sheetrock. Urban or suburban?

A sub-urban, standalone 3 stories, plus heated basement, and attick. History of failure?

I don't think, gas-supply failed in recent memory, but, as they say in finance: "Past performance is not an indication of future results". In particular, the current gas-pipe is "low-pressure" (no more than 7" of water column), which means, they'll be upgrading it in a few years... It will not be done during winter, but there will be disruptions, I'm sure...

Thanks!

The smallest electric boiler, 10KW, will draw 45 amps.

This has been interesting to watch. As far as getting a 5000 or 7000 btu space heater to keep your pipes from freezing in a 3 story house is probably the equivalant of throwing a deck chair off the Titanic to lighten the load. As healthyheating pointed out if you lose your fuel or electricity it doesn't matter how many boilers you have, they're not going to run. As far as freezng pipes go, as they said, it takes a while for your house to get to 32 deg. and stay there in order for your pipes to freeze as long as they are not exposed to drafts. Drafts will freeze a pipe before the temp. drop in a house will. If your area is like mine I can almost guaranty you will lose your electricity before you lose you gas. Did you ever consider a backup generator. More people are going that route lately because of down wires from ice storms. Good Luck.

As far as getting a 5000 or 7000 btu space heater to keep your pipes from freezing in a 3 story house is probably the equivalant of throwing a deck chair off the Titanic to lighten the load.

Let's see... The entire collection of heating fixtures throughout the house is expected to top at 120K BTUH, which is, no surprise, slightly above our total estimated heat loss.

A 6000 BTUH is 1/20th of that total, or 5%. Whether such a device exists, and would be enough to keep the water in the pipes above the freezing point is not immediately clear, but it is not unreasonable to argue, that throwing off 2300 tons (5% of the Titanic's 46000 tons) could've saved the ship... As healthyheating pointed out if you lose your fuel or electricity it doesn't matter how many boilers you have, they're not going to run. [...] Did you ever consider a backup generator.

I don't think, we'll ever lose both gas and electricity at the same time. Thus at least one of the boilers would continue to work.

If you are warning me about the electricity needed to run the gas-boiler's electronics, I was going to use a computer-oriented UPS for that... A device meant to sustain a computer (with bright screen and hard-drives) for 40 minutes, it should provide juice for a boiler's control unit for a few hours, no?

I do get the point about there being enough heat in a reasonably-insulated house to keep it above freezing for some time just wanted to see, what can be done proactively to extend that time...

I guess, the conclusion is, installing multiple boilers is not worth the extra money, space, and complexity... Now, if only somebody could speak convincingly about one boiler being better than another...

If you have the money, try the rich person's solution, that I'm sorry I did not.

Get a separate small boiler for each floor. If one fails, the others provide heat, and the boilers are so primitive that anyone can repair them and get the parts.

If you're worried about diversity of fuel source, wouldn't propane provide a simpler redundancy option?