I'm replacing my cast iron boiler for hydronic heating and domestic hot water in Massachusetts. Plans are to install a new Viessmann gas fired condensing boiler. Living in New England, it's not out of the question that we could lose power during a winter storm. I'd like to be able to keep the house warm during an extended power outage. The power requirements of the boiler and circulating pumps is going to be relatively low, so I'm looking at installing a furnace transfer switch to be able to power the furnace off a portable inverter generator that I own.

Am I risking my boiler equipment? I understand that a standard job-site generator kicks out some really dirty power, and I shouldn't plug in anything with a chip in it. But everything I've read about inverters is that they're electronics and computer safe. I'd like some thoughts if there's any thoughts or experience on this that I've missed.

I've also heard the advice, install a whole house system or an installed system serving only a couple of breakers. That's really overkill for what I'm looking for. There's nothing else in the house that I'm worried about running that can't be fed off the second outlet of the generator (an LED light and maybe a phone/laptop charger). I own the generator already, and we don't lose power frequently enough to justify an installed system.

Are there any issues with powering on/off the Viessmann boiler system. I understand the recommended sequence for plugging into a generator are to 1) get the genset started, 2) plug in the equipment. Then on the shutdown 3) unplug the equipment, 4) shutdown the genset. Is pulling power from the Viessmann going to be bad for it in some form? Again, this is not going to be a standard operating mode, but hopefully coasting through an emergency couple of days. But last thing I want is to damage my boiler during a couple of hour power outage, requiring an emergency service call once power comes back on.

Finally, I'm aware of carbon monoxide risks, and I have the requisite 50ft extension cord to put the genset outside, 20ft from the house.

Thoughts, hand-holding? Thanks.

  • Are you comfortable with your extension-cording your refrigerator? Commented Mar 24, 2020 at 14:42
  • 1
    @Harper-ReinstateMonica, I am comfortable with potentially losing everything inside the refrigerator, and moving everything in the chest freezer to a cooler outside in the shed (its winter when this is happening). Good point though.
    – A Holman
    Commented Mar 24, 2020 at 15:01

1 Answer 1


You said inverter generator , these are usually much better than the type I use, with the old fashion alternator and voltage control. I power my TV’s and microwave refrigerator, furnace and well pump with mine and have done this for over 20 years without knowingly killing anything. I have been on generator power up to 2 weeks and loose power for at least a day every year usually several.

The important part is not overloading your generator , I usually am well below the generators rating and maybe close to 1/2 of its peak rating.

The reason this makes a difference is because when generators are over loaded they start surging this causes the frequency to bounce and the voltage to spike then droop. With a proper size for the load the generator will only vary slightly.

Your inverter rating is 2kw but what is the load the boiler needs (the pump being the big draw of the system) if your recirculating pump is a fractional horse power you would probably be fine, setting up a transfer switch for this one unit may be a good idea but I would want a quality heavy duty extension cord 50’ is not that long but pump’s have a large starting current and a small extension cord can increase voltage drop , create excessive heat in the wires and damage the system where a Quality 12 gauge cord will be fine.

I would move forward with your plan it looks sound and may provide comfort when all else is dark.

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    I'll be curious to see what his pump takes. I'd be a little concerned about the starting current.+
    – JACK
    Commented Mar 24, 2020 at 15:08
  • Yes I agree but many circulation pumps are less than 1/4 hp , a 1 hp would be two big on a 2kw generator in my opinion but anything under 1/2 should be fine if just running the boiler and the pump. The smaller the pump the more lights or other devices could be connected.
    – Ed Beal
    Commented Mar 24, 2020 at 15:17
  • My quote specs for a variable speed ECM main circulator, which would serve max of 4 zones. Not sure if that gives any relevant information though.
    – A Holman
    Commented Mar 24, 2020 at 15:28
  • No we need the amp rating of the drive or motor or it’s Horsepower and voltage (guessing 120v from the disconnect you showed). Code says we need to use the size of the drive but I know I have 10 hp drives on a 3 hp motor, the real draw will be the motor size or FLA ,
    – Ed Beal
    Commented Mar 24, 2020 at 16:10

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