Most reliable heating system for a home unattended for up to 3 months in winter.

The simple boiler that you want can no longer be manufactured due the the energy efficiency requirements. if you can find someone that will be happy with converting a new boiler from electronic ignition to a standing pilot system that uses a pilot generator that requires no electricity to operate the burner. Of course the circulator pump(s) wont work without electricity but you can get gravity heat during a power outage.

As a repair service company I had several customers in your situation. So I mighr. recommend that toy find a installer that will offer your this service. monitor your heating system with a smart thermostat and have a service agreement with someone that has a key to your house. That way when you get an alarm from your smart thermostat that indicates a low temperature condition, you call the service company with your key to have the repairs made before the home freezes.

You could also install a couple wall furnaces or console heaters or even a floor furnace with millivolt systems that is big enough to keep the place above freezing if the boiler fails.

A damn good Alarm System.
Electric baseboard in critical areas.

A solar generator or computer back up would keep a security or freeze protection powered. . Probably your internet also, so you could be notified.

I'm with @mattmia2 . Install through the wall vented heater as backup. If your power is somewhat reliable you can always add in electric heaters set bellow your boiler temp. Since it is backup, the operating cost doesn't matter. You can add a wifi thermostat to monitor the house remotely.

All of this could be moot though as lot of insurance policies require somebody to check on the house every two weeks anyways.

If you’re remote, I would still drain the system, and all the domestic. You won’t need antifreeze anywhere but in the traps.

Use electric heat, and a wifi thermostat to monitor temperature and notify you.

Then if you lose power you won’t have any freeze damage.
To me, remote means, possible extended power outages, impassable roads, etc. So I wouldn’t want to rely on anyone in the above mentioned cases.
Also note you have an obsolete QB 180 burner. Make sure your service provider has all the parts necessary to fix that burner, or replace the burner before it becomes an emergency replacement.

wall furnaces can lose their pilot light and then you are still without heat

How much redundancy is enough?

Neighbors, friends or property managers you can at least stay in contact with.

Kinda what I said…

in relation to @HydronicMike 's comment abut "remote". It kind of depends on who you hire to monitor your place. I HATE tooting my own horn, but when someone hires us to monitor their place (and we have a couple dailys, several weekly/sever storm ones) that's exactly what we do. It's what we are paid to do (among other things) and so that's what we do. Three heavy duty four wheel drive pickups (with winter tires, one with chains), two snowmobiles, two tractors, skis and showshoes. Oh — and a team of draught horses and a sleigh.

And we're bonded and insured, so if something does go wrong and we should have caught it and didn't, you're covered and so are we.

Now I might add that if you bring your fancy all wheel drive SUV around and get stuck because the road was "impassable" we'll also come and pull you out…. for a very nice price…

humph

The most reliable course is a good caretaker.

Adding glycol to your heating system is a great idea. There are many items to consider (pump sizing, transfer of heat, % glycol, etc) when doing this. Make sure to hire a local professional to do this.

You may want to consider a propane boiler that comes equipped with WiFi (not Bluetooth) that will alert you if there is a problem with the boiler and/or send an email to your contractor for remote reset or immediate service. Of course this is all great, but if you have no electric none of this works anyways.

I was just at a ranger station in Colorado. The complex is run by electric BUT has a full solar electric system with battery storage. Each cabin was equipped with a standing pilot, atmospheric stand alone heater that was central to the house. These little heaters (by little I mean 40,000 mbh, stood about 3 ft tall, 3 feet wide and about a foot deep) still needed electric to open and close the gas valve and run the thermostat, but very minimal electric. The heaters actually reminded me a lot of the heaters I was used to in sailboats.

You can reset that button remotely with something called a fingerbot button pressing device. It’s basically a tiny Wi-Fi connected actuator that you mount next to the button. You can then press it remotely with your phone and an app.

Millivolt natural gas and propane fired heaters will work without an external power source. If you get one, they use a standing pilot and what’s known as a thermopile to generate enough millivolts to control the gas solenoid. You can purchase what’s known as an automatic pilot relight module, and replace the spark igniter in the heater with its electrode. If the pilot ever blows out due to wind, it will spark continuously to try and relight the pilot. It’s usually successful enough to get it relit before the thermopile cools off and the pilot valve shuts down the gas flow. I believe they have 12 VDC versions you could connect to a small 12V lithium battery, so it would work in a power outage. Keep the battery on a trickle charger so it’s always ready if it’s needed.

plumbers always seem to offer the most expensive fixes possible. for around $200 each you can get a couple of propane wall mounted gas heaters and that will take care of the problem. they have a pilot lite which can only go out if you run out of gas. main water supply should be turned off before leaving. should not depend on the boiler just the wall units. I have one in an 1200sq ft apt when set on the lowest setting keeps it close to 60

I second the caretaker option, probably cheaper than hooking up a new heating system.

I remotely manage our family's property in coastal Maine, which is sometimes unoccupied in winter. We used to drain and winterize it, but that was a real pain and had to be done very carefully and thoroughly (and prevented any use during the winter or it would all need to be re-winterized). So during a remodel with radiant heat that is really hard to drain (and no desire to use glycol), I had WiFi thermostats installed and a handful of web cameras (one in the mechanical room), and the propane tanks are remote monitor equipped (the supplier and I both can see their levels). Then we just pay a caretaker to drop in every couple of weeks to make sure nothing catastrophic has happened (broken window, fallen tree, etc), and she is also on-call if I see anything amiss via the remote systems. There is a generator as well, but that wouldn't be necessary - it's great peace of mind though that freeze ups due to power outages aren't a concern (it's also web enabled)! Keeping the place minimally heated all winter is worth the cost to be able to drop in anytime someone wants, and never having to spend time winterizing.

Trunk monkey has revolutionized the auto industry.  

I wonder when they will come out with the Boiler Baboon. That will solve all your boiler problems

I shut off the water when I'm not home.

my BIL Lake house in PA we pitched all the supply lines. Shut the pump off, open the drain, open all the faucets and problem solved.
more then once the heat went down. That gave him time to get someone there and correct it.
never ASSUME it’s foolproof!