Single Points of Failure

Over the weekend, my family had an annoying lesson in one of the basic principles of software engineering, the "Single Point of Failure". 

You see, when we built our house, we decided to go with a "hai-tekku" heating solution, and we're paying the price for it.

Normally, when you build a house you get two units installed - a hot water heater and a furnace.  For a house heated with natural gas, that means that you need to have two chimneys - one for the water heater and another one for the furnace.  That means you (a) need to have two holes punched in your roof, and (b) you send a ton of heat up the chimney of your furnace (because air is a lousy thermal conductor).  Hot water heaters are WAY more efficient than furnaces (because water is a pretty good thermal conductor, and thus retains heat better).

Instead of relying on a more traditional hot water header/forced air heater combination, we chose to go with a Polaris hot water heater, which allowed us eliminate the furnace - instead, the hot water from the hot water heater is pumped into a heat exchanger and the residential air is warmed by the heat exchanger.

This system works great - we get a huge amount of hot water (as a result of the large capacity tank and it's relatively quick charge time) and it usually "just works".


Until Saturday afternoon.  Valorie came back from riding and discovered, much to her chagrin that we didn't have any hot water.  I went down and checked the tank and sure enough, the "Igniter" light was out - the igniter had died on the hot water heater.

Well, having no hot water's not THAT big a deal - it's somewhat annoying, but it's no big deal.  We called the repair people and they came out today to repair the system.


Remember above where I talked about our not having a furnace and instead using a heat exchanger to heat our house?  Well when the hot water heater went out, so did the heat inside our house.  When I left for work this morning, it was 52 degrees Fahrenheit inside the house - a bit nippy.


Our hot water tank acted as a single point of failure for our homes environmental systems - not only did its failure take out the hot water, it also took out our heating system as well.

Fortunately, we had power and so we were able to heat water, and throwing on a couple of sweatshirts fixed things.  It was also fortunate that Microsoft has lockers with showers in them so we were all able to get showered before we went to theater on Sunday :).


For our next house, we need to consider if we need a backup heating solution or not.


Edit: Fixed awful transliteration, thanks Norman.


Comments (14)

  1. tzagotta says:

    I don’t see how a combined system is any less prone to failure compared to having a separate furnace, unless there is some inherent reliability problem with that type of system. A separate furnace is probably just as likely to need repair, and would leave you in the same situation as you described. The only difference is that you would have hot water, which is not of much use if the house is cold enough to where you want to go stay somewhere else anyway.

    The best way to reduce the probably of a heating failure is with a redundant system, e.g., a pair of furnaces. I’m not sure how that affects energy efficiency, however.

    In my view, the biggest risk, and most often cause of no heat, is due to no electricity. So for me, that is the problem to solve. Furnaces are generally pretty reliable, in my experience. I think I would consider a natural-gas fired genset in my next house. That way, when the power goes out, just fire up the generator, and you can still run the furnace, fridge, and some lights.

  2. There isn’t.  But with redundancy, if one subsystem fails only that subsystem is taken out, not both.

    To bring it back to PC-land, in NT4, when the PDC failed, all non query user operations failed because the PDC was a single point of failure.  You could still log on, but all account administration stopped until you nominated a machine to be the new single point of failure.  This was one of the major technical challenges for Win2K – the AD is a multi-mastered replicated authn system (which is way harder, but is resiliant against single failures).

  3. mahoekst says:

    What’s worse, no heat or no hot water or both? At my place we get hot water from the powerplant a couple of km’s away. (yes hot water travels under our streets :)) Talking about single point of failure. If that plant dies. No heat, no hot water and no power 🙂

    Are you going to buy backup power generator as well next time Larry? That’s a single point of failure as well 🙂

    It’s the same as with business. Uptime or SLA’s can be achieved with great backup/restore or maintenance contracts as well. You got the heat repaired the same day. Most of the time that’s good enough and it isn’t worth the extra bucks to have a seperate or maybe a 2nd installation installed.

  4. Normski says:

    In the uk we have Combi-Boilers (Combination Boilers) which is a gas powered boiler that heats radiators and supplies hot water on demand (a clever feature is that it remembers times of the day why you demand hot water and prepares the boiler so that is get red hot water the moment the tap/fawcett is turned).

    Again, same situation last winter it packed up, so no hot water/no heating and a 2 day wait for the engineers to arrive – added bonus we have 2 children under 5 years. I wasn’t too pleased with the service suppliers and ultimately got a refund for slow service.

    Great when they work, hell when they don’t.

  5. Tom M says:

    Here in the UK, (and also AFAIK in most of northern Europe), combined heating and hot water systems are very much the norm, with a sealed water system providing heat to radiators throughout the house, and a hot water system that either uses water heated on demand by a combi-boiler, or a heat exchanger that takes heat from the closed circuit. They’re usually very reliable.

  6. Norman Diamond says:

    > "high-tekku"

    Is that the mirror image of "hai-tech"?

    > Normally, when you build a house you get two units installed –

    > a hot water heater and a furnace.

    In case anyone wonders why I sometimes pretend to make assumptions about what’s "normal" and pretend it’s unimaginable that any reader might have different assumptions about what’s "normal", here is a perfect example of why.  Thank you for helping to explain that ^_^

    > instead, the hot water from the hot water heater is pumped

    > into a heat exchanger and the residential air is warmed by the

    > heat exchanger.

    Wait a minute, radiators are low-tech.  100 years ago they were high-tech.  No wait, radiators are still high-tech — last month I read a pamphlet from the maker of the heater that a friend bought, boasting of their advances in shaping the fins to improve the circulation of heated air.

    > it was 52 degrees Fahrenheit inside the house

    Actually that’s kind of ordinary in a lot of countries (not in the tropics).  In recent winters I’ve been keeping one room at about that level, but other rooms can freeze with the weather.

  7. John Elliott says:

    In the UK, it’s very common to have a single boiler for hot water and central heating. The hot water cylinder contains an electric immersion heater, so you can get hot water if the boiler’s failed or is deliberately turned off.

  8. Norman, high-tekku is the phrase my brother picked up during his year in Osaka – he (and his wife) describe everything high-tech as "high-tekku", and Valorie and I picked it up.

    Oh, and the thing that made this "unique" is that instead of a radiator in each room, you get the convenience of a forced air system with the simplicity of a radiator system.

    Also, we’ve decided not to buy a generator – our feeling is that it’s unlikely we’ll have to go 8 days without power again (knock wood).

  9. John C. Kirk says:

    As others have said, combi boilers are very common in the UK, but it is annoying when they stop working. My redundant back-up is a real fireplace in my lounge (although I’m limited to smokeless fuel rather than real coal): I normally rely on radiators, but a real fire can be nice on special occasions, and if the boiler packs up then I sleep in the lounge until it’s fixed.

  10. Duff says:

    Having a backup heating system would be a money sink… my guess is that maintaining a dormant system would be more expensive than simply getting a better heating plant.

    If in the future you are building a house, look at putting in a hot water system instead of forced hot air. Running the pipes is costly up front, but you get radiant heat that feels more comfortable and doesn’t dry the air out like a forced air system. You also don’t need to worry about filters, UV lights, etc.

    If you use a hot water heating system, you can buy add ons for your boiler that produce unlimited hot water on demand without using a hot water heater!

    My parents put a system like that in, and it works great. The hot water system paid for itself in 12 months since they live out in the country and had to use electricity or propane to run a water heater.

  11. I always love it when the operating system I run finds new ways to absolutely delight me. Yesterday,

  12. Norman Diamond says:

    > high-tekku is the phrase my brother picked up during his year

    > in Osaka

    Then it’s "hai-tekku".  (Because if it were "high-tech" then it would be an "oh you dumbo key".)

  13. The Wife says:

    Actually, we thought about a radiator or radiant system, but we wanted/needed the air filtration anyways for allergen control. Once you have to filter the air anyways, it didn’t make sense to run both pipes and air ducts.

    Hmmm, maybe next time we just need to buy our heater from the UK or somewhere else where it’s "normal". Because our system is "special", we get to pay "special" prices to repair/replace it 🙁 Of course, by the time I add in import costs, they’d probably be about the same. Still, at least if it were the "normal" system, we’d have a variety of community knowledge and usability studies to look at when thinking about replacement options.

  14. The Brother Who Lived in Osaka says:

    Norman is likely right with respect to the transliteration of the katakana.

    I chime in only to note that our first exposure to the word "hai-tekku" was also in connection with a water heater. In our case, a water heater that used natural gas but did not have a flame. It put the gas through some kind of chemical reaction that generated heat, but also had the side effect of producing a rotten egg odor. It was coupled with a system in the bathtub that at the end of the bath would transfer heat from the bath water back into the hot water tank (I’m not sure how — a heat exchanger was somehow involved). The whole system was controlled by a panel in the kitchen that had a habit of getting bumped and turning itself off. Since there was no burner, you often wouldn’t notice it until there was no hot water left, at which point you needed to wait 3-4 hours for the system to heat up again.

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