Upgrade part or whole of gas hot water system?
August 18, 2017 11:54 AM   Subscribe

We have a gas boiler and an indirect water heater. Boiler seems to date to 2002, and water heater from 2001. Water heater has a leak in the port for the aquastat, and we were told it needs to be replaced. Shall I replace just the heater, or should I upgrade the otherwise-working-but-15-years-old boiler, too?

What it says on the tin, really. The boiler, a Burnham Series 2, with about 80% efficiency when it was made, and surely less efficient 15 years later, seems to be in good shape. The water heater runs dangerously hot—it’s set to 125 or so, and water comes out at 150. It turns out this is due to a leak in the aquastat port, which has damaged the sensor relay.

I’m told we need a new water heater. We’ve been given the option of 1) replacing the whole system with a high-efficiency tankless boiler ($14-16K, all in); 2) replacing the water heater (various tiers of add ons and guarantees, with the lowest coming in at $3K).

A new efficient boiler would be, say, 95% efficient—but we have a small house and small family, and only pay about $1650 in gas total per year. At the higher efficiency, we’d pay maybe $250 less per year. We also just installed a heat pump HVAC system; our electric is cheap, so our gas needs will probably be less next winter.

There are various state incentives to upgrade the system—no interest loans and rebates, but as I write this, there seems to be no way in which the more expensive boiler makes any sense.

Am I thinking about this right? More/better insulation will surely give me much more savings than an expensive, but efficient, boiler, no?

Assuming that’s right, if I get the water heater and the 15-year boiler goes, is that just a sunk cost, or can I still use it in a new, say, 2018 installed system (high efficiency or otherwise)?
posted by Admiral Haddock to Home & Garden (5 answers total)
Installing a high efficiency boiler could very well be a difficult project depending on where your boiler is, what existing venting looks like, and whether it is easily replaced to the necessary point. (A standard flue won't work because there isn't enough energy left in the exhaust gas)

A small house with a perfectly functional reasonably efficient boiler that is already reasonably affordable to run isn't one where a super efficient boiler will create the largest energy savings. So get it inspected and serviced to make sure it is indeed functioning as well as it can and spend the rest on insulation, draft reduction, and if you're really going to seal the house up, an energy recovery ventilator to avoid the air getting stale.

The calculus would be different if you were talking about replacing a standard gas fired tank style water heater, because their standby losses are annoyingly high. A new indirect hot water tank will be much better insulated than any self-heated tank (no need for a heating element and such means fewer penetrations and more space for insulation) so standby losses aren't as much of an issue for you.
posted by wierdo at 12:17 PM on August 18, 2017

Oh sorry, I didn't notice your last question, but yes, in general the tanks used with boilers are mostly interchangeable AFAIK. You should be able to find a boiler that will work with should your existing one fail in a major enough way to require replacement, which isn't terribly likely if it is well maintained.
posted by wierdo at 12:21 PM on August 18, 2017

A standalone tankless water heater might make sense. When you replace the boiler, the water heater won't be an issue.
posted by H21 at 12:47 PM on August 18, 2017

I don't want to say that they're a scam, but the rush to install high efficiency boilers is generally not the best investment of dollars - unless, of course, the entire system is failing.

My 95 year old home has a gas boiler that is going on 45 years. We've replaced the burners, the gas regulator, the low water cutoff. We did the math on an appropriately sized, high quality replacement and the $5,000-$7,000 investment simply wasn't worth it: we'd be better off replacing some of the original windows and doors and re-insulating the attic.

Tankless systems are really cool, but I'm not sold on them being a really good value yet. We just replaced our 15 year old 50 gallon gas water heater with a top of the line Rheeam unit (sans gimicky wifi sensor) with 12 year guarantee for just under $1,000.

In your situation, I'd suggest leaving the boiler and replacing the indirect water heater with a standalone tank unit.
posted by tgrundke at 1:01 PM on August 18, 2017

Since you have a heat pump, if you get a new standalone water heater (tank or tankless) you can turn off the boiler for all but the coldest months. This will save you tons of money. Decent standalone gas water heaters cost $1000 + labor. I got a Takagi tankless for that price, but I bought direct.
posted by flimflam at 2:20 PM on August 18, 2017

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