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Combination water heater

DT437 | Posted in Mechanicals on

I have a Bradford White combination tank for domestic hot water and heating (air handler/fan coil). Looking to replace the tank, e.g., another Bradford White, Polaris, A.O. Smith Vertex. What’s best? Or should I consider tankless combi? It’s a two story townhouse in Southern Ontario (zone 6A?), approx 1100 sq ft. The tank is in a bedroom closet on the second floor, so safety is a priority.

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  1. Expert Member
    Akos | | #1

    If that is similar to the stacked townhouses around me, the heat load for those place is very small. There is very little money to be saved by going with a high efficiency unit.

    A tankless water heater would be a reasonable upgrade, but you would need to get a larger pump because of the higher pressure drop. The cost of the pump along with the install cost of the larger gas line, kills any cost savings.

    A tankless combi is a really bad idea. On most units the minimum fire is probably above your heat load which would cause a lot of cycling. Cycling on those is bad for durability and kills efficiency.

    Probably not the answer you were hoping for, but your best bet is to replace with a similar unit. Before replacing the unit, check that the tank is shot. The blower on these is a wear item and don't last much more than a couple of years. Blower replacement is cheaper than swapping out the whole tank.

    1. DT437 | | #2

      I’m told the gas line is good for a combi tankless too. What is the pump that you mentioned?

      The blower on my existing tank seems to be fine. The tank is about 10 years old.

      Any concerns with the tank being in a bedroom closet? That’s where it was built and there’s no place else to put it. Safety is a priority given its location.

  2. Expert Member
    Akos | | #3

    Your current setup has a small brass pump from pumping water from the tank to the coil in the air handler (sometimes the pump is inside the air handler but usually it is near the tank).

    A tankless water heater (not combi) has much higher pressure drop than a tank and depending on the flow rate your coil needs, you would have to installer a larger pump. You would also have to check that the minimum fire on the tankless is bellow the heat load of your place otherwise it will short cycle itself to death.

    A combi is a bad idea. To meet the hotwater needs, they need a big burner. Most are around 5:1 turndown, so even at the lowest fire the BTU output of the combi would be above the heat load of your place. Some combi have a 10:1 turndown but even those would most likely be above your heat load.

    If you have your gas usage history, you can figure out what the savings would be. Compare the gas usage form a summer bill to one in February. Your current setup is between 80% to 85% efficient for space heat. A properly set up high efficiency unit would be a bit above 90%. You are looking at a %10 gas savings. The cost savings would be the consumption delta x 10% x cost of gas including delivery.The ROI on the new unit is most likely a couple of decades.

    The location of the water heater doesn't matter. Once it is inside your house, it is inside your house. Most of these are sealed combustion appliances, there is very little chance of fumes leaking into your home. Make sure you have a functioning CO alarm in your bedroom.

    If you current tank works and is not leaking, than there is really no reason to even think about swapping it out.

  3. jberks | | #4

    Yes this is a good question. What is actually the issue? You haven't presented a problem.

    But if you really want to swap it out, a tank based Combi is your best bet, as Akos is saying. Which is what you already have.

    If you just want a brandname, I don't really think it matters much for tanked units (just my non professional advice). but I've been using HTP with no complaints so far.

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