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Replacing a BW Combi Core unit

Jubei Member Posts: 2
I moved into a 15 year old rambler last July and my luck the combi water heater just went out a couple of weeks ago. Since the furnace is still working and spring is around the corner, I decided to swap out the water heater with a unit dedicated to supplying water to the house. For my radiant floor I was originally looking at a gas boiler, but cost of running 30 ft of SS vent and lack of boiler knowledge is steering me back to a 50gal power vent water heater. The 50gal can be vented with PVC, which is a lot better on my finances.

So my questions are:

1) Would 50gal water tank be sufficient for my setup or would I need to size up or down the tank.

2) Or should I take a look back at the boiler which would be a twice the initial cost, but savings in the long run would over weigh it.

4) Would a light secondary pump on the return 1/25hp benefit the system?.

Old Setup:

Hydronic indirect heating

Basement slab heating around 1900 sqft with 3/4" tubing

Mainfloor 200 ft with 1/2" tubing

Bradford and White Combicor 75gal unit with mixing valve

Zone valves on the return

1/6hp Grundfos to circulate the whole system


  • SWEI
    SWEI Member Posts: 7,356
    begin at the beginning

    The first step is a heat loss calculation on your house.

    A modulating condensing boiler will also vent with PVC, will outlast several tank type heaters, and burn less gas.
  • Jubei
    Jubei Member Posts: 2
    Looks like I might need to do some measurements

    Thank SWEI for the suggestion, I'm going through an online calculator to figure out the heat loss which is taking a little bit more time to figure out. Would the heat loss be used in determining the tank size of the water heater?

    Sorry for my ignorance of boilers, but how much should I expect to spend on a modulating condensing boiler? I was looking at the Hydro Smart HS170, which would run $1000 for the unit and another $700 for SS piping.
  • SWEI
    SWEI Member Posts: 7,356
    heat loss calculation

    is purely for space heat.  There is another calculation for DHW demand - those are online from most of the water heater manufacturers.

    We don't discuss pricing here.  A mod/con will cost more than a tank, but less than the 3-4 tanks it will replace.  And that's just first cost.
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