why wont this work

Boston_2

I have a seriously oversized boiler used with a 3 zone baseboard hot water system. 133,000 btu. When I have been told that 50 to 60k will be fine. We now just got the whole house insulated. I have an extra 50k hot water tank. Can I use this as boiler in posse of the beast? I guess as a layman I am not clear on why a hot water tank wouldn't work.

SWEI

Short answer

While the water heater won't make a decent boiler, it could well make a decent buffer tank.  Depending on your fuel costs and seasonal heat load, a properly sized mod/con boiler might make sense.

jumper

what baseboard?

Doesn't it depend on kind of baseboard? Won't iron baseboard work with low temperature from a domestic water heater? Especially in a modernized house in balmy boston? I don't know but I've thought about using a water heater as a heating boiler.

Boston_2

Circulator

A hot water tank can get close to 150 degree's can't they?  On the lower side for the required temp, but still in the range correct? Also if i use delta T circulators with no set back on my t-stat would it be able to hold the temp?  Although the tank isn't an efficient as a boiler doesn't it have better insulation than a boiler. My concern was around if i could get enough flow out of the tank's 3/4 connection to supply 3 zones.  I guess i'm still not sure why more people haven't tried this since hot water tanks can be so cheap.

Ironman

Why

I believe that you've answered your own question. If it could be done properly, it obviously would be done. Why pay for a boiler that costs at least 4 times as much if a water heater would do?



IF, IF, you could get the water heater to produce 150* supply water, that means that the average water temp in the BB would be 140* (150 supply, 130* return). At 140*, the output of BB radiators is 1/2 of what it is at 180*.



That means if your BB's are sized correctly, you'll only have enough heat output to carry the load down to 40* outside. That's the temp at which your load is 1/2.



There are no short cuts to doing things right in this trade. If there were, men who are much smarter than us would have discovered them long before you and I discovered America.

jumper

depends on house

Suppose baseboards were sized for a leaky house with no insulation and then the house was modernized with tight triple glazed windows and ample insulation. How much is heat loss reduced? Commercial water heaters to produce 140° water are available. I figure 140° is half way to 210° for room temperature of 70°.



But you must derate the output of a water heater when used for heating. That's because the return water from heating is much warmer than fresh city water. So you probably need a water heater rated at about ten times the BTU of a heating boiler. It still makes economic sense if you can get away with it. Get your hands on a used commercial water heater that isn't leaking, clean out the scale, and it'll probably last forever. I'm guessing of course. I've never tried it.

Gordy

In the end it boils down to

Your oversized boiler being more efficient than a 50k WH, and being able to do the job of getting the baseboard the correct supply temps to heat the house.



An oversized boiler is not the best scenario, but it's better than an undersized WH that's going to operate at a lower efficiency with inadequate output.



Make sense.