boiler VS on demand water heater

Joe_13

First off, a boiler will be expected to go through hundreds of thousands heat cycles more then a normal water heater does in its lifetime. A water heater will expect cold water at the inlet at about 50-60 degrees at around 10 to 12 lbs of pressure. Output temps should not go over 120 to 130 degrees since again it's DHW. This might sound fine for radiant jobs but not enough for BB, rads, hydro. The throughput of a tankless is probably 2 to 4 GPM. Can this flow handle multiple zones and circ pumps? If you're going to feed an indirect DHW tank, the recovery rates are all based on 180 degree boiler water. The better tankless heaters can modulate to 2 or 3 fixed firing rates. Most wall hung boilers have variable firing rates down to 20% of
rated capacity.

enough to be dangerous

boiler VS on demand water heater


What is the difference between a boiler and an on demand water heater? I can see that similar BTU sized units are physically the same size and have very similar specs but the boiler always costs 3-4 times what the on demand water heater costs. What is it that the boiler can do that the heater cannot?

My application is,
a boiler/heater for hot water and radiant floor heating.

Mike T., Swampeast MO

Spent quite a bit of time at Wetstock discussing that very issue.

Manufacturers could add controls/safeties to hot water heaters to make them safe for using in space heating, but such opens new liability issues for them and they wouldn't be approved for that use in some areas anyway... Mfgr rep at the table said some "boilers" were produced in this way in the past but no more.

Sales rep from Alberta, Canada was also at the table. When proper controls/safeties are added Alberta does allow them to be used a "boilers". He said they are quite common in small, lower-cost radiant jobs--particularly basement slabs.

On the bad side: Efficiency isn't very good; even a bit lower than simple standing iron "cubes". There are some super-efficiency models but reliability seems a concern. Service life probably won't even come close to simple iron boilers. Not approved in many locations and you'll certainly void any mfgr warranty. Some will be tempted to use them as combined domestic/space heating appliances-- some internet companies even consider this the PREFERRED method--DON'T DO IT--I'm not really an alarmist and certainly not an activist but such combined systems are just plain stupid.

On the good side: Low cost, even after necessary modifications. Some systems LIKE having a significant quantity of hot water to work with. Very easy to add extra insulation to the tank. High water volume and low efficiency likely reduce condensation problems when used at low temperature.