The one for the custom built home may be larger, apart from size they are basically the same.
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The cost difference between a tankless water heater and a traditional tank water heater is typically higher upfront for a tankless water heater, but it can be more cost-effective in the long run due to energy savings and longer lifespan.
The difference between a mobile gas water heater and a standard gas water heater is that a MH unit will use an outside air intake for combustion. That is the most important difference.
You cannot, they are not manufactured with a block heater.
There really isn't except that a commercial model may have more failsafe features.
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"A holmes heater, generates its power source by being filled with oil; whereas, a regular furnace runs off of either a electricity or a gas power source."
The yearly cost of operating a gas tankless water heater is typically lower than that of an electric tankless water heater due to the lower cost of natural gas compared to electricity.
Electricity energy is converted into internal energy through work. As the internal energy of the heater increases so, too, does its temperature. Energy is then lost to the surroundings through heat transfer. The change in the heater's internal energy is the difference between the work done on the heater and the heat transfer away from it.
Radiant heaters heat objects nearby them such as furniture and are usually cheaper. Regular convection heaters actually heat the air temp in the room.
Cool touch heaters are made of ceramic and aree very safe, while mica radiators are hot to the touch but produce much more heat.
A combination boiler provide hot water to a central heating system whereas the multi-point water heater does not. The multi-point water heater provides hot water to multiple appliances or taps at the same time.