I have a heat transfer question that relates to hot water tank wraps.
A tank wrap is intended to reduce heat loss through the tank insulation, but how much of that heat is waste heat? a fraction of losses will reduce heating demand in the home. If a tank is in a basement then you will have heat transfer into and through the walls and floor, but quite a bit will rise due to convection. Certainly during the cooling season all of that heat is waste heat, but during the heating season it would contribute to some degree to heating the space.
Just curious to hear others' opinions and whether or not this kind of thing has been quantified or studied.
Correct. If the WH is in conditioned space, the "waste heat" isn't really wasted during the heating season, at least. However, it may be heating a space that maybe doesn't need the heat (such as a closet or unfinished basement). Although the heat will eventually help out where you need it, you may be better off trying to reduce the heat loss there and adding the heat where you really need it.
It also depends on what type of WH you have. If electric, modern tanks are well insulated and really wouldn't see much benefit to a blanket. It depends on the electric rate, but typically these electric tanks might have $20-$30 per YEAR in standby losses, so you can see that it is hard to justify spending much to try to reduce that.
If it is a gas WH, a lot of your losses are up the flue as well as gas WHs typically have less insulation than the electric models. The blanket may help more on these units, but you are still losing a bunch up the flue.
Next, you have to think about your heating sources and the cost to run. If an electric WH, that is really just electric resistance heat. If the rest of your house had electric baseboards or an electric furnace, than the waste heat from the WH costs exactly what the space heating costs (BTU for BTU). If your heating was a heat pump or perhaps nat. gas with a high % furnace, then your space heating might be cheaper than the waste electric heat.
So, it depends on the situation. In most cases, if newer electric WH in conditioned space, then probably don't worry about it. If gas, maybe some benefit.
What you can do, that will help in any situation, is to insulate all of the hot water piping that you can get to. 3/8" foam stuff will help, but 1/2" or 3/4" wall material is better. Doing this helps reduce amount of water wasted (and energy wasted) during repeated hot water draws (don't have to wait around as much waiting for decently warm/hot water to arrive). Water will still cool off over long periods (such as overnight), but can really help keep the water temp up in the hot water lines during the day.
What nukeman said- especially about insulating the distribution plumbing and near-tank plumbing (even ~2 meters cold water plumbing closest to the tank, and the temperature/pressure valve & associated near-tank plumbing.) Tepid water in distribution plumbing that goes down the drain waiting for hot water to arrive is truly wasted heat. You could cut that tepid-water drain loss roughly in half with a drainwater heat recovery heat exchanger, which are subsidized in most of Canada, but not necessarily economic for low hot water use households in low utility rate areas. But 3/4" wall closed cell foam pipe insulation as a DIY retrofit is cost-effective in any home with daily hot water use. (It may take awhile to pay off in a weekend cabin that's used 20 days/year though.)
Unless your heater feels HOT when you put your hand against it, a blanket is going to have minimal benefits, AND some manufacturers recommend that you DO NOT use a blanket on their heaters, especially electric ones.
Right. It sounds like even if the rejected heat contributes to heating the home, it could:
a) Heat an unused space and be disspated through the building envelope. Really, this heat is never used in a useful way.
b) Heat the home at a time when it does not need to be heated. for example, when no one is home. Over the course of the day that heat can be lost through the building envelope as well.
Thanks for the tip on pipe insulation. That was also somethig I was curious about.
My electric water heater is the storage place for old winter jackets, blankets and t shirts wrapped around the pipe. Under the jackets its as hot as the water, and a good place to dry out a wet cell phone.
I do have a fire sprinkler over the heater, unlike most, however. Seems to me the top 1/3 of the tank is only worth insulating.
With sprinkler heads down to 5 bucks and pex at .33 cents a foot, it seems all utility rooms should have one.