Combined furnace and water heater...is this any good?

I first saw an article for heat pump hot water on the EPA's site a while ago. Then I went looking a system without much luck. Now I regularly see ads for them. At least for the complete system. There are add on heat pumps for existing water heaters, which may be a better option depending on the condition. With regular maintaince, an electric water heater should last for a long time.

That was a concern I had with them. With the additional parts, how reliable would it be long term. But ACs and heat pumps seem to have good track record on their own. I could just let the condensate just go into the sump, but it is something to consider. Drying out the basement was another plus for me.

 

The biggest issue for tankless in the colder locations (those that have cold winters - you don't really get that in Clearwater) is that the temperature rise that the units can accomodate isn't always enough for the user to get the hot water they need. The "cold water sandwich" is mostly from the fact that it can't provide sufficient flow and thus, someone gets colder water. Suppposedly, this is more of an issue with electric ones as opposed to gas ones.

The electrical part is the most complicated and expensive. 220 lines and double breakers are NOT cheap. When you have to run multiple lines, it's that much more expensive. Plumbing with the new "shark bite" or "gator bite" connectors is very simple. Supposedly a lifetime guarantee against failure, although, they can break due to frozen water lines (user failure, though, probably not covered).

 

Yes I can see that might be an issue in a cold winter. I guess the method to circumvent that would be proximity of the tankless to the user with very well insulated pipes. I can say that the shower farthest away from my tankless is slighly cooler based on the faucet adjustment level but its very steady. No all of a sudden dump of cold water.

 

The cold water dump would come if someone else requested hot water at the same time. In other words, more than one faucet is requesting hot water, but the tankless can't provide enough hot water to provide to both sites. That's when you get the cold water dump. I think about the only work-around, would be to have multiple tankless in series or to get a small tank set to around 80 degrees, then have that run into the tankless to provide the rest of the temperature rise. It would save some money, as you're not keeping 40-50 gals at 120+ all the time.

Speaking on insulated pipes, I keep telling myself I'm going to insulate my water pipes, but after 5 years in my house I still haven't gotten around to doing it. I've often been curious as to how much heat is lost to the pipes while it's flowing to the faucet.

 

A modern foam insulated water heater takes very little energy to maintain water temperature at 120F+

When we lived in San Jose California our hot water run was diagonally across the house. In what passed for winter there, we got noticeably better showers after we insulated the pipes. 70 feet of bare copper pipe in a well ventalated crawl space makes a good radiator.

 

Unfortunately, I can't get to all the copper pipe to insulate it. I should have started this when I had my laundry room (where the WH is) gutted and re-done. But, there's still the issue of insulating once those pipes go past those walls. Either way, even if only 1/2 of the pipe was insulated, that's still better than none!

 

if you can at least insulate the first 5 feet of outlet pipe and also the inlet. The inlet water will get heated a little bit by convection as the water nearest the tank dissipates it's heat to nearby water. If you insulate the first 5 feet or so, you'll keep more of that heat so the next time you dump water into the tank for heating, some of it will already be pre-heated. It's not a lot, but it's something and over time, it adds up.

 

Just for the record, heating anything with resistance electric heating elements is far and away one of the least efficient uses of energy out there. The numbers that I hear is that it requires ~10 BTUs of energy in the generation and transmission of the power to produce 1 BTU of heat at the heating element. So while electric heating and water heating can be made more efficient net/net by reducing heat loss from the building or the water system, it is always a poor use of energy. (Unless you have excess power to "burn" so to speak, from solar or wind dump loading).
Another simple efficiency is to use the proper tool for the job. For example a tea kettle is probably a better way to heat water for a pot of tea than is a 50 gallon tank. So a small, point of use "hot tank" may be the perfect alternative for a small demand draw.

The higher up front costs of heat pump recovery/solar/demand gas are a way better environmental investment for whole house solutions.

 

While electric resistance is certainly an inefficient use of thermal energy -- a heat pump is considerably better -- your figure is much worse than I've seen elsewhere. 3 to 5 units of thermal input are more common, or 2 with the very best thermal plants.

Mechanical sources, e.g. hydro, should be a bit better than the best thermal plants.

 

I confess I don't have a source for a specific thermal ef % except my own feeble memory. But one has to consider that as much as 30% of generated energy is lost in transmission, so you already start in the hole. The net/net thermal efficiency would of course depend on some know transmission distance.

That said, other energy sources have transmission energy costs as well, gas compressors and pump etc, L/P transport and delivery etc. All hard to calculate however.

 

I keep going around and around all these choices for my water and air heating requirements, but I think I am going to settle on solar DHW and windows.

KISS

 

I'm not sure where you get 30% - the DOE estimated it at 6.5% in 2007.

Electricity FAQs - Energy Information Administration

 

Somewhat tangential,

I was in Lowe's hardware today and saw a hot water tank that combined a heat exchanger and backup electricity. Cost was $1500 to buy, and per the energy label about $200 a year to run. Right next to it was an old-fashioned gas based heater that cost $150, and it also had an estimate of $200 a year to run based on I think 220 therms/year.

Money was no contest, but from an enviro standpoint:
CO2: 220 therms is about 6600 kwh of clean-burning fossil fuel. If electricity is 12 cents/kwh, the combo tank was using 1666 kwh from the socket, or about 5500 kwh at the plant.

 

Like I said before, I am drawing from an ever increasingly poor memory. Something I read in the last few days somewhere cited the "up to" 30% figure. The point is that resistance electric heating is not very thermally efficient, and line lose at X% is just one contributing factor.

Icarus

 

Looks like the best heat storage water heaters have an energy factor of 0.7 and use about 220 therms/year or 18 therms/mo.

The heat pump water heaters have an energy factor between 2-2.5, with the most common ones being about 2.3. These use about 1900 kWh/year or 160 kWh/mo.

My cost / kWh is about $0.14, my cost / therm is about $1.00. So the heat pump will cost about $266/year or $22/mo to run, while the gas heater will cost about $220/year or $18/mo.

Not too surprising given that the plant generating the electricity has an efficiency between 30-60%, so the total energy usage will probably be similar (and will vary depending on your utility mix).

So it does not make sense to use a heat pump water heater unless your electricity is very "green" and inexpensive.

Now, if you don't have gas and currently only have electricity for heating, a heat pump water heater will make a BIG difference - your old electric water heater will probably have an energy factor around 0.7 so you will probably reduce your water heating bill by at least 2x and probably 3x.

That means your regular electric water heater probably uses 4000-6000 kWh/year at a cost of $560-$840/year - in other words it will pay for itself in less than 3 years of use if your electricity costs $0.14/kWh. No brainer for those stuck with electricity for heating.

 

Huh? Electric resistance heating is very close to 100% efficient. Only line losses come into play.

If you're going to refer to the "up to" 30% line loss figure, please provide a reference.

 

Exactly. You will lose a very tiny amount to losses in the house wiring, but that should really be considered part of the transmission loss. The actual resistive element is 100% efficient at turning electricity into heat.

Tom

 

I'd hope that all the electric water heaters that lossy rusted out several decades ago.

By law, electric water heaters built since 1991 have energy factors of at least 0.88. The last time I replaced mine, four years ago, the locally available units ranged from 0.92 to 0.95.

Somebody needs to turn off the water faucet before leaving the house! That is more than half the entire energy consumption of my all-electric house.

Between lower usage, lower electric rates, better efficiency than you presumed, no improvement in unit life expectancy, and having the water heater situated inside the heating envelope of my house, the stand-alone heat pump water heaters don't yet make economic sense for me.

 

I think Icarus is trying to include the generator plant thermal efficiency, which changes the system efficiency very severely.

But since I don't have access to gas, and more than 90% of my electric energy comes from nonthermal sources, that analysis doesn't work for me.