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

Just to make matters (maybe) clear. My original post was citing the net/net thermal efficiency of resistance electric heating elements from generation through end use. I believe I cited a number of ~ 1/10. (A number that I only am citing from very old memory, so, as I said, don't take it to the bank. In subsequent posts we got to talking about line loss, where I cited ~30% as a number, again with out any citation except my memory.

So back to my original statement,, resistance electric heat is a very inefficient way to heat anything, and gets less efficient as line loses increase. So feel free to post relevant information regarding both the basic thermal efficiency as well as estimated line loses, which cumulatively add up to the total efficiency.

Icarus

 

From the figures drees pointed out, average line and distribution efficiency should be in the 90-something-% range. That isn't a big deal when compared to the thermal plants -- 60% efficient at the very best combined cycle natural gas plants, and 30-something-% range common older plants.

It is the really the thermal generator plants, not the line losses, that cause the terrible net efficiencies of electric resistance heat.

 

Thanks - I didn't know what the energy factor for electric water heaters was these days - so that makes your heat pump water heater 2-2.3x more efficient. A big difference, but heat pumps should be able to get to 3x better without too much trouble.

Those numbers were extrapolated from the annual kWh usage from the heat pump water pumps I looked at on energystar.gov which estimate ~1900 kWh/year for a heat pump water heater with an energy factor of 2.3. So an electric water heater with an energy factor of 0.95 will use 2.4 times more electricity, or 4600 kWh/year.

Those numbers seem extremely high to me - when living in a condo with electric water heating our total electricity bill was about 450 kWh/mo with 3 adults living there. In my house with average gas water heater is about 7 therms/mo - which is the equivalent of about 200 kWh. I would have estimated the electric water heater power consumption at around 100-150 kWh/mo or $21/mo. Assuming the heat pump water heater costs about $750 more than a resistance water heater, the "break even" point is out at 6 years. Perhaps worth it if the warranty is that long but it seemed that the heat pump water heaters had short warranties.

Yep. From an energy efficiency point of view, it appears that a heat pump water heater at least is on par with the energy efficiency of a gas water heater when including the inefficiencies of your typical thermal generation plant. Over time, it should get better (at least until cogeneration mini plants are available for your home!).

 

When my condo had the combined water heater/heater unit, I had the temperature almost at MAX to accomodate myself, my gf, and her daughter (the little one was there about 50% of the time) with the heating requirements. I also used a programmable thermostat, and had to adjust the temp bump-up time to be earlier so the water heater had time to recover enough for my shower. Otherwise, I'd end up with a luke-warm shower at that point.

I think in the end, you probably made a good decision. I'm still debating the heat-pump water heater unit. I put it off to pay down some other debts out there first, though. I'll miss out on the 30% tax credit, but with some luck, the price will come down some over the next year or so.

I know that when I went from undersized tankless back to regular electric WH, my electric bill jumped about $20/month. I'm hoping to insulate the water pipes as much as I can soon, and bump the water temp back down a little (I bumped up from 120ish to 130ish). I feel like I lose a bit of heat due to the pipes being copper and uninsulated. I'll have to take a look at the water heater you chose. I'm always up for more conservation that doesn't affect my everyday routine (I know, I could do better, but every little bit counts).

 

Whether or not a heat-pump is more efficient than gas depends on how efficient the generation of your electricity is.

An air source heat pump will typically run a [ame="http://en.wikipedia.org/wiki/Coefficient_of_performance"]COP[/ame] between 2-5 depending on outside temperature when heating. This means for every kWh you expend, you move 2-5 kWh of heat inside.

Since your typical fossil fuel burning plant is generally 30-60% efficient (depending on whether it's an old coal plant or a new gas cogen plant) in terms of pure energy efficiency, a heat-pump will typically at least match a 95% efficient gas heater and often beat it by a significant amount.

And of course, if you get your electricity from something other than fossil fuels (like your PV panels), you're even better off.

Of course, if you're looking at it from a pure cost standpoint, it depends on your cost of electricity, cost of gas and your heat-pumps COP.

 

Me too. Actually, I was referring to both - the concept is the same either way.

The problem with ground source heat pumps is that it costs a FORTUNE to install on an existing house because of the amount of digging it takes to install the under ground heat exchange pipes.

And I've read that very often the ground source heat pump ends up being less efficient than a regular air source heat pump because of fluid pumping losses (you have to run a pump to pump all that coolant through the ground) and inadequate ground surface area so the ground actually starts warming/cooling enough to affect the COP significantly.

So IMO, you're better off with a regular air heat pump which is only nominally more expensive than your typical A/C unit (since a heat pump is just a regular A/C unit with a few extra valves so that you can run it in reverse) since the performance of these are very consistent.

IMO, it's best to combine it in a dual-fuel system so you can choose whether to heat with electricity or gas depending on utility costs - I expect gas prices to fluctuate significantly in the future.

 

Hi. I realize I'm bumping this thread from half a year ago but this thread had some helpful info I was looking for so I figured I'd add to this one instead of starting a new one.

My ~15 year-old 50 gallon electric water heater died a few days ago so I'm in the market for a new one now. Thankfully, it's summertime so cold showers are not killing me. I'm not looking to reroute gas pipes or completely retrofit my basement with some geothermal system so I'm just looking to replace the broken unit with another electric one.

I was looking at the websites of the major home appliance stores (Home Depot/Lowe's/Sears) and I saw a GE 50 gallon Electric Heat Pump Water Heater; I believe it's a "hybrid" heat pump + conventional electric heating. It's on sale for $1400. NY state tax will add ~$100 but I believe I can also get a $300 nonrefundable tax credit (IRS Form 5695) so net $1200. A conventional electric water heater would cost ~$300-$400 so I guess I would be paying a premium of ~$800 to lower my future electric bills and lower my energy footprint. Do you all think it's worth it? My broken unit's EnergyGuide sticker says $421/year vs the GE sticker's $198/year.

My main concern is that it will be in the basement where it's not exactly optimal operating temperature for the heat pump so I may not get the $198/year. Does anyone have any research/data on if the heat pump will operate efficiently in the Northeast? I guess my basement is at about the same temperature as the ground (similar to a geothermal unit?).

I assume delivery+installation will cost another $200? Please correct me if I'm wrong. I can try to ask a friend with a pickup truck to help move it and then ask an "amateur" electrician/plumber friend to install it.

I appreciate any input you may have. Thanks.

 

Well not the worst thread necromancy

The convenience of instant water is well worth the extra cost. Saving water, heating energy, and recouping your costs are just the icing on the cake. The next time you have relatives over and there are 8 showers in 30 minutes using all available bathrooms everyone will be amazed that nobody gets cold water.

Just thought I would add that I have an instant water heater that not only provides hot water to the taps, but also heats the entire house with radiant water heating. Pipes with hot water and little fins on the baseboards. Extremely efficient. $60-$100 a month to heat the house in the dead of winter and provide all showers. This month my gas bill was $29.11. The only gas is used by the water heater as I have an electric range (unfortunately). That is to heat a 3000+ sqft house, and we have had quite a few nights below freezing at night this month.