Heat pump H2O water heaters??

[Drake]

What is the amount of warm air needed to heat a tank of water? I know this can vary on temp of air available and size of tank but is it a lot? My heating season is pretty equal to non heating so heat put into H2O comes at the expense of heated space so providing that heat as cost effectively needs to be possible for there to any real advantage during that period. With a seasonal DHW demand lower(probably less that one tank/day) could the "heat" need for HPDHWH be provided by DIY solar heated air(from collectors and or surplus passive solar from living space) into zoned insulated full basement space(shop, utility, storage - non living) or with small wood/water backup stove i plan to have for hydronic radiant floor heat? My wife does not want give up "on demand" comfort but IS willing to adopt a more conserving and energy efficient life style in our new retirement home/lifestyle so that is where I will be applying my DIY experimenting. Any "input" info for these HP H2O htrs available(sale people at local stores know very little)?

[jeff5may]

To answer the simple question, a 40 gal water heater takes about 20kbtu to heat initially and 500 btu per gallon of flow, assuming a 60 degF rise.

For a turnkey, off the shelf unit, these units are very efficent. For a DIY conversion from electric resistive, these units are very efficient and very inexpensive. Being an air source device, the efficiency gain is highly dependent on ambient temp, essentialy a linear increase in COP with temperature. See xringer's thread on his A7 Airtap in the heat pump forum. He did a full install himself and has been tracking his usage and performance ever since.

Reading between the lines in your post, it seems as if you are still considering how to efficently provide multiple types of heat (zoned, hydronic, dhw,...) to fill your needs. Since you live in Minnesota, I assume you don't have much cooling demand versus heating, so you're considering a heating-only system. This greatly simplifies everything.

You have been posting for a while about a super-insulated house in the woods using earth tubes, is this that house? If so, it makes a huge difference. Just draw indoor air, exhaust the cool air outside, and let the earth tubes provide make-up air. This water heater would run independently of your zoned heating system and provide low-energy dhw. Paired with the earth tubes, total efficiency would be sky high.

Considering space heating along with water heating complicates things exponentially as the system becomes more useful. As you know, dhw uses hotter water than hydronic heating. So to keep things less expensive/complicated, you could preheat your incoming water with the hydronic heat store and trade electricity for wood or solar power. The warmer the hydronic store, the less work the heat pump would need to do.

[Drake]

Yes, that is my project. I guess I need to go get a good look at a HP DHWH and how they install. If they have an air intake/exhaust as I think you are saying that can be ducted that makes what I could do easier. I was thinking the HP DHWH for suppling the dependable low grade hot water to heat high mass floor and low grade DHW then using point source on-demand in the one full bathroom where wife will want hotter water. DW has internal htr. I would have a preheating tank before DHWH in which I can experiment with DIY heat sources and a backup if elec is interrupted. Maybe a graywater recovery tank before preheat tank because I like to tinker. I am just trying to determine if HP DHWH would out preform a standard elec DHWH enough to consider one.

[AC_Hacker]

Originally Posted by Drake

I am just trying to determine if HP DHWH would out preform a standard elec DHWH enough to consider one.

About 2.5:1

-AC

[jeff5may]

The limited btu capacity that HPWH's provide isn't enough for space heating. That duty would be better served by a waterstove and a heat store. Even with a well insulated house, space heating demands far surpass hot water demand. With your situation (essentially free renewable firewood), you could probably meet all your heating and hot water needs with only wood power (high grade heat source by nature).

It all just depends on whether the sweat equity is worth it for you or not. If it is, then your "preheat tank" becomes a long hose in the "heat store vat" that can gain heat from wood or solar sources. Burn enough wood to get the store close to DHW temp, then let solar add its heat to the store while you chop more wood. Let the HPWH rob from the hot store room at sky high efficiency if it needs to. Hang a clothesline in the store room and let that dryer collect some dust. Watch your electric and gas bills disappear for the price of chopping wood.

Sorry, I have a vivid imagination... to answer your question, heat pump water heaters generally run 250 to 400 percent as efficient as straight electric or gas. I believe xringer is using around 1 to 1.5 kwh per day to heat his water, saving like 91% versus a fuel oil burner in cash money. I believe it paid off for him the first month he sent the oil truck driver packing, then again maybe it was the second month.

[Drake]

In a well insulated/sealed home actual heat demand is not easy to pre-calculate because of how much lifestyle secondary heating can affect things. The home we currently live in I rebuilt with half the super insulation planned for the new one. It is a modest sized two story dollhouse with finished attic(very hard to insulate well and is cooler,winter/warmer,summer than first floor. But even this house has to have the window opened on zero degree days when cooking all day and adding 10-12 more people for a family meal( and the heat never comes on). But with now just two of us, gone most of the time the furnace runs more. In a super insulated space even the change from to incandescent lighting to LED may be significant. Go induction cooking, microwaving over oven and many of the other conservations that lower "waste" heat and most of the heating demand rules are in question.

As I really want hydro radiant floor heat and UL listed geo HP's are not yet readily available scaled to the low heat demand living space I see a HP DHWH as a workable alternative if I can supply the warm air it needs with out paying to heat it during heating season. I also don't want to go to the extreme of just wood boiler heating as many in my area are doing because it being "free" is relative in time or money. Being rural elec a small wood(free) or propane(getting very expensive) back up heater seems wise.

Pushing the envelope of different often means guessing your best(from the most info you can gather) and than adjusting/tweeking from there for DIYer's. We seldom have the budget for full scale engineering studies first. From this comes most real innovation(not planned profits).

[jeff5may]

I see where you are coming from better now. I am not an engineer, you may or not be.

While in college, I thought being an engineer would be cool until I actually started studying it. The thing that made me decide not to become one is the fact that engineers consider reality a huge experiment in action, there is no line. Kind of like law, the rules are always changing and nothing is ever completely finished. Success is not savored and failure is not punished unless something "unique" happens. It's just a part of the job.

I am and always have been a hands-on, do it and then figure it out later kind of person. More later....

[AC_Hacker]

Originally Posted by Drake

In a well insulated/sealed home actual heat demand is not easy to pre-calculate because of how much lifestyle secondary heating can affect things.

There is the Passive House Planning Package, which is a worksheet with built in formulas to pre-calculate.

For what it does, it is pretty cheap, but the learning curve is steep.

-AC

[Drake]

I have degrees in both the sciences and the fine arts - go figure. So I have left brain/right brain(can't ever remember which is logic, which creative) conflict all the time. Lots of intuitive leaps but glad to have some logic to help keep me hopefully leaping over the edge. I'm a hopeless modifier. My father raised me if what you got doesn't do what you want make it do it.

[jeff5may]

Okay, back from work. What put me off in becoming an engineer was the same thing you just said: engineers get paid to plan and revise things. There may or may not be an end product in this process. Their main purpose is to take something simple, complicate it severely, then try to simplify it again. Things rarely end up as they started. Just as lawyers find problems where none exist and debate them endlessly in court, engineers dictate revisions and change plans as they see fit.

You're right when you say we can't afford to do the same things as hobbyists or homeowners. Things such as comfort level, up-front cost, lifestyle, aesthetics, and value come into play. Unlike in industry, where the entire body of work may be revisited and scrutinized for who knows how long, we have very few people to please in the process, which simplifies the whole process. The cool thing is that you can plan it yourself, do it yourself, then test it yourself. When you have something put together and working, ask the wife how she likes it. When she complains, listen and fix it up. Paint it pink or whatever. Then it's done for now.

It seems to me that you have done quite a lot of homework and know what you want to accomplish, as well as having actually accomplished a great deal. The main thing to keep in mind here is that what you're planning here is only one of these and one of those. Keep it simple. Simple works.

Doing a quick search on craigslist in your area, I found the deal of the month:
RUUD 2 TON AC
If I lived there, I would have already put this unit in my yard. Add a reversing valve, a defrost control, and a BPHE wherever (indoors or out) and start tinkering. For under $300, you could have a working test bed.