(rough thoughts for now, will polish later)

There are many articles on the subject of on demand hot water heaters vs. the more common (in the USA) hot water tank, but this one is tailored to new construction of an earth sheltered home.  I personally plan to install some on demand electric hot water heaters because they suit my situation.  Your situation may require a different choice.

Check your sources.  Most of the sites that talk about how great on demand hot water heaters are are selling them.  Most of the sites that talk about how bad on-demand hot water heaters are, especially electric ones, are “electric cooperatives” who would rather sell you more electricity over a longer period of time.  If you focus on articles written by consumers groups, they tend to be more neutral and articles written by end users tend to be very pro tank-less.

Also check the logic behind the complaints.  Often they may say something general like, “On demand hot water heating costs so much that it will fall apart due to old age before it pays its self off.”  If you check the actual numbers you may find that they spent thousands on a very expensive gas unit which had to be professionally installed in existing construction.  In your situation, you may pay off your investment much more rapidly.

Retrofit vs New Construction?

If you are in an existing home with fixed wiring and you are happy with the performance of your perfectly good hot water tank (gas or electric), just keep it.  Energy is energy and the heat capacity of water is the same in both cases.  While there are probably some inefficiencies with the way the energy is imparted to the water, the biggest efficiency difference between a tank or tankless water heater is how much of that energy is lost thru the walls of the tank into the home.  Modern methods of insulation make the tanks pretty good at holding on to their heat.

If your hot water heater simply lacks capacity, think about how old it is.  Hot water heaters only last 10 or 15 years so you probably don’t need to feel too guilty about replacing it with a larger capacity hot water heater.

However, if you are building a new home and can design your electrical service and plumbing with tank-less in mind, I really think it is the way to go.

Gas vs Electric

For well-sealed underground homes (or well sealed above ground homes), you should probably try to avoid combustion.  Electric is much cleaner inside your home even if it is generated with natural gas some where else, and you won’t need a chimney.  Of course, electricity can also be generated with clean technologies and you can even create it yourself.  It is a lot harder to make your own natural gas.

Many argue that natural gas heat is cheaper than electric.  This is only because the cost of natural gas is cheaper per btu than the cost of electricity, at the moment.  Electricity is actually much more efficient at putting its energy into heating the water.  Natural gas is a non-renewable resource and the cost is expected to rise (in the United States) while the technology for generating electricity is getting cleaner and cheaper every year so the cost for electricity is expected to fall.

The cost of a natural gas on demand hot water heater is many times higher than for an electrical hot water heater, both in terms of installation and the equipment its self.  This initial cost leads to long or infinite pay back times.  Electrical equipment is much cheaper, and if your electrical service has watts to spare, the install is also relatively cheap and easy and within the realm of “do-it-yourself.”

Some people claim that natural gas has the capacity advantage, in that it can heat more gallons per minute.  But in reality, it is just that they have a larger natural gas capacity.  If you have a lower gas pressure or if you have the capacity (120 amps) for a larger electric on-demand hot water heater, things could be reversed.

Electric on demand hot water heaters win hands down in terms of temperature control and low flow rates.  People who are complaining about a lack of control or needing flow rates of 1/2 to 1 gpm are probably using natural gas on demand hot water heaters.

The Big Catch

I checked many electric on demand hot water heater sites and none of them mentioned the big catch… Powering the thing.

An electrical water heater tank uses electricity much slower, so you only need about 30 or 40 amps.  It just runs for much longer.  This means you can get by with a pretty typical electrical service and certainly save money in the short term.

Most on-demand hot water heater companies have a high end model that can produce hot water for 2 or 3 showers (in the northern states), but it takes about 120 amps.  Adding this to your your home will usually require stepping up to a larger electric service, unless you like your lights flickering when you run your hot water.

Of course, upgrading an electrical service is very expensive.   But building with a larger electrical service in mind can be done more affordably…  The equipment for a 400 amp service is a few hundred dollars more than for a 200 amp service, but the hook up (where most of the money is) is only 15% more.  You will also pay more for wire, etc.  It is expensive, about $1500 more in my case, but considering the heater its self costs less than $500, the total is not bad.  Certainly, it is cheaper than running a new construction or retrofit natural gas line.

If you have a home with 3 showers plus a kitchen and laundry and maybe even radiant floor heat, you may actually want to install two of these.  You could possibly locate them close to the points of use, which is great, but you would need 240 amps to run them all.  If your house has an electric stove (50 amps) and an electric dryer (20 amps) and a single lighting circuit (15 or 20 amps), you are already past the 320 amp capacity (80%) of a 400 amp service.  Now you are talking 600 AMP service.  The jump from 400 amps to 600 amps is significant costs thousands of additional dollars.

The solution is pretty simple…  You just need to talk to your electricity provider about adding a separate 200 amp service for your hotwater/hvac needs.  This will cost far less than going with a larger service and you will be able to separately track your hot water costs.  In some areas, you can actually get special low rates for a dedicated meter.

Solar Hot water?

I discuss solar hot water more over here.   Here I will just say that you need a tank to store the solar hot water, but you definitely don’t want to use the heating element in the tank.  Get a tank without the heating element and save some money if you can.  If you heat the water in the tank, it will be less able to collect heat from the sun.  Ideally all the heat in the tank should all come from the solar.  The output from the tank should then pass thru an on-demand electric hot water heater to add just the right amount of extra energy needed to bring the temperature up to what is desired.

See, we can all get along.  Tanks and tankless, electric and solar, together in perfect harmony…  Now if only I could afford it.

Heat pump hot water heaters?

You can get heat pump hot water heaters.  Basically, these are a small heat pump mounted to the top of a hot water tank.  They are the most expensive of the hot water tanks by a factor of 3 or 4.

They are considered very efficient if you just look at how much electricity you put in and how much hot water you get.  But if you zoom out and look at the bigger picture, you see that they have to get that extra heat from somewhere.

These devices work by “pumping” the heat out of the room and dumping it into your hot water tank. In summer, this is great because they actually help cool your home (like a small air conditioner).  As a bonus, the surface of the exchanger gets very cold and dehumidifies the air around it, which is usually helpful in an earth sheltered home.  However, in winter, these hot water heaters are stealing heat from the home which simply needs to be replaced by the homes heating system.  The energy use just moves upstream from the hot water heater to the main house heater.

One can imagine a humorous situation where a heat pump hot water heater would be used to heat hot water for a radiant floor home heating system…  It would steal heat from its self.

A slight twist on electrical resistance heating

Most electrical resistance water heating runs electricity through a resistance element which heats to over 1000 degrees.  The heat energy is then conducted to the water.  In the process, minerals from the water may be transferred to the heating elements (scale), reducing its efficiency and ultimately causing the heating element to fail.  The thermal expansion cycles of the heating element also contribute to its failure.

There is new technology on the market that uses graphite electrodes to conduct the electricity through the water.  The water is the resistor and the electrodes never need to heat up more than the water.  This saves energy and wear and tear and should be cheaper to produce (even if it isn’t quite yet).  It also operates at lower flow rates than other on demand hot water heaters.

Note: For more information, check out http://myheatworks.com/

I have two boys and I liked the idea of installing a urinal in their bathroom to save water.  I especially liked the idea of a flush-less urinal such as the H2Zero or the Kohler Steward.  I am not talking about a bucket of sawdust, I am talking about a space-age ceramic urinal that is so slippery you don’t need to flush and includes some sort of clever one way valve or fluid trap so there is no smell.  I talked more about the conspiracy against these flushless urinals in this post.  These modern wonders are now standard in public institutions, airports, etc.  The US Army Corp of Engineers has specified their exclusive use in all new military installations based on their ability to save water/money…

A variety of flush-less urinals from different vendors… Notice the lack of a flush handle ;^)

So, I did some cost analysis of my own.

I went to the Grainger website and found a 1 hp submersible well pump that would be similar to what  I would need for the new house (no city water or sewer).  This particular one says it uses 230 Volts and 9.8 Amps…  Multiply those together and you get 2.254 kWs of power required.  It says it can pump 20 gallons per minute at 100 ft depth, multiplied by 60 minutes/hr gives 1200 Gal/hr with 2.254 kWH of energy.  Divide the energy by gallons and you get 0.001878 kw/gallon, or lets multiply it by 1000 to get 1.878 kW/kGal.

Next, I needed to find my cost of power.  If I look at a power bill it adds a whole bunch of charges including nearly 9 cents per kWH for the energy and 5 cents per kWH for the “delivery charge”.  There were a bunch of other taxes and “charges” but since they are fixed costs and not variable with energy use, I am leaving those out and going with $.14 /kWH.

One quarter for 1000 gallons of water!

So then you just need to divide the $/kW by the kW/kGal to get 0.26 $/kGal.   That is just over a shiny quarter dollar per  thousand gallons…  I can already guess that it is going to take a lot of flushes to justify the urinal ;).

Then I look into how much my current toilet uses… My toilet was installed in 1992 when they thought that 3.4 gallons per flush was a good amount (sometimes, I need to flush twice, so maybe that was conservative if you know what I mean).  I have installed a low flush valve with a one-two option, but lets say my baseline is the 3.4 gpf standard.   If I imagine that each of my boys would use the urinal four times a day, and I toss in a couple of trips for myself, I get to a nice round 10 flushes a day or the less round 3650 flushes per year.  Since the alternative is the regular toilet, I could say that the flushless urinal would save me 3.4 gpf * 3650 flushes/year = 12410 gallons per year.  At 0.26 $/kGal multiplied by 12.410 kGal/year, we get 3.23$/year.

Quick table showing various options…  I assume we already have a toilet and the urinal is the additional cost we are considering.

If I figure that even a cheap urinal costs almost 300$, and then you still need to pay to have it installed, we would be talking a pay back of over 100 years.  It is easy to imagine that an army base or football stadium with hundreds or thousands of flushes per day could easily reduce the payback time to just a few years or months, but this is using well water and my own septic field…

So what if I got water from the city?  Well, I currently pay a water and sewer bill and see that it shows a charge of $3.64 per 1000 gallons for the water and then $4.83 per 1000 gallons “sewer rate”.  This adds to more than 32 times more than the cost of well water, and would significantly reduce my payback time to just 3 or 4 years.  So even if the urinal math doesn’t work out for me, maybe it will for you?

While we are on the subject of residential urinals, I am reminded of a clip I saw on TV years ago where Penn Jillett (of Penn and Teller) was giving a tour of his home and office.   I couldn’t find the clip on line, but I recall at one moment he pointed to the urinal in his home/office bathroom and said “I know what you are thinking, who needs a urinal AND a sink”.

The back of the Michigan quarter features the Great Lakes, the largest collection of surface water on the planet.

So, out on my rural property, with my low cost well water, it is hard to justify a urinal financially.  But what about for environmental reasons?What about saving water?  Well (no pun intended), I live in the great lakes basin along with a huge portion of the worlds fresh water.  It is even shown on the back of that quarter we pay for the 1000 gallons.  I have a 1 acre pond on my property and there are many small ponds, lakes and marches near by, so I can’t really say that water conservation is much of a concern.  After the water gets used, it is sent down to my own septic field where it returns to the local environment as it has many times before.  If I lived in the dry south eastern USA, I could probably justify it based on water conservation, but not in Michigan.

I came up with a weak argument that it would still work if the power to well pump were out, but it would only help the men in my family and we could just go in the woods if times got tough. ;^)

So, will I still get the urinal?  I am still working on it, stay tuned.

Overview

Solar hot water is widely considered to be the best “bang for buck” in solar.  Instead of trying to get photons to push electrons around, they just need to heat something up, which is something solar is very good (efficient) at.  Solar hot water can even be used for radiant heating.  While it works best on sunny days, it can provide a lot of hot water, even on cloudy days.

This page will look at the pros and cons of various solar hot water configurations, particularly as I consider them for heating the floors in my own earth sheltered home.

Passive Solar Hot Water systems generally have the tank at the top of the panel.  Gravity feeds the liquid into the solar collector where energy from the sun is added, causing the liquid to heat up.  In some cases, there is phase change to a gas which heads back up to the tank.  In other designs, the increased energy reduces density (increases buoyancy) and the warmer fluid “floats” back up to the tank.   It is a nice idea, but doesn’t tend to work very well, particularly in very cold areas like mine where it may not be a good idea to have your hot water tank exposed on your roof in winter.  My neighbors probably wouldn’t like the industrial look of it much either.

Passive systems usually rely on the fact that the density of water reduces with increased temperature.  One cubic ft of water weighs 62.41 lbs at 32°F, but reduces to only 59.82lbs at 212°F.  This is the principle of thermal circulation used in boiler designs for a very long time.

Active systems keep the hot water tank safely concealed in the home, but requires water pressure, typically provided by a pump, to actively push the water up to the roof and thru the solar collector.  If it is a closed system, the weight of the water coming down again counter balances (siphons) the water going up and the pump only needs to be big enough to overcome frictional and dynamic losses.

Basic Components

There are several basic components which can be assembled in various different ways.  If purchased as set, the components can easily add up to between $5,000 and  $7500 or more.  When I mentioned radiant heat to one company, the quote jumped up to over $12,000 when they more than doubled the collector area.   Of course, installation cost is extra, but much of it can be done yourself.  Many studies show that these systems will pay for themselves very quickly.  They can also be leased or financed directly from the system retailers.   It is also possible to manufacture many of these components or systems yourself, if you have the time and the know how.

Solar Collector

The collector panel goes up in the sunshine and actually collects the energy transfers it to some liquid.   There are flat panel collectors and evacuated tube collectors.

Flat panel collectors are pretty basic technology.  They usually involve a series of conductive tubes (such as copper pipe) thermally connected to a collection plate (such as copper fins) and painted a dark color.  Insulation is applied to the back and Glass is added to the front.  Fluid picks up solar heat as it is pumped thru the collector.  These can be purchased (roughly 20% less per BTU than evacuated tubes ) or even built as a “do it yourself” project.  However, they are not as efficient, particularly in winter when heat losses thru the panel can meet or exceed solar collection.  In warm climates, these can overheat.  When the glycol overheats, it breaks down (chemically) into an acid which damages the pipes and pump, but there are a variety of ways to avoid this problem, including dumping the heat in the earth, draining the glycol when it gets too hot (or cold) or simply shading the collectors.

Evacuated tube collectors are much more advanced and definitely not a “do it yourself” project.  Their solar collection in done across a row of double wall evacuated glass tubes.  Each tube is a separate collector and can be replaced separately if damaged.  The glass tubes let light enter the inner tube and heat up the collector, but the vacuum (between the inner and outer tube) isolates the inner tube, preventing conduction and convection and significantly reducing heat loss (at least from the collector tubes themselves).  The ability to collect and retain heat, even in very cold weather, improves the efficiency of these collectors and enables their use in northern climates (for temperatures down to -60°F).  The tubes themselves are “passive” solar and usually have a phase change liquid that literally boils in sunlight and transfers the energy to the antifreeze fluid passing thru the top of the unit.   If these units overheat, the steam can no longer condense and the cycle just stops without damage to the unit.  There are gaps between the tubes that can allow snow and wind to pass thru the collector.   The price is fairly reasonable at less than 1000$ for a typical installation of about 75 sqft.  

Of course, orientation of the solar collector is important, as with any solar installation and follows the same basic rules.

Hot Water Tank

The collectors only gather energy while the sun is shining.  Assuming you will want hot water after dark, you will need a way to store that energy and most solar hot water systems use a hot water tank.  Because the heat generation period is typically only a small fraction of the day, these tanks must be larger than you would find for a gas or electric hot water heater.  Most systems have a hot water tank that is at least double the size of a gas or electric hot-water heater.   These tanks can be hooked up a variety of ways (see the systems below), including with external or internal heat exchangers, with additional heating coils, etc.  these tanks are not cheap and can easily run you $2500 for a 120 gallon tank.  Extra heating elements, etc. are extra.

Some tanks have more than one heat exchanger.   The bottom one is intended for the solar heat transfer fluid, the other is for some auxiliary heat source (such as a boiler).  I recommend you use both for solar and rig up an auxiliary heat source down stream of the hot water tank.

Heat Exchanger

In freezing cold weather climates, running an “open” system where the homes water passes directly thru the solar collectors is just not a good idea.  Instead some heat transfer fluid, such as a water-glycol antifreeze mixture is used instead.  In that dual fluid setup, a heat exchanger is needed to transfer heat from the heat transfer fluid to the household water supply.  Most hot-water tanks include an internal heat exchanger, but some layouts include a separate heat exchanger between the solar panel and the hot water tank.  The commercial grade ones look like a small box, maybe 6″ x 6″ x 30″, mounted on the wall.  Pipes from the solar collector bring hot glycol thru one side of the exchanger to exchange heat with cool water from the bottom of the hot water tank.  Heat is exchanged, but the fluids are kept separate.

I suppose the advantage is that you never risk glycol entering your hot water tank.  You may also be able to save some money by purchasing a cheaper tank without an integrated heat exchanger.  However, you still need to purchase the separate heat exchanger and the plumbing will be more complicated, so it is hard to imagine that you get very far ahead.  There is also still a risk of cross contamination within the heat exchanger.

It is reasonable to assume that a separate heat exchanger probably loses more heat to the environment that an internal heat exchanger within a well insulated hot water tank.  These losses would happen along the extra tubing and along the surface of the exchanger its self.  The external heat exchanger also requires a number of extra connections and an additional pump for the separate loop.

As you can probably tell, I don’t recommend an external heat exchanger…  but I am interested in hearing more “pros” or “cons” if anyone wants to comment on this post.

Pump

An open system, where the actual supply water is lifted and pushed thru the solar collector, may not need a separate pump since the water supply is already under pressure.  However, that supply pressure may not be sufficient to lift the water and either a larger well pump or additional auxiliary pump may be needed.

Cold weather designs typically push an antifreeze liquid thru the collector.  This requires a separate pump for the separate loop.  However, since these loops are closed, the weight of the column of liquid falling back to the pump counter balances the weight of the liquid being lifted and the pump only needs to overcome frictional and dynamic losses.

If your system has an external heat exchanger between the antifreeze loop and the hot water tank, an extra pump may be needed for that loop.

(More on this to come)

Backup Heat

You can’t always count on solar, particularly in areas with cold and cloudy winters, so there are a variety of methods that can be used to provide heat to the system when solar isn’t enough.  These include wood, oil or gas boilers, electrical resistance heaters, heat exchangers, etc.  Some of these can be built right into the hot water tank.  On demand hot water heaters are also a popular component.

As mentioned in other sections of this earth sheltered site, I am concerned about burning natural gas or oil in an underground home, and wood heat just seems like too much work, so I will focus on electrical systems.

Some solar storage tanks have more than one heat exchanger.  The solar supply heat exchanger is usually at the bottom of the tank where the coolest water is.  Higher up in the tank there may be an electronic heating element and or a second heat exchanger (auxiliary supply).  Some layouts suggest hooking up a boiler to this second exchanger as a backup heat source.  Some layouts even show a closed loop hookup with an “On demand” instant hot water heater.  Many of these show natural gas boilers or instant hot water heaters, but electric on demand heaters are closing the efficiency gap and provide a lot of convenience.

Personally, I can’t imagine using an instant hot water heater to heat up my solar hot water tank.  As you can see in just about any thermodynamic transfer equation, ΔT (the temperature difference) is a key driver.  If you pay to heat up your hot water tank over night, then it will be less able to harness solar energy in the morning when the sun comes up.  The two heat exchangers are monitored by separate temperature sensors.  Proponents of this secondary heat source say that because the auxiliary sensor is higher in the tank, it will reach the required temperature and turn off while the temperature at the bottom of the tank is still cool enough to gather heat from the solar collectors.

I see the heat sources as competing rather than cooperating.  As water enters the tank, the water in the tank is mixed and the thermal stratification is disrupted.  I would not expect buoyancy to be able to maintain more than a few degrees difference between the high and low sensors.  If your tank has a second heat exchanger coil, I suggest you hook it up to the solar supply also (in series or parallel).  This will just give more surface for heat exchange and may improve your efficiency.

For the same reason, I don’t suggest using a electrical resistance heating element built into your solar storage tank.  Try to get a lower cost tank without the built in heating element, but if you tank comes with one, don’t wire it it up.  Use an on demand “instant” hot water heater instead.  Plumb it so the water comes out of the hot water tank and then passes thru the instant hot water heater only when you need it.  If it is late on a sunny day and the water in the tank is hot, the “instant” hot water heater can check the incoming temperature and is programmed to add little or no heat.  However, if it is a cold winter morning and the tank has cooled off over night, the instant hot water heater can sense this also and add heat as needed, and only when needed.

Controls, piping, etc.

And a bunch of other stuff that I will cover

(More on this to come)

Basic Configuration

Searching the web, I found many different ways to assemble the components of a solar hot water system.  With even basic thermodynamics knowledge, it is clear that some configurations are better than others.  I can only speculate that the poor configurations are primarily beneficial to the individuals who are selling them, but you can decide for yourself.

(More on this to come)

Interesting Earth Sheltered Ideas

Wind and solar energy are not “on demand” as we would like.  Sometimes they are not there when you need them, and some times they are over producing and the equipment can actually be damaged if you don’t draw off enough energy.   In an ideal world, you could store that energy for when you need it later.  This is the primary reason for the extra large and well insulated hot water tanks.  But some times, even that is not enough.  If collectors overheat, the glycol can become acidic and damage the system.  Many collectors are able to shut themselves down to prevent damage, but then you are just not collecting on your investment.

On idea that appeals to earth sheltered home owners is to sink some of that excess heat into the ground.  Windmills or waterwheels can dump excess electricity into buried electrical resistance heater coils.  Solar hot water can dump it in a similar way via a heat exchanger buried in the earth.

My particular design includes a small green house.  In the winter, it will likely loose more heat thru the glass than enters and may not support plant life as well as I would like.  However, If I ran some hot water pipe from my solar tank, by-passing the instant hot water heater, and down 6 to 10 ft under my green house floor, I could use it to warm a large volume of earth.  On very sunny days when the tank temperature exceeded some specified level, the valve would open automatically.  A small auxilary pump would push the free hot water down under my green house where it would give up much of its heat and then re-enter the tank via the cold water inlet.  That heat should slowly conduct up thru the earth and hopefully keep that green house floor warm during the winter.    Optionally, I could also run a second branch at a shallower depth with a shunt so I could dump heat at that shallower level as winter approached.