So, it's the dead of winter, and the power goes out.

[polyanarch]

My take (which isn't Joe's as I don't speak for him) is that they are a promising technology.  But being an early adopter of some technology isn't always the best Idea.  The jury is still out on thier longevity.  Test show that under certain contidions they are more efficient because you are not losing heat while a tank of hot water cools.

Cost/benifit analysis will have to show that over the long run the cost of the unit isn't going to be prohibitive especially if the lifetime of the unit is shorter than hoped.

Don't go electric either.  Electricity is expensive -evenmore in the NE than in other parts of the country and the requirements on the electrical service often mandate an update to that which will cost you even MORE money.

If you have a generator then that probably not a good idea either as the peak loads are going to be too much for it.  An electric tank-type water heater will work with a big generator (although wouldn't you rather have a gas heater and not have to run a generator to heat water if you lost grid power?)

[MaineShark]

Hey Joe, what's your take on the tankless water heaters?

The quality units (eg, Rinnai Continuum, etc.) work well in many applications.  But they are not the same as a standard water heater, which causes some people issues.  For one, some people will demand more from the unit than it can actually handle (eg, filling an 80-gallon whirlpool tub).

More importantly, though, is the way that the unit measures and adjusts for demand:

Let's say you turn on the hot faucet in your sink.  The unit has a flow sensor, so it knows someone is drawing water, but it doesn't know where or how much.  So, it cranks up to full-power for ten seconds or so, sending some pretty hot water down the pipe, to make sure that you get hot water quickly.  At the same time, it checks the differential temperature between its inlet and outlet, which tells it how much water your are drawing.  If it's getting 55 degree water in, and the full-power flame is able to heat it up to 140 in the short time that the water is in the heat exchanger, it knows that you aren't pulling much water, so it will then crank down to a much lower setting.  If, on the other hand, the outlet temperature is only 120, it knows that it is just able to keep up, and should stay running at full power.

The problem arises because few installers bother to describe this to the customer.  So, Happy Harry Homeowner turns on his faucet, gets scalding-hot water, so he turns it down, but the water heater just turned down at the same time, so now he has too-cool water, so he turns it up.  Et cetera.  You have to give the unit its ten seconds, undisturbed, to adjust.  If not, you end up fighting it.

Adding a small surge tank can reduce this problem.

Honestly, though, in a typical house with modern equipment, very little of the energy used to heat water is wasted on "standby" losses.  Modern storage tanks lose heat on the order of half a degree per hour.  So, with a typical aquastat with an 8-degree differential, the tank would only call for make-up heat once every 16 hours, if left alone.  But you will draw hot water much more often than that, so the majority of the energy used will be for water that you are actually using.

On the other hand, if you have a camp that you don't live in full-time, they can be a great idea, because you wouldn't be drawing water more frequently than once every 16 hours, on average.

Or, if you have a multi-unit dwelling, and don't want separate large tanks for each apartment, they can be a great solution.

Joe

[JonM]

My take (which isn't Joe's as I don't speak for him) is that they are a promising technology.  But being an early adopter of some technology isn't always the best Idea.  The jury is still out on thier longevity.  Test show that under certain contidions they are more efficient because you are not losing heat while a tank of hot water cools.

I think I first saw them on some episode of This Old House or something like that on PBS, from what I understood they're rather old technology, having been used in Europe for many decades.  The one they installed was gas powered I think.

[Lloyd Danforth]

I helped a guy build a passive solar house in Alstead in 1980 and he had a 'on demand' hot water at that time.  It seemed to work well.  He shared it between Bath and kitchen, but, planned on seperate units for each.

[KBCraig]

Honestly, though, in a typical house with modern equipment, very little of the energy used to heat water is wasted on "standby" losses.  Modern storage tanks lose heat on the order of half a degree per hour.  So, with a typical aquastat with an 8-degree differential, the tank would only call for make-up heat once every 16 hours, if left alone.

There's a home energy show syndicated out of Little Rock, where the host (who is an architect) advocates Marathon brand super-insulated water heaters, along with a geothermal heat pump for AC, and a heat exchanger to capture waste heat from the heat pump and dump it into the water heater. He even guarantees satisfaction from systems built to his spec.

Anyway, one day he was describing a frantic call from a family in Louisiana who had built a house and all its systems to his spec. It was a large house, and they had house full of guests for Christmas, and they were running short of hot water for showers. "House full of guests" was one of the design criteria, so the architect dispatched the local HVAC guy to check out the system. He found everything was in perfect working order... once he actually turned the water heater on.

They'd been living in the house for more than a year and had ample hot water, without the heater ever being turned on!

(D'oh! Edit to show the water heaters are Marathon brand.)

Kevin

[jaqeboy]

In a 1000 sq ft shop that a friend and I designed and built, we used a Takagi tankless water heater ~$400 and a PEX tubing kit ~$600 now at HouseNeeds in Vermont ( http://houseneeds.com/ ). Along with a Taco pump, expansion tank and some other misc. stuff, he's got a toasty warm shop all winter at bargain cost.

One key was to insulate the concrete slab with 2" of styrofoam to thermally isolate it from the earth, as much as possible. The walls and ceiling are super-insulated, too, which also is a big help. The tankless water heater only runs ~3 hours/day: from 5AM-8AM. Then there is some sun heat input and heat input from the people (~100Watts each) and some machinery. It usually is too warm in there by noon or 1 (with the heat off in the middle of winter).

Jack

[GnuAttitude22]

OK, so are tankless water heaters or on demand water heaters the same thing as "flash water heaters"?

[Kat Kanning]

No, those are the heaters in the long brown raincoats.

[MaineShark]

Kevin: I've heard decent things about the Marathon heaters.  Of course, electric is not ideal, but the nifty thing you can do is use an external heat exchanger that will heat the water in the tank by using your boiler, and then just use the tank for storage.  In many cases, the heat exchanger in an indirect water heater is the first thing to degrade, so having the exchanger outside the tank where it can actually be replaced is a good thing.  And, in the case that you run out of oil or LP or somesuch, the electric will give you backup hot water.  No one wants to pay their HVAC guy emergency-call rates in the middle of summer to get the boiler fixed solely for hot water use.  With the backup electric, you can just rely on that until it's convenient to schedule service.

I would caution that the diagram they give for how to connect to an external heat source (in their case, a geothermal heat pump is used as an example) is less than ideal.  If anyone is actually interested in it, I could try to explain the correct connections, but it's much easier to draw than describe...

In a 1000 sq ft shop that a friend and I designed and built, we used a Takagi tankless water heater ~$400 and a PEX tubing kit ~$600 now at HouseNeeds in Vermont ( http://houseneeds.com/ ). Along with a Taco pump, expansion tank and some other misc. stuff, he's got a toasty warm shop all winter at bargain cost.

One key was to insulate the concrete slab with 2" of styrofoam to thermally isolate it from the earth, as much as possible. The walls and ceiling are super-insulated, too, which also is a big help. The tankless water heater only runs ~3 hours/day: from 5AM-8AM. Then there is some sun heat input and heat input from the people (~100Watts each) and some machinery. It usually is too warm in there by noon or 1 (with the heat off in the middle of winter).

Another advantage of radiant... with the right setup, it can be used for cooling, as well.  But it needs the right electronics and a hygrometer, because you don't want the slab going below the dew point, unless you really enjoy wet floors for some reason!

While on the topic, though, I would recommed against Taco in favor of Grundfos, when it comes to pumps.  The Grundfos pump costs a few percent more, but it's extraordinarily rare to replace a Grundfos pump that wasn't installed in a grossly-incorrect manner (eg, shaft vertical, which can destroy the bearings in short order).  Taco pumps are a common replacement item.  The Grundfos UPS15-58FRC is a variable-speed pump that replaces all the standard-size Taco circulators, depending on the speed it is set to.  It also has an internal check valve that can easily be removed, if it is not needed.  I convinced the last company I worked for to stock only that pump, which greatly simplified inventory. (that is an iron pump, though; for open-system applications, the bronze version should be used - the exact part number escapes me right now)

OK, so are tankless water heaters or on demand water heaters the same thing as "flash water heaters"?

Probably.  I've never heard that term, but there are differences in what products are called, depending on the area.  I'll ask the clerk to put my soda in a bag, but someone in the mid-west will probably want their pop in a sack...

Joe

[polyanarch]

Kevin,

What is the real benifit of insulating the hell out of the bottom of the slab going to do in the long-term other than make it quicker to heat up the slab in the short-term? 

Over the course of a the winter, the heat is going to slowly leak through even the best of insulation and create a thermal gradient.  The temp of the earth is a constant and it is a great thermal mass. Nothing is going to keep the heat in forever will it?  It's just going to eventually heat up the earth below it right?

The problem I have with a lot of foam insulation is that it requires the bottom slab to be more reinforced against settling as it is not sitting on a good solid foundation (squishy insulation) and if it is not a firm floor any tile put on it will CRACK CRACK CRACK!

I am going to go with all ceramic tile in my entire house (single floor semi-sunk construction with walk-out on one side built into a hill.  I plan to insulate the hell out of the sides of the footings going down deeper but not under the slab itself.  Is this wrong with using an entire under-floor hydro heating system?

Jim

[MaineShark]

Jim, the foam really does improve efficiency.  It's worth the trouble to reinforce the slab (which you have to do anyway, as you need the reinforcing grid to support the pex tubing, so it stays in-place within the slab during the pour.

The high-density foam is pretty stiff, and shouldn't compress much.

And yes, the bigger benefit of the foam will be the ability to change the slab's temperature quickly.  In the fall and spring, there will be many times when it is warm during the day, but dramatically colder at night.  So the heating demand will fluctuate.  Without the foam, a lot of heat will be wasted doing this.

Joe

[polyanarch]

So what is a good substrata for this?  a few inches of coarse 2" rock overlaid by crushed lime compacted to hell and back with some sand thrown over it for S&G's?  Then how much foam?  Vapor barrier?  I really don't know much about it other than what I've seen done in the commerical construction industry.

I plan on having underfloor conduits for A/V and power in the center of my house (few walls -think BIG rooms with perhaps more temporary demountable walls for flexibility) so I plan on doing a good job with the reinforcing grid anyhow to keep cracks away because of the conduit in the slab -then put the heated coils in and the tile on top of that.  I want it SOLID.  I want a lot of redundant interleaved zones too for "just in case" as well as scalability of the heat/cooling circuits.

I like the Idea mentioned above using an electric water heater as a storage tank.  Having an electric backup wouldn't be the worst thing in the world.  You gotta have a storage tank -might as well have it serve a dual use.  I also want a simple forced-air system too "just in case."  The ability to use whatever is available/cheapest to heat/cool the home and water is something I need.  Initial cost isn't that important to me.  Efficiency is.  I've got money now -I might not later...

-Jim

[mvpel]

When I poured the floor of my garage and a new driveway with the help of my Dad, I learned a few things about concrete.

One is that you need to expect some cracking - control joints are put in to control where the cracks take place as the concrete shrinks during curing.  Another is to give it room to expand and contract.  We poured the garage floor as a single 4" thick unreinforced 24x28 slab, inside a raised foundation wall, on top of a minimum 4" layer of compacted sand.  We made the top surface slope slightly towards the garage doors for proper drainage. Around the entire perimiter, we put a 3/8" or 1/2" thick sheet of some sort of fibrous material, the top edge of which was flush with the slab, so that the slab would essentially float inside the foundation perimter.  Ten years later, we gather from the new owners that the floor still has no cracks whatsoever.

The metal grid in reinforced concrete doesn't prevent cracks, it just holds the pieces together and provides reserve strength when it does crack.  Concrete is very strong under compression, but not so much under tension, and the tensile loads are what the steel handles.

The cracking moment of a 4" slab of 4000psi concrete is 1.52 kip-ft, while the moment of one wheel of a 3,200lb auto is 0.24 kip-ft, or about 1/6 of the cracking moment.  So there's plenty of room even without reinforcement, hence the uncracked garage floor.  Minimal steel reinforcement provides about the same moment, 0.25 kip-ft.

[MaineShark]

So what is a good substrata for this?  a few inches of coarse 2" rock overlaid by crushed lime compacted to hell and back with some sand thrown over it for S&G's?  Then how much foam?  Vapor barrier?  I really don't know much about it other than what I've seen done in the commerical construction industry.

I think the slab design will depend on the specific application.  I've never really gotten into that; in practice, we generally just tell the engineer what our requirements are, and he makes sure that the concrete folks are given the right specs for the slab.  He who knows, knows he knows not...

On consideration with radiant in a slab is that it would be best to get that done very early in construction, and to rely on the "backup" forced-air system for a while.  If you can give the slab a year (or two!) to cure before first using the radiant, the chances of cracking it due to thermal stress go way down.

I want a lot of redundant interleaved zones too for "just in case" as well as scalability of the heat/cooling circuits.

Actually, that part is very easy.  Radiant loops have to be of relatively short length (say, 150 feet for a typical application), so you end up with a lot of parallel loops connected to a single distribution header.  Well, with the newer headers that are available, you can attach a zone valve to each individual loop.  So, let's say that you have the house split into three zones that are each served by a five-loop header.  Each of those zones will have a pump that can deliver water to the header (and assorted other equipment to regulate the temperature, etc.).  But let's also say that one of those headers has two loops which are under the kitchen, one that's under the bathroom, and two that are under the foyer (since the foyer is small, we'll install one loop in the floor, and another in the ceiling).  So, you add a valve to each loop, and install a thermostat in the kitchen, one in the bathroom, and a dual-stage thermostat in the foyer.  Run the two kitchen valves in parallel and control them both with the kitchen thermostat.  Run the bathroom valve off the bathroom thermostat.  And connect the two foyer valves to the two stages of the foyer thermostat (since the demand when someone opens the exterior door will be dramatically different in the dead of winter versus the spring and fall).

With the three five-loop headers in this example, you have the potential to deliver 15 different zones of heat, although some of the large rooms are likely to have multiple loops, so it simply wouldn't make sense to zone the loops individually, if they serve the same room.  Unless, as in the case of the foyer, you zone them in stages.  With the dual-stage thermostat, when the temperature drops a little below (eg, two degrees) the "set" temperature (eg, 70), it kicks on a single stage... when the temperature drops more than that(eg, four degrees), it kicks on both stages.

You could also combine separate heat sources.  For example, radiant in the floor as the first stage, and a water-to-air heat exchanger and blower unit as the second stage.  That costs a little more, as the radiant is a low-temp system and the heat exchanger demands high-temp water, so it will need additional piping (can't be connected to the radiant header).  But the water-to-air heat exchanger gives you faster recovery than is possible with radiant.  So, if you open your garage door and all that heated air flows out, the blower unit is going to bring it back up to temperature faster than the radiant can manage.  Some of the blower units can be recessed in walls or under cabinets, in the living space, and some are even available with condensation catch-pans, so they can be used with cold water to cool your house (ground-source loop, anyone? nearly-free a/c is good!).

I like the Idea mentioned above using an electric water heater as a storage tank.  Having an electric backup wouldn't be the worst thing in the world.  You gotta have a storage tank -might as well have it serve a dual use.  I also want a simple forced-air system too "just in case."  The ability to use whatever is available/cheapest to heat/cool the home and water is something I need.  Initial cost isn't that important to me.  Efficiency is.  I've got money now -I might not later...

Yup.  Backup is definitely good.

As, like I've mentioned, is a large storage tank that can last you a few days.  Unfortunately, that's an expense many people aren't willing to undertake, both in terms of buying the tank and in terms of the space it takes up.  But if you have any desire to burn wood or such, it's a huge benefit.  Wood can't be turned off with the flip of a switch like an oil or gas burner.  So, once you've heated the boiler itself, you better have somewhere to put the rest of the heat, or it's going to just be wasted.

Joe

[polyanarch]

I wanna burn wood as my primary energy source for heating.  I grew up in a wood-heated house, and have lived in and owned wood-heated homes many times during my life.  Cutting/transporting/stacking wood is a chore I'm used to and acclimated to.  It does suck to be a slave to the wood burner so some sort of back-up source would be great.  Usually, the back-up source I've been used to has been electric baseboard heat set very low to keep the pipes from freezing.  Cheap to install and used only rarely.  I think as a last-ditch level of defense such a system will still have a place in my future home.  But I'd rather have an oil/gas/electric-fueled water system to suppliment the wood.  Maybe being able to burn coal/corn would be good too. 

If the economy goes to hell, NH ends up secceeding and is blockaded, who the hell knows what can happen in the future.  I don't want to be cold!  Energy costs may be a lot higher in the future, shortages of certain types of fuel might happen or my fortune might be wasted/stolen by whomever.  As long as I still can hold onto my house the system will sustain me.  Initial cost isn't a concern.  Efficiency/flexibility/backups is very important.  There might come a time when we are scratching to put food in our bellies -I don't want to have to worry about a warm home on top of that.  Keeping the hearth warm in the future with a minimum fuss/cost/maintenance is the key.