Combined domestic hot water and house heating system

[mbryner]

Hey guys and gals (do we have any gals here yet?),

I came across an advertisment for this system in homepower magazine last night, so I did more research:

http://www.sunequinox.com/Technical_Information.html

It uses a on-demand propane or natural gas water heater to keep a very highly
insulated plastic 79 gal or 132 gal tank hot. Heat exchanger coils go
through this tank to peripherals such as a masonry stove, forced air blower,
solar panels, or domestic hot water. The water to the peripherals, i.e.
heater, solar panels, masonry stove, etc. is circulated by 2 built-in
Grundfos pumps. The "water" in the tank just recirculates to the on-demand
heater. Domestic hot water just passes through the hot tank via exchanger
coils. It's highly efficient (>91%). It loses only 3 degrees of heat from
the tank per day if not in use, because of the 3 inches of foam around the
tank. Can supply up to 18 gal/min of hot water. Also, it only uses 82 watts when
heating.

It does exactly what I want, it's expandable to many uses (i.e. solar which we
don't have yet), and can connect to the masonry stove, can heat the whole
house. I'm going to look into it more. This company has only been in the
USA for 1 year, but the Rotex tanks have been in Europe for about 30 years
and they seem to use good quality Grundfos pumps.

There's a video on their website.  Pretty cool idea, I think.    Seems like it would be pretty easy to hook up a Listeroid or other prime-mover cooling system to the unpressurized tank!   An off the shelf CHP system!!!!   Unfortunately my Listeroid is in its own powerhouse 100 ft from the new house under construction (because of noise abatement) and this system would be in the basement of the new house.  

Here's another link with a quick blurb about it:
http://www.greenjobs.com/Public/IndustryNews/inews06067.htm

The price is probably outrageous....

Marcus

[mbryner]

About the only thing that seems new about this is the fact that it is non pressurized

High efficiency, non-pressurized, plastic tank, circ pumps and controller built in, and 3 inches of insulation also.   I was reading an earlier thread here on indirect fired tanks (found it after starting this new thread), where the tanks mentioned great efficiency of 1 degree heat loss/hour.  24 degrees/day seems pretty high if this sunequinox system claims 3 degrees/day.

Just looking for your opinions.  I guess I hadn't seen a complete system that matches what I was looking for.   Not an expert, though, by any means!

[BruceM]

I was planning on doing something similar, but using an epoxy coated steel storage tank (600 gallons). I can get these fabricated locally for a good price.

Putting coils of copper in the tank would not be hard, since you could climb in.  The tank size was for in floor heat.  I planned on 6" of blueboard under the tank, which will be in my insulated shop.

My plumbing scheme was different, I have low efficiency, no electricty hot water heaters for backup dommestic and in foor heat.  They would be manually bypassed and shut down in all but the worst winter weather. 

The tank is on hold now, as my heating operating costs are so low that I'm not sure how much solar I need.  I may do a smaller system for domestic hot water.

[BruceM]

Glad you mentioned EPDM, Jens, it slipped my mind. I have read about EPDM lined tanks in several active solar books.  They seem to be widely used.  I have a bargain local fabricator of steel water tanks, and don't tolerate rubber well, otherwise I'd probably go that route too.

One issue that is sometimes forgotten is the tremendous force that even a modest depth of water can create.  

For example, assuming a 4x8 foot plywood box with 3.5 foot depth of water (837 gallons):

The average outward force on the bottom foot is:  3/2.2 (pressure in psi 1/2 foot from bottom) x 12" x 96"= 1570 lbs.

The total outward force on one of the 4x8 panels is:  1.75/2.2 x 42 x 96= 3207 lbs.

So you must be thinking seriously about the forces involved, especially towards the bottom of the enclosure.  

[mbryner]

Yes, I've often debated building my own storage tank.   The main concern was the forces involved in deeper tanks.    A 4' x 4' x 4' tank is 64 ft3 = 479 gal.   If you use 4 inches of foam insulation lining the inside, the volume is decreased to 367 gal.   Deeper is better because of heat stratification, but then you get those darn high pressures.   Do you think building with 2x4 studs 1 ft OC and 3/4" plywood is strong enough?   Use 2x6's, or closer stud spacing, or thicker plywood?  

One would think you could achieve tremendous efficiency by using more insulation.  The standard metal tanks that are cheaper seem to use 2" or less of insulation.   They also lose up to 1* per hour.   Why not achieve only 1* /day of heat loss?  My goal is to get it heated up with the masonry stove or on-demand water heater and not lose the heat.  (The solar panels come in to play later.)

The commercial tanks that are large volume I've found are STSS and Haase.   Both are very expensive, i.e. >$6-7k for 500 gallons.   The system I started this thread with is much smaller but it is a complete system with 3 heat exchangers, pumps, etc.

Marcus

P.S.  And what about building copper heat exchangers?  Have any of you ever made them?   I've read about using smaller diameter copper tubing, filling first with sand or salt to prevent buckling, then winding around a pipe.   Does this work well?   Multiple coils sets of a smallish diameter copper connected to form a larger heat exchanger.   Anyone with experience here, maybe when making when making Listeroid exhaust gas heat exchangers?

[mobile_bob]

depending on the size of copper, soft or hard drawn there are various tools that work well to make nicely formed bends

for 1/2" od and under, rigid makes an excellent tool for forming up to 180 degree bends, without sand or other fillers

for larger rigid copper you might try a emt conduit bender, it would work well at making 90's and up to 180 in two passes.

another option is the use of an automotive AC condensor core, such as used on a hd truck, they are sometimes fairly large
and are often made of 1/2 od material.  if you can pick up one at a truck bone yard cheap it would make a nice exchanger mounted
down in a water tank.

i am considering two home built tanks, one for hot storage, one for cold storage, and i may end up using a single unit for both
because my need for both hot and cold is very limited anyway.

for side wall ribs i am thinking of probably 2x6's and 3/4" plywood as well, with the outside insulated, so maybe the wood should be all pressure treated stuff with marine grade plywood?

bob g

[Crofter]

The outward expansion forces could be handled quite easily by using 1/2" through rods side to side and or end to end that engage the outside whalers. A tightly spaced piece of galvanized 3/4 pipe with flanges inside against the membrane could shield and allow the tension rods to be dry. Any exchanger coils placement would have to accommodate the tension rods placement. Sure save a lot of exterior work to get the same strength.

I have installed lots of embedded pipe in dam construction and often had to pipe around those tension rods. That is the method used to strengthen form work for the concrete which pushes lots harder than water. Even so I have seen the results of a few shit outs!

[mbryner]

Thanks.   Here's a good link, if anyone is interested.   This guy documented his own tank build:

http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/Tank/Tank.htm

Good idea about using a propane tank for a core, Jens.  Thank you.

[mobile_bob]

Marcus:

thanks for the link, it has some useful info that will save me from making a couple mistakes.

bob g

[mbryner]

It concerns me in that link that the guy didn't use 2x4 ribs on the side, except in the middle.   He only has the bottom 4" of the walls bound by a ring of 2x4's around the outside.    The guy in the example/link made his tank 3 ft high.  i.e. Side wall force at 3 ft deep is 187.2 lbf/ft2 or 1.3 psi, and at 4 ft deep is 250 lbf/ft2 or 1.7 psi.  That doesn't seem like very much, but over a lateral span (like 8 ft) I suspect the wall would buckle if the tank was taller than 3 ft.

[mbryner]

Hey, I was just talking to my dad on the phone about house efficiency, thermal storage, etc. etc.   He had an interesting idea:  What about lining the inside of a small septic tank w/ say, 8 inches of polyisocyanurate foam and EPDM liner?  No worry about leaks or rot, and they're not that expensive.   Wonder if anyone has tried it?

[BruceM]

Brett, Smart guy, your Dad.  I've never read about that use of a concrete septic tank, but I think it's a gem of an idea for a buried, insulated tank.  Using foam board on the inside and EPDM liner is what makes it doable- a spray foam job on the outside would be expensive.   I don't know about EPDM prices.

Jens- do you have a link for the proven design you mentioned?  I googled and didn't find one as you described, which is beefier than most I've seen.  

I agree that the ply box Brett posted a link to looks a bit puny for a 4 foot deep enclosure.  Adding more vertical braces would help and is cheap.  The top rail then carries a lot of force, I'd want at least doubled 2x6s, bigger if the length got to 8 foot.  Or increase the number of steel braces across the top. 

[Geno]

The guy in the link buried his 36" tall tank 24" into the ground. Maybe that's why he didn't make it as strong as what's being discussed here.

Placement
"The tank should be placed on a flat and level surface that will take the 4200 lb water weight.  A concrete pad, gravel pad, or well compacted soil.  I think that it is best to keep the tank on the surface, and to provide drainage to make sure that the wood stays dry.  I violated this rule with my tank, in that it is sunk in the ground about 2 ft.  In my case, the soil is dry, the tank is inside a shed that protects it from weather, and the tank bottom sits on 3 inches of open gravel plus 4 inches of insulation -- I'm still not sure this is a good idea, and I would avoid it if you can."

I've been thinking about thermal storage as well. Especially in the summer when I dump most of the hot water my Listeroid makes. In general though I just don't use that much domestic hot water. I do understand you guys with families are in a much different situation.

Thanks, Geno

[BruceM]

Thanks for pointing out the earth burial, Geno.  That's very important, and  means that this is not structurally proven as a freestanding tank.  

I've used Tigerfoam and Handifoam on a number of projects. (I can't spray the stuff myself but it cures fast and is not deadly to me the next day.) They are the same isocyanurate foam that is in the blond foam boards (usually foil faced) which is what's recommended for the layer of foam next to hot tank.  I have seen articles showing Isocyanurate foam used for spraying the outside of a (to be) buried tank.  It is not approved by the vendors for constantly saturated (wet) soils.  (Occasionally or seasonally wet is OK.)

The Tigerfoam or Handifoam system is really good for DIY stuff.  It's easy to use, just a bit expensive.

[Lloyd]

Hi group,

I've been lurking for a while and plan to get more involved. I come from the boating world where efficiency/co-gen is an important way to design and function...especially if your the type that reels at the thought of burning north of 10 gals per hour.

Now onto topic, My design proposes as follows: Dig a 6 foot deep hole laydown a bed of crushed rock, then cover with sand,  (don't forget the drain) then cover with 6" blue foam...top that with a 5in reinforced slab. Then form concrete walls to height, add a grid of DOW poly 55 drums interconnected with cpvc tank fittings. Then a perimeter lining with blue foam. After all connections are made, tested for leaks. The voids between the walls, and the tank grid is filled to depth with peat moss. The peat is an outstanding insulator, as well as generating some of it own heat.

Build a covering deck, that you can lift panels to top off the peat, and service. All circ pumps http://lainginc.itt.com/LG-pump-DC-Solar-Pumps.asp are operated from a dedicated mmpt controlled PV

It's that simple bet very effective.

Lloyd