Best hydronic heat storage applications and ideas.

Thaddeus

Greetings.

I have recently tabled a hydronic dryer project due to the knowledge I gained here. As a collateral issue from that discussion, I am interested in hearing from anyone here about heat storage.

Ways, ideas, actual installations, even backyard engineering ideas that might have failed.

My heat source is a ODWB Central Boiler Classic Edge 750 240,000BTU with 3 supply/return ports, only one is in use.

My goals are: 1) heated walkway 2) heat my hydronic dryer 3) heat my garage (20,000 cubic feet) 4) heat my hot tub


So if you have experience with hydronic heat storage I am interested in hearing from you.

Thanks for anything you might contribute

Jamie Hall

Not specific to hydronic heating -- in general. There are really only two good ways to store heat: in a phase change substance or in mass. Phase change is great -- provided that you can store or use the heat at a temperature which is suited to your phase change compound. That's not so easy, and so it is rarely used in room temperature applications (there are compounds which do change phase or have usefully high solution/precipitation heats at usable temperatures, but most of them you really don't have around. Or cost a lot.

Mass, then is the obvious one. What you want is a suitable mass of something simple, and the something simple is water -- which, conveniently, has a remarkably high heat capacity and is easy to obtain and manage. As is well known, a pound of water will absorb or release one BTU for each degree Fahrenheit change.

So some real numbers. Suppose you want to store the entire output of your boiler from 24 hours. That's about 5,760,000 BTU. Further, let's suppose that you can start with water at 50 degrees Fahrenheit and bring it to 190. That's 140 degrees temperature change. You would need about 40,000 pounds of water to do that, 5,000 gallons or 660 cubic feet -- more or less. That's a lot -- but water's cheap. Or a tank 8 feet deep by 8 feet by 10 feet. Not so bad. And then insulate it heavily.

Swimming pools work, too. It's all in the insulation.

And, of course, the size can vary to taste -- it's unlikely that you would need that much water, as it's unlikely that you would want to store your entire output for a whole day.

And, of course, being water you can pump it from your boiler with no trouble, and back to where you need it.

GroundUp

The unused ports in the OWB won't do you any good if the heat is required in and around the house. If storage were remotely located (garage or whatever) you'd still need to run insulated underground lines to the house for the loads so at that point, you'd be WAY ahead financially to dig up your 25mm Thermopex and replace it with 1-1/4", and forget about the storage. In order to have enough heat stored with your existing 25mm, you're talking several thousand gallons worth of water in your house.

If it were mine, I'd replace the Thermopex with piping properly sized to the load and forget about the storage. The 25mm will always be a problem for you. Not to mention the flow rate through your OWB is rapidly killing it with spot boiling. Those Edge units need 10+ GPM to be happy

Thaddeus

Groundup. May ask is your 25mm reference to the OD or ID because I measured my Thermopex at 21mm ID, which I provided in a picture above.

Digging up my system is not viable and the current configuration heats the house and domestic hot water with no problems.

I have calculated that I am only using approx 25% of my boilers capacity. I orginally bought it to do much much more including heating my garage off the second loop.

So a heat storage tank would work for me in my situation without adding a "replacement cost" to my scenario. Then I could use the heat storage to feed ancillary things like my hot tub, walk way and dryer.

Jamie Hall

The heat storage tank is the way to go -- but I'd be very inclined to put it near the boiler with it's own separate loop, pump, and aquastat to serve as a dump tank, as well as piping it so that you can draw for your other loads.

Where you are only using a small fraction of your boiler's capacity, both you and it may be much happier if you run it much closer to capacity, but for less time, and use that tank to store the heat. Just insulate everything. Thoroughly.

GroundUp

25mm is nominal size. As stated above, they only make 3 sizes of Thermopex and what you have is 25mm. I have some out in the shop I can measure later but I'm guessing that is an OD measurement.

I don't think you're letting the storage idea sink in. If you add a hot tub and snowmelt to the existing house load, your 25mm lines will never feed it unless you have several thousand gallons of storage and seriously limit the usage of both. More water does not create more BTU, so you will need enough heat to charge the storage prior to drawing said heat for use. If you can only move 40k with your existing lines, it would take an entire day to charge up your storage before you can draw from it. Is the garage attached or detached? Where would you locate this storage? The boiler's rated output doesn't mean anything if you can only transfer 40,000 BTU through the lines. You need more flow, plain and simple. If you were to use another set of ports on the boiler, you would need a way to get that heat to the house anyway- meaning another set of underground lines in addition to the ones you already have, OR simplify the equation and replace the ones you have with some that will do the job.

GroundUp

This is 25mm Thermopex. Appears to be an OD measurement

Thaddeus

Jamie Hall, I have a garage under construction that when finished and insulated will be approx 19500 cubic feet and thanks to this discussion I know now the line I planned on using for the modines is way to small.

The layout of my yard vs garage and house and hit tub and where I want to put a heated walkway is very conducive to an underground storage tank which if I do install it will be only 20 feet from the boiler.

With 2 unused ports/circuits on my boiler, one was/is slated for the garage I can use the 3rd for the heat storage/dump. Thus maxing the boilers total capability.

Thaddeus

Groundup, I will not (after what I have learned here and from you) be using the house loop for anything but what it is being used for now I am going to remove the dryer circuit.

As I stated to Jamie Hall, I have the other 2 unused circuits on the boiler, and by my calculations I am really only using a fraction of its capacity.

Correct me if I am wrong, but it seems to me that installing a heat storage tank on it's own loop with 1-1/4" supply and return, that I could run a hot tub, walk way and dryer as these things are ancillary to my heating needs and therefore their demand can be controlled.

By leaving the house alone and putting the separate garage on it's own circuit (and the garage will not be running on auto, it will be on demand/manual, where, when I want to warm it up I'll load the boiler accordingly to meet the demand.) I beleive that I can manage the loads on the boiler and meet my needs while remaining efficient and meeting all of my heat requests at appropriate times and loads.

After everything I have learned from you and others in this discussion I am reasonably confident to meet my goals the addition of a storage tank (right now I'm thinking 1000gal) would greatly assist me.

GroundUp

There is no need for the storage, is my point. It will gain you nothing but a large expense unless you plan to batch burn. You can run the walkway, hot tub, and dryer all off of a single 1-1/4" loop with no storage at all and still be well under the rated 240k assuming this walkway isn't 500 feet long. Figure 150-200 BTU per square foot for snowmelt, and the 1-1/4" Thermopex will carry about 150k at a 20 degree delta so you could max out your boiler by adding the garage and have no storage at all- that's how they are designed to run. They are designed with enough water to do their job- storage is of no benefit.

PerryHolzman

The problem with storing heat... has always been inadequate insulation and heat leaks....

My background is both someone who seriously studied heat storage from the 1970's on (all those articles of rooms full of rocks, etc) for long term heat storage... and know of dozens of failures of those systems - and why they failed; and also someone who became an Engineer with heat exchanger and heat transfer expertise... who actually designed a workable heat storage battery for my house (and other houses) which would have allowed me to use solar energy in the summer to recharge the storage battery - to have enough stored heat to heat the house for a colder than normal winter (to be recharged the next summer).

Building that designed storage battery would have cost about $100,000. 2 key design issues: How to effectively store the heat in a way that is storable and recoverable at the provided and needed rates. How to keep the battery from loosing too much heat due to losses to the room around it.

First problem would be solved by using phase change materials inside tubes; assembled in a matrix to look like a heat exchanger. Thousands of tubes. Actually, I had 4 different phase change "temperature" materials so that phase change would start to store heat at 90 F and end at 140 F. Tubes would only be partially filled to allow for expansion/contraction of the phase change material. Tubes would be immersed in water - the same water circulated around the house (you could add a heat exchanger if you wanted to separate the system).

Tube and support materials would be long term galvanically compatible with each other and the house heat loop piping (Idea is 100+ Yr life); and not degrade with normal heating loop water chemistries. I have the knowledge to figure out the material selection; and know the heat exchanger shops that could build it (only new technique for them would be putting phase change material inside a tube and crimping/seal welding the tube plugs at the end.

Then everything had to be insulated massively (and the feet had to sit on insulated supports, which sat on another set of insulated supports, and the water in and water out pipes had to run downhill at least 4 feet to prevent heat from constantly rising out the top.

My memory is that this heat storage battery would have been about 6 ft x 6 ft and 24 ft long for my house; to be housed in a new "basement" room built next to the house.

There would have been more than 1 ft of high R insulation on all sides (and the supports, and their supports, would have sat on the same kind of ceramic thermal tile as was used for the space shuttle heat shields).

The main failure of almost all attempts to store heat long term is the lack of recognition of how much insulation is required to prevent long term heat loss to the house and ground, and how to prevent natural convention heat loss due to air (or water) naturally moving up through it when the system is not being used to supply heat.

I believe my "Thermal Battery" was beautiful design and I'm totally convinced it would have worked. Just not financially viable. I looked at some other houses and came up with a standard rule of thumb to make this work... An appropriate sized storage battery to store heat from summer solar heat for winter heat in southern Wisconsin would cost about as much as the house cost to build. That would not include the cost of the solar collecting system or the radiant heat system for the house.

You would be looking at most likely a 40-60 payback.

That's why you don't see them...

That's also why I know of dozens of rock storage and attempts at tank storage that failed. Every one of them forgot to account for heat loss of the system - even if they had enough mass to make it work.

Insulate, Insulate, Insulate... including the support feet (there are ways to calculate how much insulation is needed: Then add 50% and overlapping insulation joints to ensure all joint heat loss is adequately controlled. Stop vertical convection by eliminating it in the design.

I wish you the best,

Perry

ps: I'll design you a battery if you are willing to pay me to do it.

Rich_49

https://www.latitude51solar.ca/commercial-solar-water-heating/2-uncategorised/148-cocoon-large-solar-storage-tanks

hot_rod

I've chased thermal storage my entire career

I've found the best bang for the bucks, easily sources is old LP tanks with plain water. Vertical mount is best for stratification.

If you are handy or have access to a welder should be able to build and insulate a tank like this for 800 bucks or so.

Buying a manufactured insulated, pressure rated tank over 119 gallons size probably runs 12 bucks a gallon or more!

500 gallon LP tanks are most common around here, on Craigslist find them for $350- 500 used.

I wrap the tank with multiple layers of bubble foil type insulation, Insultarp, white side out look pretty good.

My tank provides storage for my 5 solar collectors, wood boiler, LP backup. It supplies radiant floor heat and DHW with a plate HX. My goal is a day or two of no-burn or no solar input, depending on weather conditions.

You will get a lot more useable energy from an insulated tank inside the building envelope compared to an OWB with a large chimney hole in the top siphoning heat away 24/7

I like the 2 or 3 pipe method best for simplicity and overall best performance.

The water with hydronic conditioner in the tank is the solar water as the tank is also a large drainback tank with an air bubble up top. The only heat exchanger is for DHW on demand.

clammy

Kinda off topic but hot rod are u still using the hydronic wheel or have u moved on to a new one of a kind hydronic distribution center experiment at the homestead . Always impressed w your creative imagination and making it come together like no other and make it function . Cool stuff Peace and good luck clammy

hot_rod

clammy said:

Kinda off topic but hot rod are u still using the hydronic wheel or have u moved on to a new one of a kind hydronic distribution center experiment at the homestead . Always impressed w your creative imagination and making it come together like no other and make it function . Cool stuff Peace and good luck clammy

The circle primary loop is going, and the one in the shape of a ∆
Moved on to buffer tank/ separation concepts.
I hope all is well with you.

kenjohnson

I'd probably check in at hearth.com, specifically the boiler room forum. You are going to find a lot of people there who have solved the storage problem for indoor and outdoor wood boilers using a variety of propane tank and home-built solutions. There is a lot of great, practical advice there.