Freezer as hot water storage?

[joecool85]

I have a 20cu ft chest freezer from the 70's available for free. Doing the math, it should hold about 150 gallons of water. I was thinking I could line it on the cheap and have a pre-made unpressurized insulated storage all ready for me.

Goal: minimum of 8 hour "run time" with my Memco MW100 burning pine, willow and similar wood. From my research, it appears that with a little bit of modification I should be able to get 4 hours burn time, so I would need 4 hours of heat on storage during the coldest part of the month.


First question: is a chest freezer a good idea to use as storage?

*Second question: is 150 gallons "enough" storage?

*To answer the second question, here's a bit of data: My house is 1200sq ft, I live in Central Maine. We use 650 gallons of oil/yr to heat our house via baseboard and dhw. Our system's max draw on baseboard is 40,000btu/hr right now. Our baseboard circ pumps average 8 hours of run time/day in January. This means we are consuming 13,000btu/hr every hour during the coldest month of the year.

**My math:

Running my tank 150/190 at 150 gallons would give me ~50,000btu/hr of storage. At 13,0000btu draw, that would be just under 4 hours. So most of the winter I would be fine, and on the coldest of cold days I would need to load my boiler more often or just let the oil kick in and keep the house warm.

 

[Nofossil]

Your math is right, but would you really want to build a 4 hour fire every 8 hours? Most folks want to build one fire per day (how ever long it needs to be to get a day's worth of heat) and then have enough storage to cover the balance of 24 hours. 500 gallons is often considered a practical minimum, and by coincidence that would give you almost exactly enough to burn once per day.

The heat output from baseboards drops off sharply as supply temps get below 180. I run mine all the way down to 120, but they can't keep up with outside temps below 150 on a typical day.

With open storage you'll need a heat exchanger. Count on at LEAST 10 degrees delta when heating storage AND when drawing heat from storage. That means that 190 degree water from the boiler will not get storage above an average of 180 at best, and storage at 180 will give you baseboards at 170.

I also expect that the plastic liner isn't rated for 190 degrees, and that the metal shell might not stay rectangular with the weight of the water in it.

 

[stee6043]

Interesting idea. My first thought is that I wonder if a normal freezer would actually survive being filled 100% full of water? I tend to think it might buckle, crack or break at the seams. If it survives the first filling I would bet it will not tolerate high temperatures before something gives it up.

But alas, if it survives the fill and survies some heat - my last thought is that with unpressurized storage you will be hard pressed to acheive 190 degrees consistently with your storage temps. Depending on the type of HX you use I would estimate that you'd be more in the ballpark of 175-180 max temp (others with non-pressurized may correct me here). So worst case if you're heating from storage from 175 to 150 this severely limits the usefullness of this amount of storage.

I'm surely not trying to rain on your parade. It's a neat idea. But I think 150 is on the light side to begin with and using an old freezer may not be a particularly long term solution...

 

[joecool85]

Yeah...starting to think you guys were right in that I should probably just use it without storage for a year or two, then when I can afford it, add storage. If I'm going to build a tank or buy a tank I'd like to go large, 800-1000 gallons.

And you're right, building a fire twice a day is a bit of a pain in the butt. Doable, but a pain. Ideally I'd like to build a fire maybe twice during the coldest part of the winter, and once during the rest. And ideally I would like to run my storage tank at 165/185 so that I get decent heat from my baseboards and also I don't fry my tank (home-built or otherwise).

So that would mean I would need 9 hours of storage (3 hour min burn time gives me 12 hours with storage) at 15,000 btu/hr (to play it safe). Doing the math would mean I would need 810 gallons. So a 820 from Tom would work, or I could build a similar sized unit.

And the plus side of a large tank like that is that if I want to use it for dhw in the summer, I should get about two weeks or so per firing. Sweet

Then later down the road if I upgrade my baseboards (add more, and swap to high output) I could bring my tanks lower temp down to 155 and have 50% more btu storage available. Actually, in the summer I could do that anyway and then I could have like...a month worth of hot water per firing!

Yeah, I've talked myself into it. I'll wait and use a plywood tank or buy one from Tom.

 

[joecool85]

More math. Going on the lower side (13,000 btu/hr average on cold days), at 8 hours (I think I can get 4 hours burn time from boiler with a bit of work) and running the tank 155/185 for a 30 degree swing, would mean I would need a bit over 400 gallons. Which would be nice, because I have a corner in my basement that would hold a 8' x 3' a 4' tall tank perfectly. With 4" of insulation on the inside of the tank, that would give me 7.3' x 2.3' x 3.3' interior. This is 415 gallons. Due to the small size of my basement, my want to do this cheaply, me being handy and a bunch of other things, I think this would be the ideal situation.

I have a few ideas, when I get them all together I'll post another thread so I can get some feedback. You guys have been a huge wealth of knowledge!

**edit**
If I was to make the tank a little taller, 56" instead of 48", it would hold 500 gallons and this would meet my needs perfectly.