Coil length for HX

[joecool85]

It seems that everyone here that builds their own HX using copper coil is using hundreds of feet of coil. I figured this was normal. I had to replace my heating coil on my oil furnace last year and decided to do some math.

It has 28' of 1/2" finned copper tubing. It is rated at 100 degree rise, 3gpm with 180F boiler water.

3gpm x 60 = 180gph

180g x 8.33btu/per gallon = 1500btu/per hour per degree x 100 degrees = 150,000btu/hr

So how is it that this 28' length of 1/2" copper can move 150k btu/hr, but people building their own hx use hundreds of feet of copper and figure they are moving 100k btu/hr ?

 

[WoodNotOil]

It seems that everyone here that builds their own HX using copper coil is using hundreds of feet of coil. I figured this was normal. I had to replace my heating coil on my oil furnace last year and decided to do some math.

It has 28' of 1/2" finned copper tubing. It is rated at 100 degree rise, 3gpm with 180F boiler water.

3gpm x 60 = 180gph

180g x 8.33btu/per gallon = 1500btu/per hour per degree x 100 degrees = 150,000btu/hr

So how is it that this 28' length of 1/2" copper can move 150k btu/hr, but people building their own hx use hundreds of feet of copper and figure they are moving 100k btu/hr ?

Because of the fins. That increases the surface area by a lot. You can have finned coils made and they would be considerably shorter to accomplish the same btu/hr rate. The other thing you have to plan for with storage is that the transfer rates not only depend on the boiler temp, but also on the tank temp. It is really easy to transfer heat when there is a 20+* difference between the two, but as you approach the same temperature, you need more surface area to transfer the heat.

 

[joecool85]

I figured as a rough idea the fins maybe doubled the effective surface area. Then I remembered about the delta T. We rate coils on 20 delta T, not 100.

So, if you multiply the 28' x 2' = 56'. Then multiply that by 5 and you get 280' of 1/2" copper tubing to move 150k btu/hr at 20 delta T.

Do those numbers sound about right?

 

[Gooserider]

Not really - the fins do considerably more than double the effective area - but don't offer a lot of advantage in a storage tank environment - or so claims Tom from ME, our resident coil HX expert...

Secondly that's a ΔT of 20°F across the entire coil, but what is more of concern is the "instantaneous ΔT" across any given point on the coil, as the temperature differential gets smaller as you flow through the coil, you get less heat transfer, so you need to make the coil longer to compensate.

Third, in order to get the maximum benefit out of the storage, it is really nice to have less than a 20°F ΔT the ideal would be total transfer, but that isn't possible because of thermodynamics - Nofossil says 5-10°F is about as good as one can realistically get, but again that takes a longer coil.

I've seen some math on how to figure it in other threads, but bottom line seems to be that the several hundred foot lengths you mentioned have been shown to be necessary by practical experience...

Gooserider

 

[joecool85]

My current plan is to use 3 x 50' sections of 3/4" (max 12gpm - 29.43 square ft of transfer, $450 in copper) or 5 x 50' sections of 1/2" (max 10gpm - 32.71 square feet of transfer, still $450 in copper).

"Refrigerator" copper coil should be fine right?

I'm hoping for 100,000 btu/hr transfer minimum. Using 3/4" I would need 23.59 btu/square inch for this to work. Using 1/2" I would need 21.23 btu/square inch for it to work.


Using math from this thread: https://www.hearth.com/econtent/index.php/forums/viewthread/45843/P15/

I should get 70 btu/hr per degree of delta.

3/4" hx at 150'
29.43 x 70 x 10 = 20,601 btu/hr when the tank is running at 180F and wood boiler at 190F
29.43 x 70 x 40 = 82,404 btu/hr when the tank is running at 150F and the wood boiler at 190F

1/2" hx at 250'
32.71 x 70 x 10 = 22,897 btu/hr when the tank is running at 180F and wood boiler at 190F
32.71 x 70 x 40 = 91,588 btu/hr when the tank is running at 150F and the wood boiler at 190F


It does really seem like the 1/2" is going to do the best job for the dollar. And these are average numbers, the middle transfer number could be 60 to 80. So I'm sure that companies selling an hx would run the 80 to inflate their figures. Even with the same tank temps this would bring my 3/4" up to 93,760 btu/hr and my 1/2" up to 104,672 btu/hr.

 

[jebatty]

How do your calcs relate to your boiler btuh capacity and other heat demand? As your calcs show, btuh transfer capability falls off rapidly with closing Δ T. If you don't have other heat demand, your boiler will do lots of idling during the tank charging process, and increasingly so as tank temp rises. And figure in that you won't be able to squeeze that last 10F or more of boiler output into the tank. If peak boiler output 190F, maximum tank likely will be 180F and maybe less, but your boiler is unlikely to maintain peak output, so figure an average high output and subtract at least 10F from that to guestimate your maximum effective tank storage temp.

Depending on application/needs, this 10F+ loss factor can really impact effective btuh capacity of storage to meet demand. If you need 180F for baseboard, then being able to heat storage only to 180 does little if any good. If you only need 100-120F for in-floor radiant, then 180F storage has considerable potential. Even pressurized storage, with the ability to actually load storage to 190F (which I can do with my storage), will not provide much capacity if need is 180F, but it is %-wise a considerable increase in ability to store btu's over a lower maximum storage capacity.

 

[joecool85]

I should have posted it in here, but I have it on other threads.

My wood boiler has a max of 120k btu output (Memco MW100). I plan on running 1" insulated pex to it (it will be installed in an insulated shed 30' from the house). Planned storage is 500 gallon unpressurized.

My house has a max draw of 42,000 btu/hr plus dhw right now. Before storage goes in I will be adding more baseboard and my max draw will be more like 55,000 btu/hr plus dhw. That max draw is at 180F baseboard temp.

On a normal January day (the coldest day of the month here), my house only consumes an average of 15,000 btu/hr. I figure I should plan on twice that for overnight temps.

The high output baseboards I'm going to be running put out 750 btu/hr per foot at 180F and 500 btu/foot at 150F and put out 280 btu/foot at 120F.

So with 74' of this (which I will have), that means that I can heat my house with as little as 140F supply water. I plan on keeping the tank 150F as a minimum, so heating my house shouldn't be an issue.

With the current 600 btu/ft at 180F, 200 btu/ft at 120F baseboard I have now (67' of it) I have to use 160F water to heat my house.

In spring and fall I could use water temps as low as 120F to comfortably heat my home, bringing my min tank temp down to 130F if I want to.


This house is 1200 sq ft, pretty well insulated, in Central Maine. Most January nights are about 10F, 30F during the day or so.

 

[jebatty]

On a normal January day (the coldest day of the month here), my house only consumes an average of 15,000 btu/hr. I figure I should plan on twice that for overnight temps.
***
So with 74’ of this (which I will have), that means that I can heat my house with as little as 140F supply water. I plan on keeping the tank 150F as a minimum, so heating my house shouldn’t be an issue.

Good info. Your 500 gal storage will provide about 40,000 btu per 10F of available storage. So on an overnight basis, you will have 1-2 hours of stored heat for each 10F of available storage. Looks like it may get you through a cold night if you have the tank charged up to 180F.

 

[joecool85]

On a normal January day (the coldest day of the month here), my house only consumes an average of 15,000 btu/hr. I figure I should plan on twice that for overnight temps.
***
So with 74’ of this (which I will have), that means that I can heat my house with as little as 140F supply water. I plan on keeping the tank 150F as a minimum, so heating my house shouldn’t be an issue.

Good info. Your 500 gal storage will provide about 40,000 btu per 10F of available storage. So on an overnight basis, you will have 1-2 hours of stored heat for each 10F of available storage. Looks like it may get you through a cold night if you have the tank charged up to 180F.

That's what I figured as well, and thats on the coldest of cold nights I should be just fine. Worst case, it is -20F all night and the oil kicks in for a few hours before I wake up. I can deal with that.

 

[joecool85]

So 250' of 1/2" will do it for my space heater coil, it seems from my math that 100' of 1/2" would do it for my dhw. What do other folks use for their hot water coils?

 

[Vtgent49]

My opinion is a bit different. Most storage tank coils seen on this site wind down from the top of the tank to the bottom, or vicea-versa. This is a waste.

Coils that heat the tank should all be coiled near the bottom to get max delta T, and heat extraction coils should all be near the top. Maximize delta T to minimize cu pipe. Stratification will still occur, as gravity cannot be denied.

I have a finned 25' coil in my Vaughn Solar DHW tank, and it has no trouble transfering up to 40 degrees on any of my 3 speeds of my Grundfos circulator. It is all wound up in the bottom of the tank.

 

[webie]

My opinion is a bit different. Most storage tank coils seen on this site wind down from the top of the tank to the bottom, or vicea-versa. This is a waste.

Coils that heat the tank should all be coiled near the bottom to get max delta T, and heat extraction coils should all be near the top. Maximize delta T to minimize cu pipe. Stratification will still occur, as gravity cannot be denied.

I have a finned 25' coil in my Vaughn Solar DHW tank, and it has no trouble transfering up to 40 degrees on any of my 3 speeds of my Grundfos circulator. It is all wound up in the bottom of the tank.

That would work if all we were doing is chargeing the tanks or recovery from the tanks with seperate loops .
Our coils that go from the bottom to the top are used for both chargeing and recovery by way of us reverseing the flow through the coils . So we heat from the top down and recover from the bottom up . Reason is to get and keep our hottest water near the top and colder water at the bottom to maintain even recovery temps for our heating systems even though we may not charge our storage tanks fully .

 

[joecool85]

Kind of a moot point now that I know a 250 gallon propane tank would fit in my basement better than (or the same-ish as) a 300-400 gallon home-built unpressurized unit, and it would allow me to run 140-180 in the tank, and actually use that. If I charge an unpressurized tank to 180, I can only use 170-140. Plus I can get a tank and expansion tank for $400-500, the coil alone for an unpressurized setup would cost me that. I didn't realize that most folks were using unpressurized tanks for two reasons: small entry into basement, or needed 1,000+ gallons of storage (a pain to do with pressurized). I have a small basement, but a relatively large entry via bulkhead and I measured out, and I won't have any issues bringing in a 30" x 92" cylinder