Buffer Tank Minimum Size

[mr.fixit]

Would a 100lb. (24 gallon) propane tank work for a buffer tank-hydraulic separator?
I plan on redoing some of my plumbing this year,mainly because I have more than one heat source heating the 500gallon tank and loads. It worked fine with just the EKO, but with the addition of a pellet boiler,and my diesel CHP , things don't always flow where they should.
So thinking of having everything feed the buffer then another pump to heat storage.

I have been watching for a used tank but haven't found one close,but I can buy a brand new 100lb. tank for $120.

Any thoughts?

 

[Bob Rohr]

24 gallons will not add a lot of buffer capacity, but it would be a dandy separator. Top port for a vent, drain cock on the bottom, and the 4 connection ports.

Plan on some insulation, or you end up with a 24 gallon round, steel radiator.

 

[tom in maine]

Some simple math would tell you.
A gallon of water holds 8.32 btus for every degree F it is heated. 24 gallons heated from 100-200F is going to hold 24x 8.32x100=19,968 btus. If you have a 100k btu boiler,
that is about twelve minutes of heating at peak output.

I would suggest at least 200g to start. It still is probably not enough for Wisconsin, but would be a start.

 

[maple1]

Some simple math would tell you.
A gallon of water holds 8.32 btus for every degree F it is heated. 24 gallons heated from 100-200F is going to hold 24x 8.32x100=19,968 btus. If you have a 100k btu boiler,
that is about twelve minutes of heating at peak output.

I would suggest at least 200g to start. It still is probably not enough for Wisconsin, but would be a start.

That big - just for a buffer/separator? I see he already has 500 gallons of storage.

The big scrap yard where I found my 330 gallon LP tanks had a mountain of 110 gallon tanks. They were cheap ($60 at the time). Have you checked all the scrap yards around? They're 4' high, so maybe one on top of the other? That would get you more storage too. Mine had a couple top fittings, but I can't remember if it already had a bottom one or I had to get one welded in.

 

[mr.fixit]

Yes I have 500 gallons of storage,so it would really be just a hydraulic separator. I should of titled the thread differently.
Guess my main concern was if that size of tank(24 gallons) be large enough for adequate separation with multiple inputs and outputs.

 

[ewdudley]

Any thoughts?

In one of Seigenthaler's slides there a setup where all the heat sources feed into storage and the loads draw from storage so there is no need for a separate separator. The trick was that the return for the oil/LP/pellet boiler was drawing from a point somewhere near the top of storage so that the oil/LP/pellet boiler was only working with a couple dozen gallons of storage.

In my case the oil/LP/pellet boiler is 50 feet away from the wood boiler and storage, so I used a buffer/separator tank so that I could bring heat to the buffer/separator from either storage or the oil boiler and then loads can draw from the buffer/separator without any complications.

The buffer/separator also reduces short-cycling of the oil boiler.

I used a 30 gallon LP tank because it worked out as plenty big enough to lengthen the oil boiler duty cycle, and it is big enough so that the pump that pulls hot water from storage can run for a minute or more when replenishing the buffer from storage.

Also it is plenty tall enough to provide a zone of hottest water at the top for DHW, then a zone of water for baseboard and radiator loads, and then a zone for 'spent' water that the in-floor PEX zones draw from.

However you end up you should consider a return temperature protection valve for the pellet boiler, if needed, which can really simplify your design in certain cases.

[Edit:] A much larger buffer could simplify integration of solar thermal.

 

[Bob Rohr]

In one of Seigenthaler's slides there a setup where all the heat sources feed into storage and the loads draw from storage so there is no separate separator. The trick was that the return for the oil/LP/pellet boiler was drawing from a point somewhere near the top of storage so that the oil/LP/pellet boiler was only working with a few dozen gallons of storage.

In my case the oil/LP/pellet boiler is 50 feet away from the wood boiler, so I used a buffer/separator tank so that I could bring heat to the buffer/separator from either storage or the oil boiler and then loads can draw from the buffer/separator without any complications.

The buffer/separator also reduces short-cycling of the oil boiler.

I used a 30 gallon LP tank because it worked out as plenty big enough to lengthen the oil boiler duty cycle, and it is big enough so that when the pump that pulls hot water from storage can run for a minute or more when replenishing the buffer from storage.

Also it is plenty tall enough to provide a zone of hottest water at the top for DHW, then a zone of water for baseboard and radiator loads, and then a zone for 'spent' water that the in-floor PEX zones draw from.

However you end up you should consider a return temperature protection valve for the pellet boiler, if needed, which can really simplify your design in some cases.

The separator, or separator function built into piping or a storage tank, is helpful when you start mixing circ pumps of various sizes from the multiple heat inputs.
It allows them to all work at the same time, one at a time, or any combination without hydraulic interference.

The separator tank, properly built adds excellent, dirt, air, and possibly magnetic separation, so a true 4 in 1 device.

It's not large enough to add much buffer, but you have that. A bit generous for a separator, but no harm in that, for the price it sounds like a fun project.

We regularly sell 25 gallon ThermoCon insulated tanks for this purpose. But not at that price

 

[mr.fixit]

Thanks all, if I get some time today I will draw up what I am thinking for piping things and post it ,and see what you guys think.

 

[velvetfoot]

Speaking of things not flowing where they should, eliminating two directional supply flow to/from 120 gal buffer tank, (pellet boiler-no bulk storage), substituting separate connections to the tank for loads and boiler, may help my setup too since higher flows from the feed pump seem to warm zones when not calling for heat. Can also see advantage if/when adding solar. Won't hijack post. Will post a diagram on a new thread and hope you guys comment too.

 

[mr.fixit]

Here is what I am thinking,I drew the load side separate just to keep it less cluttered,boy it sure looks complicated when you draw it out on paper. No wonder the wife just shakes her head.

 

[ewdudley]

(Good schematics.)

How far is it from the wood boiler to the storage tank?

On the load side I think it would be possible to eliminate the "High Temp Load Pump" and its neighboring check valve, plus eliminate the "differential bypass valve" so there would be no primary loop, just a supply and return manifold. Then substitute a simple check valve for each of the zone valves on the shop building loads, and add a pump to feed the makeup air FPHX. (The FPHX is needed because the makeup air subsystem is running antifreeze?)

Won't the load of the makeup air subsystem be fairly large although intermittent? Maybe a buffer would be a good idea there so you could get ahead of the game when the makeup air subsystem is idle, which would mean you could use smaller piping to feed the shops.

 

[mr.fixit]

Storage is right by the boilers,so 5-6ft. It only has one fitting tap on the bottom,and I was in a hurry when I first put in the system and didn't add any.(dumb)
I was only planning on redoing the near boiler plumbing,didn't really want to mess with the plumbing in the shop buildings.That has all been in service for several years.

Correct on the antifreeze in the makeup air coil,and yes it is intermittent,once or twice a week for 3-4 hours at a time.Works great even in sub-zero temps but sure takes the wood!

That high temp load pump only runs when the make up air coil is running,the shop infloor pump works alone.So when the infloor pump kicks on,it will draw from the lower portion of the buffer,and when the make up coil runs it will draw from the top of the buffer. When they are both on at the same time(very little) I guess it will be a free for all.

 

[Bob Rohr]

Storage is right by the boilers,so 5-6ft. It only has one fitting tap on the bottom,and I was in a hurry when I first put in the system and didn't add any.(dumb)
I was only planning on redoing the near boiler plumbing,didn't really want to mess with the plumbing in the shop buildings.That has all been in service for several years.

Correct on the antifreeze in the makeup air coil,and yes it is intermittent,once or twice a week for 3-4 hours at a time.Works great even in sub-zero temps but sure takes the wood!

That high temp load pump only runs when the make up air coil is running,the shop infloor pump works alone.So when the infloor pump kicks on,it will draw from the lower portion of the buffer,and when the make up coil runs it will draw from the top of the buffer. When they are both on at the same time(very little) I guess it will be a free for all.

If you don't want to add so many ports to the tank, you can "header" the inputs into the tank. That header should be sized to the max. flow rate of all inputs running, with a 2 fps velocity to keep all the circs happy. This schematic is from Idronics 15.

Somewhere I have a drawing of how to connect loads so they can be supplied without needing to first heat the entire buffer. Basically on the load side treat the distribution as a parallel circuit.

 

[Bob Rohr]

This drawing shows how to provide boiler return protection with two circs, the next revision to my system. I use a thermostatic mix block now, with delta T, ECM pumps sitting around my shop, I'd like to try a pumped solution for next season.

 

[mr.fixit]

Wouldn't be a big deal to weld in as many fittings as needed,other than insulating around all of them. Might be a good use of one of those diy foam kits with the 2 tanks.

 

[velvetfoot]

I wonder if an indirect water tank could be used as the mixing tank and the coil used to provide domestic hot water?

 

[ewdudley]

Storage is right by the boilers,so 5-6ft. It only has one fitting tap on the bottom,and I was in a hurry when I first put in the system and didn't add any.(dumb)
I was only planning on redoing the near boiler plumbing,didn't really want to mess with the plumbing in the shop buildings.

The wood boiler should feed storage directly, using the near boiler loop same as ever. Then the 'storage pump' loop goes the opposite direction to bring heat to the buffer/separator as required. Otherwise you would need two pumps on the 'storage pump' loop, one going either direction and you would need controls to figure out when to pump from buffer to storage, storage to buffer, or neither, which wouldn't be pretty.

Having only one connection to the storage tank top or bottom is not a problem at all. The lines to and from the buffer/separator can tee in close to the storage tank and it will work right.

That high temp load pump only runs when the make up air coil is running,the shop infloor pump works alone.So when the infloor pump kicks on,it will draw from the lower portion of the buffer,and when the make up coil runs it will draw from the top of the buffer. When they are both on at the same time(very little) I guess it will be a free for all.

I think I see now, pretty neat. I wasn't appreciating the part about the in-floor zones pulling from the bottom of the buffer. Taking that into account all the zone valves seem like they're needed.

 

[Bob Rohr]

I wonder if an indirect water tank could be used as the mixing tank and the coil used to provide domestic hot water?

The coil in the indirect would need to be copper or stainless for DHW. And it takes a LOT of coil to generate DHW from a tank. I think the smallest reverse indirect from TurboMax has 20 or so square feet of HX surface, maybe 3 copper coils. these make nice buffer and DSHW generators.

Unless you just want some pre-heat, a typical single coil indirect would not exchange much..

Much better performance with a plate HX external to the tank and a small circ pump.

 

[mr.fixit]

The separator tank, properly built adds excellent, dirt, air, and possibly magnetic separation, so a true 4 in 1 device

Do you find that the magnetic separator catch much? Is it like the magnets they put in the oil pans of automatic transmissions? They always are loaded with fine iron particles.

The wood boiler should feed storage directly, using the near boiler loop same as ever. Then the 'storage pump' loop goes the opposite direction to bring heat to the buffer/separator as required. Otherwise you would need two pumps on the 'storage pump' loop, one going either direction and you would need controls to figure out when to pump from buffer to storage, storage to buffer, or neither, which wouldn't be pretty.

Yea I am seeing that now. Leaving the wood boiler as is will make things easier anyways.

 

[Bob Rohr]

Do you find that the magnetic separator catch much? Is it like the magnets they put in the oil pans of automatic transmissions? They always are loaded with fine iron particles

The magnetic separators add a lot of value if you are using or considering ECM circs. We have been getting more and more feedback on the amount of particles the mags are pulling out of systems. It's not a difficult addition, on a steel vessel, just add a brass dry well and slide a magnet up inside it.

 

[Karl_northwind]

Ken, I'd love to see your setup. Mind if I look you up if I'm in the area?
karl

 

[mr.fixit]

Ken, I'd love to see your setup. Mind if I look you up if I'm in the area?
karl

Sure anytime. I am here most everyday unless I am out doing an install.

 

[mr.fixit]

Well after thinking about this a bit,I may just uncover the existing 500gal. tank and add some more ports to the side of it. Seems it would be much easier and simpler than adding a separate hyd separator tank,that would need to be insulated also. It would be easy(well fairly easy) to gain access to one side of the horizontal tank,and drill and weld the fittings. It has plenty of ports on the top already. Down side is the tank has to be drained and refilled.

What I am thinking is having the pellet boiler return draw from somewhere near the top of the tank like EW suggested,so it only works with a small amount of water.

In some of the diagrams in the Biomass-Hydronics pdf sticky they show a "upside down"dip tube where the aux. boiler draws from. Anyone know the reasoning behind this vs. just putting the port in the tank higher?