Some thoughts on a new storage setup

[mousebndr]

Hi everyone,

We are in country Australia and there are not many heater people around we can talk to so hoping you all might have a few ideas.

We currently have an open system with a small 400L tank on the roof for DHW. We want to increase this to around 5000L to use as storage so we dont need to feed the fire all day.

Question is, we would like to thermosyphon into the tank, is this realistic? Looking at installing a Seton W130 since we can build one pretty easily, so the boiler will produce around 130k btu

With storage our idea was to have a non-pressurized tank with a heat exchanger for the pressurized heating loop and a second hx for the DHW. What sort of hx would people recommend? Just a 3/4 copper tube spiral or are there better options?

Thanks all

David

 

[ewdudley]

We currently have an open system with a small 400L tank on the roof for DHW. We want to increase this to around 5000L to use as storage so we dont need to feed the fire all day.

Question is, we would like to thermosyphon into the tank, is this realistic? Looking at installing a Seton W130 since we can build one pretty easily, so the boiler will produce around 130k btu

With storage our idea was to have a non-pressurized tank with a heat exchanger for the pressurized heating loop and a second hx for the DHW. What sort of hx would people recommend? Just a 3/4 copper tube spiral or are there better options?

Sounds like your storage tank will be the highest point in the system, so I don't see why you need the HX between the boiler circuit and the storage tank. All you should need is an expansion cistern above the storage tank and you'd be all set.

If the tank is six or eight meters above the boiler you'd have a lot of pressure difference to drive a thermo-siphon. If you ran the numbers I wouldn't be surprised if 30-35 mm pipe was big enough. Getting a mixer valve situated correctly to provide boiler return temperature protection might take a little thought, but there's no reason a Danfoss or a Termovar wouldn't work in a thermo-siphon system.

--ewd

 

[benjamin]

Welcome!

Please give us a little more information on your current system and needs. Australia is a big place and we don't have a clue what your heating needs are? or what your current heating system is? or what you plan to add along with the storage? SOLAR???

Thermosyphoning is very realistic, just make sure every part of that pipe circuit is designed with thermosyphoning in mind, ie it all slopes correctly to eliminate air and maximize flow, the pipes are big enough, and the turns are minimized.

http://www.heatinghelp.com/article-categories/332/Gravity-Hot-Water-Heating
This is an excellent site to learn more about hot water heat. If you understand all of this then you'll have no problem.

builditsolar.com is a great site all around, but especially for do it yourself solar systems with open storage tanks.

Just taking a wild guess here without more info, but I'd suggest that a much smaller tank will be more effective for domestic hot water, maybe 1-2,000 liter, to reduce the standby losses and the time it takes to get it up to temperature.

 

[jimbom]

"A pint's a pound the world around." ~ 5000L ~ 10,000 pounds. I am sure you have thought of this, but it was the first thought I had. I have wanted to thermosiphon, but the structural always puts me off.

 

[mousebndr]

Too right... our system is as follows.

We are in what is known as the central tablelands, on the mountain range between the east coast the the plains. 1000m above sea, temps in winter hover around 5C during the day and -2C at night. So no where near as cold as some of you folks get. In the shoulder we see around 12C in the day and 3C at night.

Our system has 21 radiators combined total of just above 40kw. Then there is the DMH on top of that. With our current boiler we turn the DMH off during winter as its just too much load on the boiler.

The boiler is a real pain. We have to feed it every few hours to keep the house anywhere near 20C . Overnight it will give effective heat for a few ours then drop off. Its basically a firebox surrounded with a bladder.

So what we want to achieve is to have enough storage to last about 12 hours in peak winter and 24 hours in the shoulder. So we only start the fire once to twice day and run it flat out till the storage is up to temp.

I should add, the current tank is one the roof of the house but we cant put 5000L up there without someone getting squished. The new tank would be located to the a garage next to the house (fully enclosed) with the boiler directly under it. I would say we would get about 1.5-2m clearance from the top of the boiler to the bottom of the tank.

Thanks heaps for your help.

 

[jimbom]

What I can calculate with the givens is you need about 14 GPM at a delta temperature of 20 °F to move 140,000BTU/hour. If your tank is five feet above the boiler, the driving pressure would be about 0.04 psi at a this difference of 20 °F. Once you know the pressure losses in your valves, els, and connections, you can figure the pipe diameter that would flow 14GPM at this pressure. It is do able, but the devil is in the details at such low driving forces.

If you have 180 °F out and 160 °F return on a 5' straight run up and back with no valves or restrictions; 14 GPM will carry 140,000 Btu/hour with normal size pipe. Of course, you need to get a qualified local to give you the exact size for the conditions that will exist after all the complications that arise in any real job. Getting engineering off the internet is not recommended.

 

[mousebndr]

What I can calculate with the givens is you need about 14 GPM at a delta temperature of 20 °F to move 140,000BTU/hour. If your tank is five feet above the boiler, the driving pressure would be about 0.04 psi at a this difference of 20 °F. Once you know the pressure losses in your valves, els, and connections, you can figure the pipe diameter that would flow 14GPM at this pressure. It is do able, but the devil is in the details at such low driving forces.

If you have 180 °F out and 160 °F return on a 5' straight run up and back with no valves or restrictions; 14 GPM will carry 140,000 Btu/hour with normal size pipe. Of course, you need to get a qualified local to give you the exact size for the conditions that will exist after all the complications that arise in any real job. Getting engineering off the internet is not recommended.

Whats normal pipe size?

 

[ewdudley]

...our system is as follows.

...

Our system has 21 radiators combined total of just above 40kw.
...

So what we want to achieve is to have enough storage to last about 12 hours in peak winter and 24 hours in the shoulder. So we only start the fire once to twice day and run it flat out till the storage is up to temp.

With a Seton 130 putting out 38 kW and a load of 40 kW you wouldn't need any storage at all.

If your boiler had enough output to put out 12 hours worth of heat (480 kWh) in four or five hours, then you'd need enough storage for seven or eight hours (~300 kWh). With as much as 20 K deltaT in your storage you're still looking at more than 12500 liters of storage to feed a 40 kW load.

I should add, the current tank is one the roof of the house but we cant put 5000L up there without someone getting squished. The new tank would be located to the a garage next to the house (fully enclosed) with the boiler directly under it. I would say we would get about 1.5-2m clearance from the top of the boiler to the bottom of the tank.

To do what you say you want to do, it sounds like you need a boiler on the order of 2.5 times as big as a Seton 130 and storage about 2.5 times as large as well. If 'normal' pipe is up to DN50, then you'd probably need pipe larger than that for thermo-siphon.

Thermo-siphon or not, I would think you'd want to feed storage directly from the boiler, even if storage is unpressurized, and then you can use smaller heat exchangers to pull heat out for your loads.
--ewd

 

[jimbom]

...

Whats normal pipe size?

Something you could buy off the shelf. Size would be dependent on the actual layout, fittings, boiler tap options, etc. I think it is do able, but the devil is in the details. A 14 gpm pump at this head would be relatively inexpensive.