radiators for domestic heating - thermosyphons

[MrReds]

Hi everybody !

I am contacting you concerning this issue:
Is there, according to your knowledge, ad advised water velocity in a radiator used for domestic heating - the thermosyphons -?

Should it be high, so that we get an higher internal heat transfer coefficient, or low, so that it would have time to exchange heat with air ? Are there any reference values ?
But in this second case the length of the path would affect this value or not ?

Is there any useful link where to look into this subject ?
In particular, which are the equations regulating the phenomena of heat exchange of a radiator ?

Thanks !

 

[willard3]

What kind of system are you envisioning, a pumped system or a thermosyphoning one?

Pumped systems generally stay at or above 3 ft/sec to keep the film coefficient down in finned radiation.

Cast Iron radiators operate at very low water velocities, ie, you only need finite velocities.

 

[MrReds]

Sorry for not being precise.
I am referring to thermosyphoning utilized for domestic heating.
That is:
you have a pump that makes hot water pass into tubes.
Outside of this tubes, there is air that is made warmer by the heat of the water into the tubes.
It is what is generally used in houses.

Hope this is more clear.

 

[chicopee]

I do not think that you are using the term thermosyphoning properly. The term that you are referring is mostly used with convective heat transfer of water within lets say a firetube boiler You are describing convective heat transfer when air is traveling between the fins to pick up heat for the rooms and while there is thermosyphoning within the heated environment it is not really used in such application.
As far as the velocity of hot water thru piping it will be based on the type of baseboard radiators. If you check some of the most current literature from manufacturers, their specifications will be either based on 1 or 4 gallons per minutes of hot water from which you can easily calculate the velocity that you require.

 

[moltenmetal]

Old (pre 1950s in North America) home heating systems used density-driven (thermosiphon) circulation between a "boiler" in the basement and cast-iron radiators in the upper floors. Flows in these systems were very low indeed.

Finned tube baseboard systems came later and did not work via thermosiphon.

Thermosiphon systems are out-moded and NEVER used in new construction. You can buy a wet rotor circulator pump for $100 which uses 1/12 hp or less, and which will produce a water circulation rate far higher than you'd get by density-driven (thermosiphon) circulation alone. The benefit of higher flow is that it will drive down the water temperature you need to feed to your radiators to match your home's heat loss, and in so doing will improve the thermal efficiency of your boiler. The circulator permits the use of water supply/return lines which are far smaller in diameter than required for thermosiphon circulation.

What are the velocities due to density-driven circulation in one of the old systems? REALLY LOW- less than 1 ft/s. The effective head difference between hot and cold is only a few inches of water column. Hence you need very large lines to permit adequate circulation rates: in my home, the individual radiator drop lines were 1" NPS and the headers were up to 2". Return lines were larger than supplies for obvious reasons. With the circulator, these lines have been replaced with 1/2" ID O2-barrier PEX tubing...

Want design info for the old systems? Check out

http://www.heatinghelp.com

. Their forum is excellent for this stuff.

 

[chicopee]

Moltenmetal,are you referring to steam or hot water heating for pre-1950's?

 

[moltenmetal]

In old homes you can find two-pipe steam/condensate systems, one pipe steam systems (ie. steam is supplied and condensate returns via a single pipe), and hot water systems.

My own house (built in the 1930s) had a coal-fired hot water thermosiphon system, which was standard for Ontario homes of that era. The "boiler" was just a coal firebox with a water jacket, later retrofit with a natural gas "conversion burner"- terribly thermally inefficient.

Steam was more common for larger homes and institutional buildings, churches etc.

 

[willard3]

I will now advocate for thermosiphoning systems.....if you loose elec power, you can still have heat. Not true with pumped systems.

I design wood-burning thermosiphoning water systems is rural areas with frequent power failures. Not needing electricity for heating is an advantage.

 

[moltenmetal]

So is having a heat source that's cheap, or free...to the point where thermal efficiency is a secondary consideration...

Sure, my old thermosiphon system was immune to power outages. But it was ~40% less thermally efficient than the pumped system with modulating condensing boiler that I've got now. The 30% fuel savings would pay for a home backup generator right quick. In my case, a $100 deep-cycle battery and a $30 inverter will keep my boiler and 1/12 hp main circulator running for a very long time indeed.

 

[willard3]

Home generators in the 5-7.5KW range (common for residences) from $1000-3000 and you will need a transfer switch for $1K to 3K. Lets go low and say $3000 for a generator/transfer and this doesn't count the 1-2 gal/hr fuel cost ($5-10/hr)or nasty noise and fumes.

I can buy a lot of pipe in a residence for $3k and, if you check old ASHVAE volumes, system efficiency of pumped systems is not that much higher than siphoning systems, certainly not 30%. Also, the cost of elec for running the pump is not inconsiderable.

 

[moltenmetal]

The 30-40% efficiency gain I stated was relative to an archaic coal boiler fitted with a gas "conversion burner".

There's no need for a whole home UPS just to keep the furnace running. True, my house could freeze if nobody were there and we lost power for a couple of days, but the same would be true without someone to stoke your wood-fired unit.

If you're going to heat with wood, we agree that you're unlikely to lose much efficiency going thermosiphon versus going with pumped circulation: I would imagine you can't let your wall temperatures fall too low in your wood-fired unit anyway so your thermal efficiency is limited by that fact. The same is not true of a natural gas-fired system, where you can take advantage of both load-matching modulation and condensation.