Coil versus exchanger tank for baseboard loop

jimb245

I'm needing to replace a very old steam boiler with something like a McLain SGO Series 3 131 MBH. The old system is wimpy on the baseboard loop output because it uses an unsubmerged second coil and is undersized for its load (1/2" ilne, 48ft of radiator, 800 sqft in NH). I'm hoping that having a proper coil in the new system helps a bit, but just wondering if there is any reason to use an external heat exchanger instead, since it would probably cost more? There is already an electric storage tank for domestic hot water so it doesn't necessarily need a coil to preheat.

dopey27177

No Coil is needed.

Tried and true hot water loop for base board heating from a steam boiler.

See attachment

Jake

Jamie Hall

If you use the concept with no heat exchanger, honestly... good luck. I wouldn't, if I were doing it.

mattmia2

Without an HX, the loop is essentially an open system and needs components designed for an open system, correct?

Jamie Hall

Correct

STEAM DOCTOR

@jimb245. Is the baseboard above or below the boiler? If above, then use a coil. I have successfully used the other method but the coil is close to foolproof. 

dopey27177

The use of a coil is not needed. extra bucks for a closed lop system.

When you look at the enclosure sent you will see that after the system is charged you have a closed loop system. The only time the system is n open loop is when the boiler is drained and emptied at that juncture you will have to recharge the system again.

I installed more than 20 plus of these systems in my 50 plus years in the business.

A heat exchanger only adds a big cost to the new installed system.

What you really need to do is send us pictures of the boiler piping, steam main and return to ascertain if you have a piping problem that affects the operation off the one pipe stem system.

Jake

Jamie Hall

I give up. But don't ask me to install or approve an open system of this type.

mattmia2

A tankless coil is a great way to achieve the separation...

hot_rod

seems the elevation to the upper most radiator or fin tube above the water line X .433 put that upper system under a negative pressure, at least when circulator is not running, so then boiling point is determined by that negative pressure?
This vapor pressure graph shows that any condition below the curve, water boils.

Not sure if a steam boiler is considered an open system like the outdoor wood furnaces which have a vent hole in them

mattmia2

The crud that collects in the returns would tend to indicate that it is an open system.

Isn't there a cautionary tale that Dan wrote about the condensate boiling in one of these.

Jamie Hall

There is one minor consideration with systems of the sort being advocated by some here: the pump. If the vacuum is broken in the elevated portion of the system -- and it will be -- then the pump for the system must be chosen to be able to develop enough head to refill the system, not simply to circulate the water around a closed system as we usually do. For example -- the Taco 007e, a rather common pump these days, has a shutoff head of 10 feet. It will not be able to fill the system if the highest point is more than 10 feet above the boiler water line -- i.e. anything but a one story and basement ranch. A pump with a greater shutoff head must be provided. This is unlikely to be the best pump for circulating...

dopey27177

To Jamie Hall

Unlike a regular hot water system this system will not lose its prime as long as the water level in the boiler does not drop below the feed and return connections to the boiler.

Note the bypass valve and the thermometers.

The by pass valve is to be opened until the water temperature leaving the boiler is adjusted to 180 degrees f.

If you do not believe what I am saying check with Dan's writings on this system.

I for one never used more than 3/4" pipe or installed this in a system requiring more than 40,000 BTUH.

Jake

Jamie Hall

optimist

ethicalpaul

If the vacuum is broken in the elevated portion of the system -- and it will be -- then the pump for the system must be chosen to be able to develop enough head to refill the system

In Dan's article I linked above, it explains that you can fill the system with a garden hose if your pump can't do it.

What will break the vacuum (allow air in) to the elevated system? Do you mean like a valve or pipe failure, or something more common?

Jamie Hall

ethicalpaul said:

If the vacuum is broken in the elevated portion of the system -- and it will be -- then the pump for the system must be chosen to be able to develop enough head to refill the system

In Dan's article I linked above, it explains that you can fill the system with a garden hose if your pump can't do it.

What will break the vacuum (allow air in) to the elevated system? Do you mean like a valve or pipe failure, or something more common?

Pick your poison. First -- and perhaps most important -- remember that it is common for leaks to be selective: that is, they may appear when subject to a vacuum, but not under pressure (even quite high pressure) or vice versa. This makes it much more interesting. However, to directly answer -- the problem could be from a variety of things. Most common would be a valve packing. Another could be a valve seat, such as a bleeder -- but to be fair, those are usually selective the other way: they will leak under pressure, if at all, but nut under a vacuum (one of the more common devils would be a bleeder which isn't (or can't be) quite shut tight). Small leaks in threaded fittings are usually visible and usually aren't selective -- but it takes much less of a leak to cause a loss of prime than it does to cause a puddle on the floor. Many types of fittings which depend on rubber gaskets or o-rings are only intended to really seal one way -- but those are rare in heating and plumbing (nut uncommon, though in hydraulics or fuel supply).

Outright failure of something would be much rarer -- and more obvious.

ethicalpaul

Thanks. It would be really annoying to have an invisible negative-pressure leak slowly adding air to the top of the loop!

dopey27177

Typically the piping for that system is type L copper with soldered or brazed joints.

Before the system is put into operation a pressure test is commonly done to assure there are no leaks in the system.

The two ball valves used for a shut off must be at the lowest level of the boiler section. These valves can be shut off at the end of the heating season, this assures that the prime or columb of water does not drain.

Additionally, these valves must be shut off when the boiler is drained for service.

System installed is much cheaper than using heat exchangers and coils.

Jake

jumper

Jamie Hall said:

I give up. But don't ask me to install or approve an open system of this type.

Agree with Jamie. The suction head available is steam pressure plus height of water in boiler above pump. You need special low NPSH pump. Coil is far less expensive and more energy efficient.
Unless you have a fifteen foot deep pit to locate circ? If you insist on open system talk to a cooling tower rep about NPSH. Or a fountain guy.

Even in closed systems the circulator requires pressure. We see rooftop pumps and heated slabs work for years. Then for whatever reason pressure is lost and pump destroys itself.