Heat exchanger

Bill_39

to use a heat eschanger for a hot water loop off of a steam boiler? I have to replace a boiler with a hot water zone that is on an external tankless. I was going to get rid of the HX and just pipe it off of the boiler directly unless for any reason it is better to use a HX. The loop is about 22 feet higher than the boiler and I am adding a superstore.

Ken_8

The biggest problem with

using the boiler directly is the scale and rust that screws up the pump. A bronze three piece circulator type is best but will cost a fortune. Nonetheless, a wet rotor type or ferrous model will get jammed up with rust and scale on short order. Even with wye strainer we find them a pain.

I would re-use the external HX and stay away from steam boiler water.

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Tom S.

I'm potentially in the same situation with choosing to use a HX or not.

I'm just a homeowner trying to educate myself, so here is my stupid question: if you pipe it directly to the boiler, don't you use a coil inside and have a closed system for the hot water zone? I mean not using the boiler water directly, but using the heat from the boiler water to heat up the coil/closed loop for the hot water heat.

Tom

Bill_39

Bill_39

ok

Who makes a good one? I have never done a hot water zone off of a steam boiler.

Bill_39

Ken

If I use a heat exchanger I still need a pump don't I? Then it is the same situation on the boiler side

Mark Eatherton1

How to without a HX

http://www.heatinghelp.com/newsletter.cfm?Id=29

As Dan says, it pays to wander away from the Wall once in a while..

I've done numerous systems this way with excellent results.

Just make sure your boiler is not short on capcity or it may not produce steam when needed.

ME

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Bill_39

Mark

What kind of pump do you use when you do it that way?

Mark Eatherton1

Good ol' Blood and Guts

(B&G series 100 3 piece circulator)

Bullet proof, bronze to resist oxygen. Does require user maintenance. Which reminds me of a bumper sticker I want to develop.

"When was the last time you hugged your boiler???"

ME

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Bill_39

Mark

I understand why it should be bronze but why a b&g 100 instead of a regular bronze cartridge circ. I'm not arguing just wondering why?

Mark Eatherton1

Flat curve...

You shouldn't need a whole lot of head, and the B&G is a low head, flat curve , high starting torque circulator. "Old Dependable" as the old timers (wait a minute, I'm almost an old timer..) say.:-)

The reason to not use a cartridge circulator is bearing failure. The water lubricated bearings will see a lot of ferrous oxides in suspension (rust) and the bearings will fail. The B&G's bearings are not water lubed.

ME

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Bill_39

thanks mark

Mark Eatherton1

Things to consider...

Unless the new steam boiler comes equipped with an immersed, internal heat exchanger, you will still have to deal with the pump being exposed to rust. I personally don't see any advantage to the external heat exchanger route. It just adds additional cost and takes up space that is usually not available.

You will need to modify the control logic of the boiler such that if only the hot water zone calls, the boiler is limited to 180 degree F operating temperature. This will require an immersion type aquastat that watches the condensate temperature. When the steam zone calls, you need to incorporate some method of limiting the supply water temperatures to below 180 degrees F, because the upper end of the hydronic system will be running at or near sub-atmospheric conditions, which can cause water to flash to steam, and you DON'T want that happening in your hot water side of the system. Dans method of blending the cooler return water with the hotter supply water works OK for metallic pipes, but if you intend to use any plastic or composition tubing, you will need some method of guaranteeing that the supply side will not exceed 180 degrees, or your tubing may try and crawl out of its 02 barrier like a snake in molt.

Make sure that the boiler has excess capacity. This means a complete EDR survey of the radiators, and exposed, uninsulated pipes if possible. If the EDR math is close, you should consider installing TRV's on the whole system to insure even heating, and even comfort. Make sure that the boiler is also being properly fired to the loads its carrying.

Make sure that in your design, you incorporate necessary isolation valves to make filling and purging the hydronic loops a snap. Also make sure the homeowner understands the operation of the system so that he doesn't allow some unsuspecting first year heating tech to come in and undo your doings, or YOU will be the one getting the no heat call. These systems are a little more difficult to understand and operate than the typical closed loop hot water heating system.

Done properly, they can provide years of trouble free comfort. Done wrong, they create more hate and discontent for our industry than you can believe. I know, because I've had to deal with a number of them...

Good luck. Take lots of pics.

ME

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