high limit and return temp

Alan(CaliforniaRadiant)Forbes

does it?

I'm not familiar with that boiler, but those are good questions for your boiler rep. or the manufacturer.

Weil-McLain (219)879-6561

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harald

high limit and return temp

I have a simple question about the high limit setting and return temp in my hot water gravity conversion system. The boiler is a Weil-Mclain CGA-3 with a simple system bypass arrangement. The manual says that the return temp should never drop below 130 degrees. The boiler arrived with the high limit set at 140 degrees with a 30 degree differential. For a return temp of at least 130, then does this mean that the high limit should be increased to at least 160?

Also, I find it hard to imagine how a return temp of 130 can be achieved in the spring and fall when the boiler water itself may drop to 70 degrees during the long gaps between runs.

Any information would be welcome.

Plumbob

probably no problem

The return can drop below 130; it does that in every cast-iron boiler at startup (as you said). This may happen between runs at most times, not just in spring/fall.

However, the real rule is not as stated in the manual; it is that the return MUST GO ABOVE 130 at some point in each cycle. This boils off the condensate that would otherwise corrode your heat exchanger.

Now for your boiler, I don't know if the aquastat senses supply or return. if return, which is what I would guess, it means the return does get to 140 at the top of the cycle, so you are OK. What it does is heat the water to 140 (return), then go off until it cools to 110, then starts heating again. This assumes the thermostat is constantly calling for heat during this time.

It is also sort of reasonable that the dial is designed not to go down to an unsafe temperature. I suspect that 140 is the minimum temperature on that dial.

Mike T., Swampeast MO

Some Things to Consider

The low temp "warning" is mainly about damage from condensation in flue gasses causing corrosion.

There are still MANY gravity conversions, most of which I daresay are using a cast iron atmospheric boiler--frequently the ORIGINAL even--long since converted from solid fuel.

Many of these systems have no form of bypass (your system bypass is there to at least help the boiler to get up to operating temperature).

Some of these systems have been using outdoor reset for decades--in these even the supply temperature will be below 130 degrees a significant amount of the time.

A few have TRVs added to the radiators--in many? most? of these cases, the return temperature will never exceed 130 degrees.

For years, I've asked for photos (and anecdotes) regarding condensation damage in gravity conversions with cast iron atmospheric boilers. Unless everyone is hiding them for unknown reasons they don't exist--even in the flue piping--let alone the boiler.

In my area, EVERY boiler I have seen has the aquastat setting at 140 degrees or below. They usually have no bypass and the thermostat controls both the boiler and burner in concert--a sort of "indoor" reset. The 140 degrees essentially becomes a safety limit!

All this immunity seems to come at a cost--efficiency.

Once your new boiler dies (it may take many decades) or once fuel becomes so expensive that you feel you must take action, use a condensing, modulating boiler and TRVs.

harald

high limit and return temp

Thanks, RPG for your help.

The aquastat is plugged in to the boiler just below the supply outlet. And yes, 140 degrees is the lowest setting.

What would a reasonable high limit setting be? Does it depend on how cold it gets outside (i.e. how much heat is demanded from the system)?

I am under the impression that the system bypass setup is to prevent cold shock to the boiler. To prevent cold shock I imagine that the temperature differential between supply and return is important and should be within some limit. What should that differential normally be?

Plumbob

?

> The aquastat is plugged in to the boiler just below the supply outlet.


I'm not sure what "plugged into the boiler" means. Is it sensing the supply or the return temperature? This may not be obvious from its position if it uses a remote sensor. If supply, I guess it is possible for the return to be below 130 at all times. But your system bypass probably reduces or eliminates the chance of that. I'm not really sure.

Older boilers had thick heat exchangers and could take many years of abuse; newer boilers have flimsy ones, which is better for efficiency, but they will fail more easily if the return stays below 130.

>

> What would a reasonable high limit

> setting be? Does it depend on how cold it gets

> outside (i.e. how much heat is demanded from the

> system)?


Yes, and also how much radiator capacity ("radiation") you have. If you have lots of radiators (many older homes have an excess because they were uninsulated and had leaky windows when the radiators were originally put in, but were upgraded later), then you can manage with 140 water all winter, and that is better for efficiency and comfort (it won't overheat as much). But if your newer house has only enough radiation to heat the house on a very cold day if there is 180F water, then, when it gets cold you'll have to turn it up to 180F manually. An electronic device that varies water temp automatically based on outdoor temp is called an outdoor reset controller.
>

> I am under the impression that the

> system bypass setup is to prevent cold shock to

> the boiler. To prevent cold shock I imagine that

> the temperature differential between supply and

> return is important and should be within some

> limit. What should that differential normally

> be?


Don't know.

Gordy

I'am only sharing my experience as a homeowner highly interested in radiant. I have a WM CGM 7 which has a simple bypass piping arrangement into a mixing valve. My radiant is low temp ceiling 120* max. The return temps never get above 95*. Once the call for heat starts and the boiler gets past the 130* critical temp for condesation. The boiler should refire at 140* and then shut down at what ever your high limit setting is and cycle until the T-stat is satisfied.I'm under the impression that the shorter the time to get the boiler up past this 130* mark the better off you are. I also read in the manual that a DT of no more than 50* is recommended.

Finnie

Boiler to hot???????? makeing steam

Have oil fired hot water boiler for home heat.High limit 180 low 160 diff 15.The boiler is heating itself up over 200 at time's an making steam which in turn is opening pressure valve.

I have then let is cool down release all the pressure an then I have had to open water valve manually ,which is suppose to be auto valve an then I refilled boiler bled all air out an tried it again,and after a week or 2 of operation it does this again?????

 Could it be my auto water valve not refilling the boiler as it is being used,does a boiler use that much water,or could it be my controlor ,which is expensive as it was just replaced a year or 2 ago????

Thank you for the help.