Conventional Boiler Protection

Santilal

Hi

I have just been shown how to do boiler protection with a two-way thermostatic valve. I have not seen a two way valve used for boiler protection in any of the literature I have read. Can anyone see why it would not work or what the drawbacks of this method are. If it does work why is this not commonly used - it looks cheaper than using a TMV. See attached drawing (I'm sorry it is not very clear, but you should be able to see how it is used). It's called a Bypass Valve in the drawing.

Thanks.
Santilal

Santilal

Hi

No, I don't think it's a cast iron boiler. It is a New Zealand made steel boiler, but little other technical information is available. It is supposed to deliver 30kW for a temperature differential of 15*C, so I think the flow will be about 30LPM.

Cheers.
Santilal

Santilal

Hi

Now I am a bit confused. I thought the purpose of the valve in this example and a TVM in the examples you see in the text books was to keep the inlet temperature to the boiler above 60*C, to prevent condensation. Without a valve the inlet temperature could be anything. This boiler is a coal boiler by the way.

Thanks.
Santilal

Santilal

Yes, I now understand. This seems such a simple approach and I am not sure why I haven't seen it before in my reading. So in effect, it doesn't matter what the return temperature actually is because the aquastat will just open, stopping the circ. The flow stops and the stationary water in the boiler will quickly reach a higher temperature. Once it is hot enough, the circ will start to move it again.

You said that you only need a valve if the flow through the boiler needs to be continuously maintained. What types of boilers need to have continuous flow. I am an amateur just trying to learn about these systems to repair a system that was poorly installed and is not working very well. Thanks for the input.
Cheers,
Santilal

Santilal

Hi
I have just found that the boiler capacity is about 34 litres. I presume that 34 litres is enough mass?

Cheers.

Santilal

Zman

If you look at his design, http://forum.heatinghelp.com/discussion/154857/any-comments-appreciated#latest he has a significant amount of in slab radiant.
I would argue that when that slab is cold and calls for heat, it can and will drag the boiler down below the condensing point for an entire heating cycle.
If the system was all low mass radiant panels, I would not be concerned.
I would recommend some type of condensing protection on this system.
Carl

Jason_13

This is know as a system bypass which many US company's did away with when cast iron boilers went to lower input and cold start. This idea was talked about many times in the 50's by Gil Carlson from B&G. He later penned a similar idea named a boiler bypass. It works opposite the system bypass.
The system bypass will allow hot supply water to blend with cool return water to try to pre-heat the return to try to get above 140f within a reasonable amount of time. The problem is how long does it take? It also reduces the system flow rate that will slightly reduce the radiation output. The problem with this type of bypass is large water volume systems. How long until the return temps get above condensing temps.
The change to a boiler bypass takes cool return and adds it to hot supply water thus reducing the flow through the boiler so water temperature picks up faster. With the use of a boiler bypass the return water temperature is less important than with a system bypass. Reducing the flow in the boiler allows the water to stay in the boiler longer thus more temperature rise. With a lower supply temperature to the system a boiler bypass also acts like a poor mans OD reset.
Gil Carlson stated in a white paper discussing flue gas condensation and thermal stress, problems exist with extremely low return temperatures or cool water at a high flow rate.
I am not fond of the idea of a control stopping and starting the pump. The more you quickly change temperature in a cast iron boiler the better chance of a nipple or gasket leak you experience or even thermal stress cracks. The taller the section the better chance of the stress cracks.

Zman

Hatterasguy said:

Jason said:

I am not fond of the idea of a control stopping and starting the pump. The more you quickly change temperature in a cast iron boiler the better chance of a nipple or gasket leak you experience or even thermal stress cracks. The taller the section the better chance of the stress cracks.

When the pump stops, the boiler temperature is permitted to climb. Once it climbs by 5 degrees, the pump restarts. The process repeats until the RWT is above 140°F.

An aquastat set to 140°F with a 5 degree differential performs the task.

There won't be any nipple leaks..........gasket leaks...........or thermal stress cracks.

I guess this approach would all depend on the location and lag time of the sensor. Personally I don't prefer it.
The OP has been working on this for over 2 years and has received tons of good advise.
The simple design we discussed a year ago is still better than
this recent one.
I think he is working on redesigning the wheel....
IMHO
Carl

Harvey Ramer

Hands down, best boiler protection you can get is with a caleffi thermomix or similar device. Simple and extremely effective. Not talked about much but it can also enhance and maintain system efficiency by a decent margin.

hot_rod

The best way to protect for return temperature is to have a valve or injection system that is operated by temperature.

All those manual bypass valves or pumps are just a guess if they cannot sense and react to temperature at the boiler return.

The 3 way thermostatic is the most simple, no power required, no programing. If you get a high flow (high Cv) they don't cost much pumping power.

Mark Eatherton

Having worked on many cast iron boilers in low temp applications, and having seen both aquastat protection versus anti condensing valve protection, I can tell you that as HR said, the anti condensing valves are far and away worth the time and money to install.

Have pictures somewhere showing the difference. The aquastat did show a significant amount of condensation, whereas the anti sweating valve show absolutely NO signs of condensation.