HHW boiler modulating burner control 2

[kico]

Heating hot water system has a boiler with modulating burner control and a 3-way valve on supply pipe to reset bldg supply water temp based on outside air. I have two questions:

1. My understanding is that the conventional way of controlling the burner,whether there is a 3-way valve or not, is by leaving boiler supply water temp; is this correct? The contractor is using return water temp to modulate the burner.
2. The boiler keeps shutting down on hi-temp limit. Could this be happening because the 3-way valve bypasses so much water around the boiler that the flow switch opens up shutting down the boiler?

Thanks for your help

 

[imok2]

It sounds to me that when the valve goes to bypass the boiles becomes a closed vessel with a lot of residual heat( no water circulating) heats up and opens the hi limit. Sounds to me like the control is out of sync with the heat load or the boiler is too big for the application.

 

[Cooky]

You should not control the water temperature supplying the building by altering the boiler supply temperature.

The boiler must operate above a minimum temperature to avoid the problem of the flue gases condensing within the heat exchanger and boiler flue system. If the flue gases do condense, the result is hydrochloric acid, this will result in rapid failure of the boiler heat exchanger and the flue system.

To avoid this occouring, the boiler should be operated at a higher temperature. If you wish to reset the heating water circulating temperature, then you use the 3-way valve arrangement. Ideally, you should create primary and secondary water circuits.

Cheers,

Cooky

 

[ChrisConley]

I agree with Cooky, except that for the most part condensed flue gases form water and carbon dioxide. Which can then form carbonic acid, not hydrochloric.

 

[quark]

Third acid is coming into picture. The main concern in boiler flue is SO₂. It's condensation point is relatively high thus forming H₂SO₄. Generally the lowest flue gas temperature is maintained depending on the sulphur content in the fuel. If your flue temperature is not below 200⁰C (more precisely 180⁰C corresponding to a maximum value of 4.5% sulphur) you shouldn't have any problems.


Kico!

I think the control scheme you already have, seems to be ok except that your boiler capacity is high as noted by Imok.

Regards,

 

[TFurst]

Keep in mind that the high limit control on a hot water boiler is mounted on the outlet side of the boiler and if you are controlling the burner cycle and modulation from return water your setpoint needs to be offset to account for the design temperature rise through the boiler at the minimum firing rate. Based on over 25 yrs working on boilers and boiler plants with the 3 way valve resetting the loop temperature I have found that the boiler will behave much better if the boiler is controlled based on leaving water temperature. As an added safety the control scheme can be modified to use the 3 way valve to maintain a minimum return water temperature. As a general rule for natural gas the minimum temp is approx 140 deg F and for fuel oil about 160 deg F.

Hope this helps a bit
Tony