Flow rates through a mod/con

Paul Rohrs_5

I want to apply higher flow rates to a mod/con boiler.

If I couple a mod/con boiler with an established BTU output, but double the flow rate, I will reduce my delta T(with the same BTU output of the boiler) thru the boiler. So with that in mind, shouldn’t the boiler recognize that and modulate down even farther as it only has to meet a the programmed Delta T ? It seems the opposite could be true if the fixed output of the secondary zones limit my boiler and make the boiler increase the firing rate because it thinks the reduced Delta T = a higher secondary heat loss?

If the 1st premise is true, I will increase the flow rate with a higher head pump which will equate to more electrical consumption, but less gas consumption because of the down-firing and modulation. The boiler can afford to “artificially down-fire” because it will still want to operate off of the least Delta T possible. (?)

It seems we should be able to amortize out the potential gas fuel savings for the increased electrical consumption. Or….we can use zone valves on this and come out ahead across the board.

Is there any rhyme to my reasoning or am I overlooking something? This would be a great module in Siggy's HDS software to demo how Mod/cons will react to flow and Delta T scenario's

Regards,

Paul


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S Ebels

ummmm ahhh

If you extrapolate that logic, you could zing 1000GPM through the boiler and the burner would hardly fire at all. Got to be another side of the coin somewhere. Btu out = Btu in.

Input is a function of the btu's lost out in the system which the boiler will try to make up regardless of the combination of flow and temp differential.

Floyd

I'm struggling to remember....

but somewhere in one of my conversations with a WM tech. I belive that they told me that they actually had a logic programmed into the brains.....and that the DHW logic and the space heating logic were not the same. I think it was when I complainined to them that the boiler modulated better when making hot water than it did when heating. Can't remember exactly what the excuse was but something about they thought that the DHW flow rates were likely to be more predictable than the heating, didn't really try to think it though, then but it may apply here. I do know that if you take a boiler that is supposed to runan 0011 and run say a 007, it will short cycle like crazy due to not getting enough BTU's out of it fast enough. Now if you throw an 0014 on that boiler, like one that I checked out htis morning. A WM 155 that had a 0014 on it instead of the normal 0011. Hooked the 'puter to it. Could'nt notice any difference...it ran just fine, made a few reset adjustments and set the differentials a bit different to give it more time to mod. down when it overshot if a large zone shut down.
Guess what I'm tring to say is that I think your over thinking this a bit. Even if your runing the train at 100 mph and only getting half the people on the train that you would at 50 mph, your only going to get rid of the amount of people that can jump off at the intersection..(closely spaced tee's)
The WM mod/con that I'm familiar with not only checks the delta T, but also the difference + or - from setpoint, rate of increase or decrease in water temp. and incorporates that all into the brain (logic) and controls the firing rate from the information.

I may be all wet... but that's how I've got it figured.

Floyd

Marty

It will ramp up

Assuming primary secondary piping,,Double the flow rate through the boiler and it will go to a higher firing rate.

Marty

Backwards

I would think It would ramp right up tossing fistfulls of btu's at all that water flying through the boiler. Slowing the water down would increase the Delta T through the boiler and it will modulate down. Sorta backwards from higher flow for more capacity on whatever is putting the heat into the space.

I don't know exactly how they modulate but it has to have the target supply temp in the algorithim somewhere.