Relation of mod con fan RPM to combustion rate

Gordan

Let me use an example to illustrate. My TT PS60 has a minimal fan speed of about 1900 RPM, which should correspond with its minimal modulation of 15 kBTU/hr. The max fan speed is about 6800 RPM, which should correspond with its full input of 60 kBTU/hr. Can one interpolate at points in between, so, say, 3200 RPM (which, coincidentally, is where it's running during our lovely cold snap of low teens here in PA) would correspond to 15000 + (60000 - 15000) * (3200 - 1900) / (6800 - 1900) = 27000 BTU/hr (roughly)? Or is there a strongly nonlinear relationship between fan speed and output? I'd suspect that it's nonlinear toward the upper end of the fan's performance range but somewhat flat below that.

Mark Eatherton

Sounds lie a good question...

for the manufacturer. My guess is that its a linear proportion, but I'm not 100% sure. Could be the dreaded Hockey Stick curve, raising its uglyhead again...:-)



ME

Gordan

Yes, had the same idea...

So, having just gotten off the phone with TT tech support, I can relay that they say that it is roughly linear. Not sure how that works for other boilers.



I may have had my top RPM wrong, though... he said 5200 (I was going from memory, don't often get to see it) which would put my input rate at 15000 + (60000 - 15000) * (3200 - 1900) / (5200 - 1900) = 33000 BTU/hr. Not bad for a house that used to have a 169000 BTU/hr input Burnham atmospheric monster.

Tim McElwain

I looked into this

a while back on several different boilers. What I found is that it is pretty much linear but also depends on the design of the combustion blower/gas valve connection and how the valve itself reacts to air flow. But I would venture most are pretty close to linear.



I have found some differences on Mod/Con forced warm air however because of the difference in how they are configured. It includes dealing with the resistance of the flow through the secondary heat exchanger and the cleanliness of the system itself.



I am sure the lab rats from the different companies would perhaps dispute some of this depending on the design of their equipment. I certainly have not done extensive research but mostly trying to answer questions asked in class about the 5 to 1 ratio as related to actual BTU's in each step.

Gordan

Good enough for me

Thanks, Tim and Mark! Basically, this means that the fan RPM (relative to min-max) is a pretty useful means of estimating BTU/hr input, which was what I was looking for.

bob_46

Gordon

It would be great if you clocked your gas meter at various RPMs and plotted a curve and posted the result on the Wall .



bob

Gordan

I agree, Bob

That would be great because it would mean that I would have a gas meter that I can clock, which would in turn mean that I have gas, and not propane, and that would in turn mean that my heating bills would be a fraction of what they are! :-)



Incidentally... I've been checking the average retail and wholesale propane numbers for various regions. Why do we in Northeast/MidAtlantic regions get reamed out? The wholesale prices are not that much higher in this region than the Midwest, yet the retail prices are two times higher!

Mark Eatherton

I could do this...

I still have a gas meter (pulsed output) connected to my boiler here in Denver. I could put the boiler in "Service Mode" and manipulate the fan RPMS while doing a DHW load, and see what those numbers come out ot.



I agree with TIms' assertion that fan RPM doesn't always necessarily reflect air and gas flow through a dirty Gianonni heat exchanger tho... If it can't breath it can function correctly...



If and when I find time, I will clock my little Munchkin and post results.



ME

Tim McElwain

Answer concerning

cost of Propane, it is because it is getting more and more difficult to get delivery directly to many areas in the Northeast. Several companies I know of in Mass are driving all the way to Southwestern PA to get product. When Propane providers have attempted to build large storage facilities in the eastern part of the US politicians have found it a convenient way to get elected or stay in office to oppose building those facilities. That in addition on the Natural gas side opposing LNG facilities. They oppose, we elect them, we pay the price.