Munchkin BTU usage and NG BTU content question

Jim Cheung_2

We have a Munchkin T80. I ran it flat out without reset and timed the litle spinning disk on the gas meter.

The Munchkin is drawing 1.6 cft per minute, or about 96 cft/hr (20% more than the rating, if 1 cft = 1000 BTU).

Possible explanations:

(a) The NG has 20% lower caloric content than it should, but the Munchkin is smart enough to draw NG based on BTU rather than by cubic feet. (I know the NG energy content can vary, but 20% seems extreme. The gas company's correction factors applied on the gas bill are less than 1.01.)

(b) The Munchkin is conservatively rated; the 80KBTU model actually generates 96KBTU.

Which is correct?

jp_2

timing gas meter

how long did you time the gas meter?

I would suspect the gas meter is off? I've watched the little dial move and it doesn't move very smoothly, slows down, speeds up......

if i time it for a minute, i'll get one cf/h, if I time it for 10 minutes I'll get a different cf/h calculation.

the boiler is looking at incoming gas pressure. it has no clue whether gas has 80 kbtu's or 110kbtu's. it modulates gas volume only.

never heard of gas meter getting recalibrated?

Jim Cheung_2

Timed it for a minute about five times, essentially the same answer each time.

Yes, a third possibility is that the gas meter is not properly calibrated.

jp_2

timing

time it for fifteeen minutes, see what you get.

also, what does esentially the same mean? lets see some numbers. remember, any error in your reading are then multiplied by 60.

Jim Cheung_2

gas co won't calibrate meters

My gas company (Nicor, Chicago suburbs) says it will not calibrate meters, but I can get it replaced if I pay $100.

Per-minute readings were within the range 1.5-1.7 cft per minute, so 90-102 KBTU for a 80K boiler.

I will do a longer test tonght. It is a pain because one has to watch the dial carefully so as not to miss a turn: there is a half-foot dial and a two-foot dial, but then there is a 1000-ft dial, nothing in between. I don't know if all gas meters are built like that, and if so, why.

Guy_6

Munchkin

The boiler does not actually even operate by "monitoring" the BTUs; It operates under the ASSUMPTION that it was set up for proper combustion. That point would be maximum BTUs for that particular boiler. It then modulates based on the difference between the target temperature and the return water temp.
Another T80 fact is that it will operate at 110,000 BTUs for hot water production, which may be throwing your figures off.

Jim Cheung_2

> The boiler does not actually even operate by

> "monitoring" the BTUs; It operates under the

> ASSUMPTION that it was set up for proper

> combustion. That point would be maximum BTUs for

> that particular boiler. It then modulates based

> on the difference between the target temperature

> and the return water temp. Another T80 fact is

> that it will operate at 110,000 BTUs for hot

> water production, which may be throwing your

> figures off.


Thanks. This is interesting...I think...

Modulation is not an issue, I made my measurements while it was running flat out. The difference between return and target was over 30F.

You are saying that it is capable of 110 KBTU for DHW? If so, why not for central heating? (I don't have DHW.) Assuming that is it set up for proper combustion, and that the NG has the correct thermal content, what is the maximum BTU for central heating?

How can I set it to produce the full 110KBTU output if I want to? Maybe somehow fool it into thinking it has DHW?