efficiency change as weather gets older?

Chuckles_2

[Oops, I meant, as the weather gets COLDER. I can't edit the main title, apparently.]

Our old CI boiler would get more efficient (on a therms per degree-day basis) as it got colder. Now we have a new Munchkin, and right now it is using about 40% less than what the previous boiler would have used in the same weather.

(By using therms per degree-day, and not dollars, my calculation is not affected by the price of nat gas, and also I subtract the gas consumption of the water heater, which I measured during the summer.)

However, if I compare the October efficiency of the Munchkin to the January efficiency of the CI boiler, I am saving only 20%.

Am I right in assuming that the Munchkin will NOT get noticeably more efficient as the weather gets colder? So it is reasonable to take Oct Munchkin efficiency and assume that the Jan Munchkin efficiency will be about the same? Lets say we're always in the condensing range.

Constantin

I think you hit the nail on the head...

...I am going to go out on a limb here and assume a couple of things.

#1, your old CI burner probably had huge standby losses or a coil. As such, it would be very inefficient until you hit real heating weather, short-cycling despite having a large mass to heat, and all that.

#2, As the weather is getting colder, the Munchkin has to inject more and more heat into the pipes to keep you happy. As such, I would not bet on condensation always happening. Are you measuring return temps to be sure?

#3, The flue gas to inlet temperature Δ is going to widen as it gets colder outside. There is no way for the gases you're expelling to drop below 70-90°F once it exits the Munchkin because that's about as low as it'll go in normal home (flue gases cannot be lower than the returning water temp going through the secondary HX).

Thus, if your exterior temp is -10°F, then you have a 100° Δ between the inlet and the outlet. During the summer months, this Δ will be much smaller (sometimes even negative). The wider this delta, the more energy will be going up the flue instead of being added to your system.

Your old CI boiler will have had a much higher flue stack temperature to start with (300°F+). Thus, the effect of flue stack temps on combustion efficiency as a function of season would not have been as noticable. In fact, the efficiencies gained from warm-start and longer cycling in the wintertime (i.e. lower standby/startup losses) probably far outweighed the efficiency drop due to flue gas temp losses.

Kal Row

simply put.

when it's real cold and the supply water is 180 and the return water above 136, the only thing the new boilers have over the older ci ones, is cumbustion efficency and the faster heat exchange of stainless/copper/aluminum - even 20% is a lot, probably because you old boiler was half dead, if it was a new ci, you would only get a 12-15 diff tops, and if the new ci was a sealed combustion type like a slanfin VSPF, then the diff is only 7-10, and if it's piped pri/sec/vari-injection/outdoor-reset then the real world running diff is down to 3-5,

there you have it, the instalation is more important then the boiler, the boiler is just a heat source, makes you wonder - doesnt it!!

(ps i base the above, on a real world experiance with an inexpensive ci boiler, piped pri/sec/vari-injection/outdoor-reset a-la tekmar 361 - the new slantfin boiler had the same eff rating as the old hydrotherm it replaced, yet the old ran 12min out of 15 and the new one 2 out of 15 - with same outdoor temp - same ibr rating on both also - to be fair though, i also added zone valves on the install)

Dale

steady state

Older boilers with correctly setup burners and clean heat exchanger surfaces ect could and did give about 78% effeciency steady state, your new boiler can give 92% IF the return water temp is cool enough ( lesss than 140) to condense. So that 14% difference steady state if the munchkin can condense. If the boilers run non stop which they should on a design day and if the return temp is over the amount needed to condense you may theroretically only see a 5% difference. This is why savings claims should be conservative. This is also why and IO reset is a good idea when adding a munchkin, you may be able to get enough heat spring and fall to get the boiler in condensing mode.

Chuckles_2

> Thus, if

> your exterior temp is -10°F, then you have a

> 100° Δ between the inlet and the

> outlet. During the summer months, this Δ

> will be much smaller (sometimes even negative).

> The wider this delta, the more energy will be

> going up the flue instead of being added to your

> system.


More, yes, but not a higher fraction since the home's heat loss goes up as well. The efficiency should remain about the same.

Weezbo

there are alot of ways to pipe a boiler and the fields..........

and alot of ways to Control them.......... now when it comes to natural gas i think they have even more chances of extraction of nearly every btu fired into them to be displaced and redistributed into the fields....or perhaps KW is a better measurement of energy...or Joules:) it is interesting to note that joules sound like Jewels and with the price of available fule sources always going up never down it may be best we start thinking of converted energy as Jewels:) flue losses can be modulated also they help cut down on these jewels escaping out the largest available openings.....the colder it gets out side does indeed have a distinct impression on fuel "Efficentcy",we arent doing bad though by piping, controling and modulating the stand by losses....i once had a physics prof explain why we really have no machines on planet earth function at better than 60 % i still belive him. despite whatever hype the manufacturers present us with AFUE and the rest of it.

Constantin

No..

Think thermodynamically, specifically WRT to the conservation of energy... IMHO, the efficiency should not be about the same. Think about it this way: At steady-state, what separates a condensing heating appliance from it's non-condensing peers?

Much lower flue gas temperatures.

Where a CI boiler will be happy when it's flue temperatures are around 350-375°F, the condensing boiler will operate with flue gases as low as 90°F. The lower flue gas temperatures means that more energy is being kept inside the house, not expelled to the exterior along with the combustion byproducts.

To recap: the low flue temperatures is where condensing appliances get their steady-state efficiency advantage over CI technology. Lower the exterior temperatures, and the makeup air temperature drops also. All that air is heated to at least 90°F as it runs through a HX, then it is expelled out. The greater the difference between the inlet and the outlet temperature, the more energy is being vented to the great outdoors instead of being added to your home.

So, with a large delta between the inlet (makeup air) and flue gas temperature, the combustion efficiency cannot be as high as when the delta is low. Remember, this is not a car where the denser air makes for a more powerful engine by having more oxygen molecules in it than warm air. These are non-pressurized combustion appliances...

S Ebels

What's happening

Your old boiler did in fact become more efficient as the weather got colder. This is because it was forced to run at a more steady state condition. (constant on) This is always the best operating efficiency for any burner in any appliance. Cycling on/off as it would in shoulder seasons or moderate temps would increase stanby losses dramatically.

This is why outdoor reset and constant circulation of water do so much to lower energy comsumption. The boiler is not bringing the water all the way up to 180* everytime there is a call for heat. The constant circ helps to lower the number of burner cycles/hr by keeping the home at a more even temp and extracting all the heat from the water before calling the boiler on again.

Is your Munchie running on reset? Are you running constant circ? Both of these will help get you boiler in its max efficiency zone regardless of the weather.

15-20% reduction is typical of what I see going from a CI boiler with standard control system (on/off operation) to a condensing boiler using reset and constant circ.

Brad_9

Flue temp

So, if you have a concentric flue, won't some of the heat in the exhaust gas give up it's heat to the colder incoming air? Couldn't this reduce the exhaust temp, where it exits, to lower than house temp? I don;t know if you can get further condensation after going through the secondary hxcher in the boiler.

Mike T., Swampeast MO

I believe yes, but it's probably not much.

Here's a strange question as well...

If the heat is mainly going into the HX via radiation does this mean that the flue gasses aren't really very hot to begin with?

Plumbob

yes and no

> Is your Munchie running on reset?


Yes.

> Are you running constant circ?


Not 100%; we have two zones so it's hard to do 100% circulation.

> 15-20% reduction is

> typical of what I see going from a CI boiler with

> standard control system (on/off operation) to a

> condensing boiler using reset and constant circ.


Too bad! I was expecting somewhat more, based on claims I read here and elsewhere.

Constantin

That depends on a couple of factors, no?

Consider these four factors that affect HX efficiency:

• How efficient is CPVC at conducting heat? Not very.
• How big is the "tertiary HX" surface area? Not very.
• Is the gas flow laminar? Yes
• How fast are the flue gases running past this "tertiary HX"? Quickly.

At least one wallie has tried to wring some extra % of heat by putting another HX on the flue gas pipe. However, it was constructed of copper (great thermal conductivity) and it could have been baffled/treated on the inside to create a turbulent airflow that would slow gas speeds and increase surface area (think corkscrew/rifle-patterned fins on the inside).

Naturally, the folks building the boiler wouldn't be too happy if the static pressure drop across this HX in conjunction with the rest of the flue system would overpower their combustion-air fan.

I think this is where the Vitodens scrounges some additional efficiency from combustion gases... a much larger HX than the one found in the Munchkin will probably increase efficiency marginally. The question is whether all that additional material translates into a net positive investment for the consumer.

Constantin

Not just radiation...

The flue gases impinge on the HX, even if the flame does not. Therefore, with airflow you have a lot of conduction/convection inside the HX. It's just using air as an exchange medium instead of direct coupling.

Also note the ways in which folks try to increase efficiency. IIRC, the Monitor FCX oil boiler uses a spray system in the secondary HX to mist the flue gases entering the secondary HX. You can create a ton of surface area with droplets, causing all that heat to stay in the HX instead of escaping to the great outdoors.