Munchkin experiment, reset vs no reset

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We've been having a string of 30-35 degree-day days, so I measured gas consumption per degree-day on a couple of days with reset, then just as a test, I disconnected the outdoor sensor and measured the gas consumption per degree-day for another couple of days.

The supply temperature with reset is about 125F to 135F depending on the exact temperature outside. Without reset it is supposed to be 190F, but I don't know if it actually gts there during a heat call.

The therms per degree day numbers were exactly the same, maybe a touch lower without reset but the lowest and highest values for these four days were within 3% of each other.

So why am I using reset, exactly?

ALH_3

Reset

Outdoor reset has other benefits. Without reset you're back to on-off heat which can lead to expansion noise problems and overshooting thermostat setpoints. In my opinion it's not each of the features included in these modulating, condensing boilers...it's all of them working together to complete the package. Tekmar makes great controls for cast iron boilers, but it's really still just an adaptation of a less than ideal boiler to provide an ideal output to the system. I still wonder how much condensation really buys. To my mind the burner modulation is where it's at. Being able to fire the boiler only to the degree required makes the boiler work directly with the system rather than independent of the system.

-Andrew

Mark Hunt

Remember


The Munchkin 925 is watching supply and return temps, so even without the reset, it will still modulate according to the delta T it was programmed with.

The OD reset function just gives the Munchkin a target temp and it modulates accordingly. W/O the reset, it modulates on HWS vs. HWR.

Mark H

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PS

I tend to agree that modulation is the more important factor in consumption savings. It will provide benefits 100% of the time, while many condensing units' benefits will only be provided for a fractional period.
The OAT reset is a single, artificial determinant factor of load and does not account for humidity, solar, infiltration, etc - the HWS vs HWR accounts for more of these factors. Not to say that OAT doesn't have its benefits - especially with non-condensing boilers - but it may be a bit overblown.

Jay_12

Reset

How efficient is your condensing boiler at 190F? I'm sure that it is no longer as efficient as when it is operating below the condensation point. That is why reset is imporatant on these boilers.

Jay

Chuckles_3

Chuckles_3

>The Munchkin 925 is watching supply and return temps, so even

>without the reset, it will still modulate according to the

>delta T it was programmed with.


Yes, but it is still trying to get to the setpoint (190F). The only thing is, if the return tmperature is 100F it runs on high, if the return temperature is 183F it modulates down to low, but its objective is still to get the supply to 190F. And 190F is a noncondensing temperature.

PS

I may be misunderstanding you (I have no specific knowledge of your controls) but I'll give my 2 cents. With the OAT reset, your target temp is much lower and readjusts according to delta temp., but runs more because you are constantly providing minimum BTUs to replace heat loss. W/o reset, your target (default?) is much higher but boiler will cycle (= less run time) due to higher BTU delivery than load. Seems like you are operating (and analyzing) at design conditions in which case the differences in input are negligible? When conditions are such (shoulder months = warmer)that the boiler will be able to cycle, modulate and condense - you will see the differences in consumption.

I am trying to wrap my brain around this too, so I hope more knowledgeable folk chime in with their opinions.

Chuckles_3

> Seems like you are

> operating (and analyzing) at design conditions in

> which case the differences in input are

> negligible?


I do understand that in design conditions there may be no difference between reset and no reset. But 30-35 degree-days is not design conditions; a design day is here 70 degree-days. As I said, the water temperature with reset is 125-135F (condensing range). Setpoint without reset is 190F.

PS

Got it - conditions under which you have been testing are such that you should see at least an appreciable difference in input. Is this a case where increased efficiency doesn't necessarily provide decreased costs?

This is precisely why I am undecided on condensing vs non-condensing, modulating, OAT reset, etc. What type of distribution/emitters do you have? DHW?

Chuckles_3

No DHW. Radiators (excess of radiation), plus small addition with baseboard (no excess).

PS

Keep experimenting under other DD conditions and let everyone know what you find (if possible). Hopefully we are missing something - or something is amiss on the piping or boiler/controls that needs adjusting - and others here can help out. Otherwise - you are providing real world ammo to riddle the current state-of-the-art boilers/controls full of holes (at least for your application - which is similar to mine). I am not willing to accept the conventional wisdom without proof these costly systems will deliver for my application.

Mike T., Swampeast MO

Can you tell if the burner was modulating during the test? I suspect it still is, but don't know for sure.

Converted gravity systems hold so much water that they tend to have an automatic indoor reset via the thermostats. Return temps stay naturally low because you're moving a lot of water through them. Also remember that your new boiler is sized more in line with the actual heat loss--you couldn't get the system side up to 190° if you tried with any rational indoor temp. With the boiler wanting to maintain a set delta-t and with more water flowing through the system side than the primary side, the return temp to the boiler just can't rise very high. Delta-t in the old gravity portion is probably very low--like around 5° or so. Even if the boiler is working at full output, it just can't raise the supply temperature very high because the system is accepting the heat as rapidly as it can be made. By the time supply temp starts to rise significantly, the thermostat is satisifed and the burner shuts down.

I suspect your results would be quite different were it not for the nature of the old gravity portion.

Chuckles_3

> Can you tell if the burner was modulating during

> the test? I suspect it still is, but don't know

> for sure.


I didn't watch it, so I am not even sure it ever got to the target of 190F before the stat shut it down.

> Also remember that your new

> boiler is sized more in line with the actual heat

> loss--you couldn't get the system side up to 190°

> if you tried with any rational indoor temp. With


You may be right about this. What you are saying is that the supply never went to 190, it stopped at (say) 150 because at that point the radiators were already putting out all the heat the boiler can produce (80MBTU/hr). Which would also suggest that the boiler never got a chance to modulate.

And while 150 (a wild guess) is more than the 125-135 reached with reset, it is not that much more (compared to 190) and there would be some condensation since deltaT=20.

> I suspect your results

> would be quite different were it not for the

> nature of the old gravity portion.


As I interpret it, its not so much the quantity of water in the converted gravity system, but rather the excess radiation that could keep the water from ever getting to 190.

PS

So because of the limitations of your existing system you can expect the same boiler consumption regardless of OAT reset? He tested both scenarios at ~ 30-35 DD. This is discouraging if you can't realize SOME savings with all these extra efficiency bells and whistles.

Chuckles_3

Well, one could call it advantages rather than limitations. Excess radiation is a good thing, and a condensing boiler with stainless steel HX is able to stand the lower water temperatures that resulted from matching the boiler to the heat load rather than to the radiation. If I'd gotten a CI boiler matched to the heat load, condensation would have destroyed it. (The house previously had a CI boiler of twice the size, matched to the radiation, and it got to 190 without a problem.)

And I did get higher efficiency when the weather was even warmer last Oct-Nov, in the range of 0-20 d-days. Of course not much gas is consumed during those warm days so I didn't save very much.

Jay_12

efficiencies and temperature

I was simply stating that at 190F water temperature, the boiler is no longer condensing and extracting latent heat from the flue gas vapor. Using the latent heat of the water vapor adds roughly 5 to 8% in boiler efficiency according to boiler manufacturers.

I can't explain the marginal fuel consumption change when your outdoor reset was removed. Perhaps the outdoor temperature was the same, yet due to other factors such as wind loading and less solar gain, the resulting heat loss of the building was not the same.

You would think that at a lower target temperature the boiler would modulate the gas valve to match the heating load, resulting in long efficient burn times. At a high target, the gas valve would open rapidly, with the boiler reaching temperature quickly and then the burner would shut off resulting in many less efficient on-off cycles.

That said, it would be interesting to use a data logger to monitor supply water temperatures and the number of on-off cycles with and without outdoor reset and also monitor gas usage.

Jay

PS

I assume a new, properly sized CI boiler would have been non-condensing type and would run at non-condensing return temps, thus no HX thermal shock. If you added an OAT reset, you could repipe near boiler to prevent low return from entering HX. All probably for much less than your Munchkin? Glad to hear you had some savings - but will the payback be enough to justify the extra cost? I have nothing against these boilers - just trying to squeeze as much info as possible from real world experiences so I can make an educated decision on my own system -)

Chuckles_3

> I assume a new, properly sized CI boiler would

> have been non-condensing type and would run at

> non-condensing return temps, thus no HX thermal

> shock.


Nope. If one has excess radiation, as I do, but the boiler is properly sized, it will never be able to heat the water to 180F, because even at 150F (say) the radiation will be radiating all the heat that the boiler can generate. Once that happens, the boiler is physically unable to heat the water any further, because if it could, the radiation would be radiating MORE heat than the boiler can generate! This is true for any boiler, condensing or not. That 150F will be enough to heat the house, since the boiler is correctly sized, but the return water might be 130F and that will cause condensation and thus corrosion. (Thermal shock is not the issue here.)

So it is not actually true that boilers should always be sized to the heat loss. With CI boilers is may be necessary to size them to the radiation.

hydronicsmike

I always thought that mod boilers aren't supposed to shut off

unless I am mistaken, a modulating boiler shuts off only when the heat call goes away. Your remark about short and inefficient cycles seems unfounded. Thats the idea behind modulating boilers, isn't it?

hydronicsmike

I believe what Jay was refering to....

was that the boiler may have been off for a while and cooled down and when a call for Heat comes in it may be modulating up to 100% to achieve the Target of 190°F. With Reset, the Supply Water temp could have been lower and the zone ON longer therefore. So if Reset would have calculated a lower Water Temperature and the call for Heat was ON longer, then the boiler may have been able to modulate more and give you additional savings. With maintaining a higher temp, the call for heat comes and goes more frequently and this hinders the modulation.

Jay?

I hope this helps.

Mike

chuck shaw

Just a thought

have you checked the status menu on this boiler? Is the out door sensor reading right? Would make a differance. Should be installed without any interferance from line voltage. And on the north side of the home.

Chuck

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Sure; the outside temperature read by the Munchkin is correct and the water temperature in normal operation is what it should be based on the curve. The sensor is on the NW side of the house, shaded by the house on the W side of ours..