bigger exchanger = higher efficiency?

R. Kalia

I read somewhere here that with modulating boilers, the reverse of the conventional wisdom holds true----it is actually better to buy a higher-BTU boiler, because if there is the same (modulated) burner output in both cases, the bigger one is more efficient because it has a bigger heat exchanger.

Did I get that right? Specifically, in our house a Munchkin 80 would be just right in size, but I could also get a Munchkin 140 to allow some leeway for possible future addition of indirect hot water. My question is, will I also get any noticeable increase in heating efficiency with a Munchkin 140? How much? Worth the price difference?

I guess my question assumes that the Munchkin 80 and 140 have different size heat exchangers---is that true? (I am sure the wall-mounted models have smaller everything, but I am asking about the floor model 80 as compared to the 140.)

Many thanks for your help,

don_34

I guess

there some truth to that.If you look at today ac unit and
how big they gotten,then you would think thats to be the case.But you dont want to go by how they test in the lab.
This is real world.

So I guess with a boiler if you modulate the btus this will
keep the boiler running longer,keep the cycle efficiency
up,and give the boiler water more chances to pick up the btu.

Thank goodness for moudulating equipment.As fars as any
saving in your fuel bill,where that depends on mother nature and how long of a shower you like.

If it be me I would do the 140 and go with the indirect
and try to match the boiler as close as possible to load.
Any more then that its a waste.

the boiler is all ready set up to see higher cycle efficency
dont defeat it by going above and beyond.JMO.

R. Kalia

not sure I follow...

> If it be me I would

> do the 140 and go with the indirect and try to

> match the boiler as close as possible to

> load. Any more then that its a waste.


???

The 80 is the Munchkin size that is best matched to our space heating need. Adding DHW to it is a bit risky.

140 is way too much, even with DHW. But if I gain some significant efficiency, it may still be worth it. Hence my original post.

hr

What is the load now?

and how much will you add. I think the 50 and 80 Munchkins step up their output in the DHW mode? So unless you have a huge DHW load, the 80 should be adequate.

Efficiency will be better innthe unit that cycles less. If the 80 is adequate at DESIGN day, it will be the best choice. Keep in mind how many days you are at actual design temperatrure.

Even with modulating equipment you want to size as close to the load as possible. Else they would sell only two sizes

I'd think you would have less standby loss with a smaller HX, and combined with less cycling, the smaller unit would be my pick.

It all depends on how much additional load you plan on adding, I suppose.

If you go with the larger unit, add a buffer tank to store BTU's, extend the cycle lengths, and help with micro loading, until the additional load is added.

hot rod

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Cliff Brady

Lower modulation point with the 80

T50M 17,000 btu/hr lowest fire input
T80M 19,000
80M 27,000
140M 46,00

You will cycle lower and less with the wall hung T80M, and have 100,000 btu/burst for DHW.

R. Kalia

the load now

> and how much will you add. I think the 50 and 80

> Munchkins step up their output in the DHW mode?

> So unless you have a huge DHW load, the 80 should

> be adequate.


Currently our old cast-iron boiler draws ~85,000BTU/hr averaged over a sub-design day. (I believe that figure much more than a load calculation, which comes up with about 130,000BTU.) So even with only a 10% increase in efficiency, we would need less than 80,000 BTU/hr input. Of course the actual efficiency increase will be higher, since the current boiler is 36 years old. With a 25% efficiency incraase we would only need 65,000 BTU.

Our water consumption is not much, requiring less than 1 therm a day or 5000 BTU/hr average. So you are right that we can probably use the 80.

R. Kalia

burst?

> T50M 17,000 btu/hr lowest fire input T80M

> 19,000 80M 27,000 140M 46,00

>

> You will cycle

> lower and less with the wall hung T80M, and have

> 100,000 btu/burst for DHW.



Yes, I do realize that the wall-hungs have 5:1 modulation, while the floor models have only 3:1.

What is BTU/burst? is that different from BTU/hr? Anyway, it does not help because I was really talking about an indirect hot water tank, not 'instantaneous' hot water.

The consensus seems to be that we should go for the smallest boiler that does the job, not a bigger one even if it has a larger heat exchanger.

I read here that the floor model Munchkins will also have 5:1 turndown before next heating season. Any confirmation on that?

Uni R

Burst

Think of burst like take off power for a jet engine. It can ramp up to these higher BTUs when a call comes from the indirect water heater so that it can get it satisfied sooner and get back to heating the home.

hr

Ideally

you would do a heatloss calculation on your home as it sits today. With this you could accuratly size a boiler. Doesn't matter what the current size or efficiency is on the exisiting boiler.

The homes construction and outdoor temperature dictate the load.

Does the current boiler do an adequate job? Does it short cycle or turn on/ off a lot on a design (coldest) day? It could be grossly oversized, especially if you have made insulation or window improvements, over the years.

Could be you need less boiler than you think. A free calculator appears on the bottom of this page if you want to do some calcs.

Thet Munchkin will fire up to 100,000 when the indirect tank calls for heat to speed recovery. It is not an instantanous type dwh provider, it just ramps it's output up.

Hope this makes sense.

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Uni R

the load now

Two questions. On the coldest day of the year (at or below design temp), can it keep up? If it can keep up, how long does it have to run throughout the day to do so? If it's only running 10 or 14 hours on a design day, then you know you still have the fudge factor dial pointed at oversized. I wouldn't get a 140M when you need an 80M or lower.

R. Kalia

you misunderstood

I didn't say the existing boiler is sized at 85KBTU. It is a 150KBTU(!!) boiler. But the existing boiler size is irrelevant.

What is relevant is that this boiler USES 85KBTU/hr averaged over a sub-design day, as read on our gas meter. (Over 24 hrs it uses 21 therms if you want to do your own calculation.)

R. Kalia

you misunderstood

> Does the current

> boiler do an adequate job? Does it short cycle

> or turn on/ off a lot on a design (coldest) day?

> It could be grossly oversized, especially if you

> have made insulation or window improvements, over

> the years.

>

> Could be you need less boiler than

> you think. A free calculator appears on the

> bottom of this page if you want to do some

> calcs.


See my previous post also titled "you misunderstood". I never mentioned the size of the current boiler.

And I HAVE done the load calculations (or rather two contractors did the load calculations). They both came up with much higher numbers compared to our actual usage with the old boiler.

Uni R

we not do good communicate - L

Sorry... Yes, I completely misread what you were saying. We don't know what the efficiency of your current system is, so it is tough to gauge the differences on the input side and then factor in whether or not the system would actually be considensing on a design day. However, given your gas consumption for a **subdesign** day I would still not see any benefits for choosing the 140M unless you need a ton of DHW.