Oversizing a modulating boiler - why not?

Unknown

That might be correct... but not installing the vision control on a munchkin is a pretty silly idea.

Cary Alon

Mod/Con Boiler - why not next biggest size?

I did a heat loss for my house, came up with about 50K BTU.

I'm looking at boilers to replace my FHA furnace (125K BTU).

I'd like to do DHW with the boiler also.

So here's my question: Say I buy an 80K boiler instead of 50K, in order to leave overhead for expansion (of the house). If the boiler modulates, wouldn't the 80K boiler do the same job as the 50K boiler? Am I missing something?

Thanks in advance for your help!

Cary

Ron Schroeder

But do you expect to

add on at 2.5 times your current house size then probably an 80K unit would be more in order and some modcons that fire at 80K jump to 100K for hot water or from 50K to 80K for hot water. Size for expected heat load and then the lower it fires in the off season the more the savings in fuel, I think its 1% savings for every 3 degrees you drop water temp in heat mode

Brad White_9

The 50K

is an input number so you would be short, assuming your heat loss is accurate. If the heat loss can be justified to be in the 42-45K range and the boiler can boost for DHW, I would go with the 50. If your heat loss is sound then yes, go up a size.

The goal is an ideal, the closest you can get and not too much over (increments being what they are). The reality is though, if you have an 80 MBH input boiler with a 74 MBH output, you are still almost 150% of your heat loss on the coldest day. Your range of modulation is limited.

Better than any on-off bang-bang boiler, but still, for the effort, each facet should do it's best. This is a system, not just a boiler.

My $0.02

Brad

Ross_7

Priorty

If you have DHW priorty incorporated into you control scheme, you should even worry about installing a correctly sized boiler for your house.

Unknown

Minimum Modulation Rate.

Typically, that minimum modulation rate gets bigger with bigger boilers.

When you are under that rate, the boiler cycles instead of modulating. You want it to modulate, so the lowest minimum modulation rate you can use will be best for your boiler.

That said, it's certainly not as bad as it used to be to oversize, but it's pointless to pay more for a unit and get less efficiency unless you have to. If you know you're adding on to the system, then you might have to, of course.

Cary Alon

In looking at Munchkin, for instance,

the T50 modulates from 16K BTU to 46K BTU, the T80 from 17K BTU to 74K BTU.

If they both start at the low end, there doesn't seem to be much difference to me.

I understand that this is a system that works together, and I want to be as efficient as possible. I guess I want it all :-)

It just seems that there is little or no downside besides the initial cost difference which in and of itself is minimal. Am I missing something glaring?

Thanks!

Unknown

between those two particular units, not a big difference other than initial cost.

John Ketterman

The wall-mounted T80 is a special case. The floor model T80M goes down only to 27K.

The general rule is still to not oversize even a modulating boiler. But if you are really going to use the T80 rather than the T80M, in this special case there is no problem.

Mark_46

Cary

I BELIEVE the boiler would be too small if you will use an indirect. Any indirect DHW tank (30, 40, 50 gallon) is a larger BTU load than your 50k heat loss for the whole house! A 50k boiler will spend more time in DHW mode and recovery rates will not be achieved as specified by the water tank manufacturer with an undersized boiler.

I was in a similar situation. I have a 55k spaceheating load which is right on the edge in the Heat Transfer product line. I chose an 80M and a 30 gallon superstor tank.

Unknown

Mark, the T50 boosts to 80k for domestic demands. It's a pretty cool little unit, really.

Cary Alon

So as a cursory review,

The WM Ultra smallest model is 80K, the Buderus is 84K, the Knight is 80K, the Veissman 200 is 91K.

Guess that smaller is not that popular, regardless.

It seems that all of the above are substantially more expensive than the Munchkin, which has been ascribed (by some) to enhanced controls.

Any input as to reliability/longevity? Should I be concerned about buying the Munchkin?

Cary

Unknown

I would let local installer preference and track record speak for itself. good boilers are only as good as their installer.

Ron Schroeder

I personally like

the W/M Ultra but it is more important that you find someone that is omfortable with the product that the sell and service. The best boiler is not worth is salt if not installled and serviced properly

Mike T., Swampeast MO

If this is a heat loss calculation based on Manual J (most are), there is already a significant safety margin.

Maintaining space temperature at design conditions will always take less energy than the calculation. 30% - 60% less.

Nothing at all wrong with this overstatement as conditions do change. Weather can be unusually cold or windy, some may want higher than normal room temp, weatherization degrades, etc.

What matters is that you already have a cushion factor via most heat loss calculations. There's truly no reason to increase the size of the boiler beyond.

If you need more domestic hot water than can be supplied by a boiler giving priority to such, then consider other methods of supplying the hot water. Make certain however that the indirect is capable of completely absorbing the full output of the boiler.

Also, if you're using daily setback and your recovery period coincides with your peak DHW demand, use "smart" thermostats, built-in setback schedules and available DHW settings to avoid the problem. With a "smart" thermostat, set for your wake temp at least an hour before you wake. Set the DHW tank to heat especially hot while the boiler is "down" for setback.

ALH_4

modulating

The Vitodens 6-24 is 20,500 to 81900. Any of the boilers you mentioned works fine if installed correctly. Though, I have three complaints about the Munchkin. Reset controls are not included, and therefore their cost is masked. Second is the fact that the heat exchanger rests on a fiberglass batt inside a plastic jacket, and the jacket is not exactly positively sealed. It's inexpensive because it's cheap.

Any modulating, condensing boiler runs more efficiently at lower firing rates. Of course you want to minimize the time the boiler runs below low-fire for longevity. But you also want to minimize the time the boiler runs at high fire for efficiency.

-Andrew

Mark_46

Isn't that only if

the Vision 1 control is used? I believe the Munchkin boilers cannot boost as equipped 'out of the box'. Correct?