Proper mod/con boiler sizing

Brad White_9

The reason modulating the temperature down makes for longer run times is because the cooler water takes longer to heat the space. Yes, it takes less time to make that cooler water setpoint, but in the end it is what goes into the space that counts. Beyond that point indeed the boiler cycles. Fact of life.

(This whole thread makes me want to design a system with a tiny little, oh, 10-15 MBH boiler to trim the house during really mild weather before a bigger Mod-Con takes over :)

Bill Spencer

Proper mod/con boiler sizing

I apologize if this is a repeat for some of you, but I posted this previously to a thread which might have been considered closed. Here is a repeat of it from the MOD/CON thread...

Hats off to Dan H for such a great site and all the other extremely knowledgeable and kind people devoting their valuable time to this cause. I am by no means an expert so I'm hoping those in-the-know will educate others like myself about how to deal with a minimum sizing problem I see with mod/con boilers which is likely to affect quite a few installations.

While the virtues of a proper heat loss calculation are espoused repeatedly here for maximum boiler output sizing, one thing not frequently mentioned is that minimum output is important too when multiple zones are used.

Here is an example of the mod/con minimum sizing problem:

Design day heat loss: 120,000 BTU/hr

Number of zones: 6

Turndown ration: 5:1

Boiler input: 30,000-150,000 BTU/hr

Boiler output: 24,000-120,000 BTU/hr

Heat emitters: copper/aluminum fin baseboard

Emitter output per zone: 20,000 BTU/hr @ 180 degrees.

If only one baseboard zone calls for heat, it can only give off only 20,000 BTU/hr. The boiler can only modulate down to 24,000 BTU/hr so it is going to quickly overshoot the desired 180 degree supply temp. This occurs repeatedly until the heating call ends or more than one zone calls for heat.

Although this boiler is perfectly sized for total heat loss it still short cycles. Outdoor reset will make things worse since even less heat is emitted per zone and the cycles will become even shorter.

While I don't have quite this situation, I do have one zone like this and another on the borderline. There are more problems with my situation, but I want to stick to this aspect of boiler sizing so others avoid this problem. This leads to the following questions:

1) What can be done to address short cycling due to boiler output exceeding emitter output in severe cases and borderline cases?

2) Would higher output emitters help the problem or make new problems?

3) What solutions will work with outdoor reset?

Thanks.

Brad White_9

Thoughts

I think you laid out the parameters well, Bill.

First I have to ask how often a single zone will have a full call for heat and the others absolutely zero? Possible but the hours of occurence are so few that the annual implications are fairly minor even if cycling does occur.

You are right to say that OD Reset will exacerbate cycling by lowering the operating high limit at that time. In such cases an on-line/off-line parallel buffer tank might make sense.

Higher output emitters similarly will have a similar effect of reducing the required water temperature at almost any outdoor temperature.

Still, in the end, I have to think that any heat load will be fairly evenly required across a given house at any time. Exceptions would be a positive shut off or a sunny room. So say your total heat loss is 120,000 on a zero degree day and at 65 degrees you have warm weather shut-down.

Your heat loss is roughly 1846 BTUH per degree delta-T. Each 20% of modulation is good for about 13 degrees temperature drop. Your low-fire should run steady-state if the outdoor temperature is 52 degrees and the whole house is calling in proportion.

And if it cycles? I would not worry too much. 90% of the boilers in America do that all the time and have for years. The tide is changing fortunately but for the few hours, is it really a concern?

Again, a buffer tank will buy you some time.

My $0.02.

Brad

Plumbob

Yes, if you have multiple "microzones", you'll get short cycling. Condensing is completely irrelevant. Modulating makes things better, not worse, compared to standard boilers, but of course, multiple zones still means shorter cycles if the zones call independently.

Tekmar makes controllers that manage heat calls to decrease short-cycling; it makes a zone wait until another zone is calling too. You can do a version of this yourself if you know, for example, that two zones call for heat about the same amount each day; wire them so they are one zone. (Just because they are physically two zones, with separate pumps, doesn't mean that they can't be electrically one zone.)

It is also possible to wire up relays such that when (say) zone 1 calls, both zone 1 and 2 will get heat, but if zone 2 calls, only zone 2 will get heat. Obviously this is for a case where zone 2 needs more run time than zone 1.

If you have enough money you can also use multiple staged boilers with the appropriate controls, eacn boiler being much smaller than a single boiler would be.

But for heaven's sake, short-cycling is bad but it's not worth obsessing about. All boilers short cycle in warmer weather. It reduces efficiency slightly in shoulder seasons, when you are not using much fuel anyway, so the slight loss of efficiency doesn't lead to such a big increase in total seasonal fuel consumption.

PS Actually, reset will greatly increase run times for all zones, and so it will sharply decrease the probability that only one zone will call. Reset is a good thing.

Mike T., Swampeast MO

Outdoor reset will make things worse since even less heat is emitted per zone and the cycles will become even shorter

Yes and no. With outdoor reset your emitters themselves will be more suited to the current load because they give off less heat with lower supply temp. So, heat calls from a zone become progressively longer the more closely the reset temp is suited to just maintain the desired conditions. As heat calls from each zone become longer, the chance of a single zone calling diminishes.

Once the heat loss of the structure as a whole approaches minimum boiler output, the burner will be forced to cycle.

Uni R

Reset makes for longer run times?

"PS Actually, reset will greatly increase run times for all zones, and so it will sharply decrease the probability that only one zone will call. Reset is a good thing."

How so? Wouldn't it take less btus from a boiler to satisfy a lower reset water temp? Modulating obviously increases run times ideally to a continuous basis but I fail to comprehend how resetting the water temp down makes for longer run times or do you mean using a temperature setback on the t-stats?

John Ketterman

Lower water temperature = very long heat calls = multiple zones calling for heat at any given time = larger load = longer burner-on time.

Mike T has said the same thing in his first post above.

(You can have reset with or without modulation. I am talking only about reset; let's say reset without modulation compared to no reset and no modulation.)

Uni R

No you can't...

You need to go use your mathematical skills on the Monoflo circ sizing question.


I would argue that although reset is a control type for modulation, it is the modulation that increases run times because the BTUs out of the boiler match the BTUs escaping the envelope. I questioned the validity of saying that reset increases run times because on its own it doesn't and that PS seemed like a standalone comment and didn't seem right.

Brad White_9

OK, I'll go to my room...:)

I think maybe to make it a more complete statement, it would read such as, "reset with [edit: or without] modulation increases run times compared to on-off firing type control".

And I was avoiding that Monoflo circulator sizing question because I am having too much fun here. I may take that on yet...



Thanks

John Ketterman

Modulation is not required. See my answer below.

Brad White_9

Duly noted.

Edited to suit. Ever since Pinnocchio died, I do what I can to please you, sir!

Mike T., Swampeast MO

But without modulation you really can't maintain a low temperature AND have long boiler run times.

Boilerpro_3

The reason why

reset helps is mainly because you have more water mass in operation (more zones running at one time) and therefore it takes the boiler longer to heat up the water. A single zone may only hold a couple of gallons of water, but if most zones are on, which a tightly tuned reset system will typically see, you probably have 10 to 15 gallons of water to heat up on each boiler cycle. It takes alot more btu's to heat up 10 gallons of water 20F than it does for 2.

Boilerpro

John Ketterman

Gee, I didn't say modulation wasn't a good thing. I just said that reset lenghtens boiler cycles (in a multizone system) whether there is modulation or not. We were originally discussing the role of reset, not modulation.

Reset (by itself) lengthens boiler cycles in a multizone system. Modulation (by itself) lengthens boiler cycles in any system. Reset and modulation together are even better.

Mike T., Swampeast MO

Thread title regards sizing condensing AND modulating boilers...

Mike T., Swampeast MO

BUT the boiler has NO IDEA how many BTUs are escaping the envelope!

Modulation certainly increases run-time but the boiler has to know, "What makes me modulate?"

In most cases some sort of reset (indoor or outdoor) answers this question.

(The exception is the basic Munchkin/Trinity that modulates on delta-t across the HX. Such a scheme is utterly inferior if you want to truly save energy as the boiler is forced to fire to a fixed high limit at full output before it begins to modulate.)

John Ketterman

> BUT the boiler has NO IDEA how many BTUs are

> escaping the envelope!

>

> Modulation certainly

> increases run-time but the boiler has to know,

> "What makes me modulate?"

>

> In most cases some

> sort of reset (indoor or outdoor) answers this

> question.

>

> (The exception is the basic

> Munchkin/Trinity that modulates on delta-t across

> the HX. Such a scheme is utterly inferior if you

> want to truly save energy as the boiler is forced

> to fire to a fixed high limit at full output

> before it begins to modulate.)

John Ketterman

> (The exception is the basic

> Munchkin/Trinity that modulates on delta-t across

> the HX. Such a scheme is utterly inferior if you

> want to truly save energy as the boiler is forced

> to fire to a fixed high limit at full output

> before it begins to modulate.)


With current firmware, the Munchkin starts at low fire and then modulates up as needed.

Also, it does not exactly "modulate on ΔT across the HX". Since the primary flow rate is fixed, the modulation level is what determines this ΔT (20F at full fire, less at low fire), so you can't use this ΔT to set the modulation level. Rather, it modulates based on the difference between the target temperature (from the reset curve) and the actual return temperature. Which is a pretty good way to modulate, IMHO.

hr

It is so simple to have both!

modulation and a great built in reset program is loaded on most of the ModCons. If you are the tinkering type, like Mike you can fine tune these babies to infinity. Heck you can even watch and adjust settings when you are away on vacation.

This leaves the question of if and when to buffer to handle those micro loads. Some times the cost to add the properly controlled buffer is worth it, some times not. Depends on how far you want to chase the "ultimate control" package, and how much green you want to spend to get there.

hot rod

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Mike T., Swampeast MO

Which is a pretty good way to modulate, IMHO

Agreed. Does the basic (e.g. without Vision control system) Munchkin now have some sort of sensor to establish a target via some sort of reset curve?

Brad White_39

Of course you are right, Hot Rod-

I was just humoring him. I will stick to my guns from now on

Mike T., Swampeast MO

And if you're the master plumber/welder/fabricator/downwinder type like Hotrod you'll buffer a modulating boiler to the point that it efficiencly supplies just your heated bed frame and night stand

Plumbob

Sorry, my mistake. I was describing the Vision system, which is actually built into every Munchkin, although they charge you for the wires to insert into the connector so as to attach the outdoor sensor.

I wasn't thinking about the basic Munchkin without sensor wires. Those modulate based on the difference between the fixed target temperature and the return temperature (still not based on the difference between supply and return, though).

Bill Spencer

Wow what a a response. Thanks to all.

J. Criket,

Yes, having two smaller boilers would be an ideal heat source for my situation but two boilers means more parts to break down and less savings in the bank :-)

My situation is not quite as simple as what I mentioned at the start of this thread since that was simplified for the sake of discussion. While I'm not obsessing about short-cycling, I would like to avoid it if not too difficult. I think I will only have one zone which might short cycle. This could be handled by several methods:

1) Upgrading the baseboards to higher output ones.

2) Joining zones or use relays as you mentioned.

3) Utilize a smart zone controller such as a Tekmar to delay calling for heat until two zones call.

If possible, please post part numbers for any Tekmar parts needed to accomplish 3).

Would 1) be the best approach or 3)?

Thanks.

Brad White_39

Don't forget the buffer tank option!

That hydronic piggy bank can take you over the hump between cycles. Many ways to do that and some long threads on that recently. Do a search for "buffer" and see what you get.

Mike T., Swampeast MO

My bad as well. Said "supply" instead of "fixed target".

Plumbob

> If possible, please post part

> numbers for any Tekmar parts needed to accomplish


http://www.tekmarcontrols.com/lit/d.html

Find zone controls 367/368/369. Look in the manuals for "Zone control staggering and synchronization".

May not be exactly what you need, though. It isn't exactly what I said it was!

Frankly, for one zone you are better off ignoring the problem. Reduced efficiency is not a big problem for a microload because it is a few percent of a very small energy consumption.

Bill Spencer

Short cycling adds wear and tear to the boiler's moving parts no doubt and probably even some non-moving parts. I'm not going to worry about it since you've all educated me in ways to address this should it actually occur on my smallest zone. Thanks for your advice and thanks to all for their help.