Mod Con Boiler sizing

leed

I have looked through the comments / recommendations on boiler sizing. I know that Mod Cons will run at a needed output. There are differences in the lowest outputs between similar size (high output rating) manufacturers. Is it preferable to have boiler which will run at 20K as a low when compared to boiler which low limit is 30-32K?   Heat loss for the house is estimated at 50-55k

CMadatMe

Low End

Don't think 10,000 btu/hr is that big of a deal. You using 30 as a low end so I'm thinking your looking at a Viessmann Vitodens...I'd be more concerned with condensing then low end. What's more important, condensing at 30,000 btuhr btu/hr or not condensing at 20,000 btu/hr?

icesailor

Mod Con Sizing:

I'm the fly in your soup.

Being a licensed Master Plumber AND a Heater, I'd be more concerned about a Mod Con that has the ability to give you decent domestic potable hot water performance if you are using an indirect. Today, August 12, 2012, there is no heating load. But my sizable DHW load is 24/7/365. In the last few hours, there were two showers, the dishwasher and the clothes washer. If my wife got in the shower and didn't have adequate hot water to get the soap out of her hair, she would not care how efficient the new Mod Con boiler was. As far as she is concerned, I can put the whole thing into an unmentionable place and get something that will give her decent hot water. WHENEVER SHE WANTS IT!!!.

Properly sized domestic hot water systems are an important part of the equation that isn't always looked after.

When most folks run out of domestic hot water, they don't call the Heater Guy, they call The Plumber Guy. I know. They call me. Sooner or later.

Robert O'Brien

DHW

Much better off to oversize indirect and run it hotter with a mixing valve than to over size the boiler just to make DHW

icesailor

Sizing

If the boiler is too small, it doesn't matter how big or small you make the tank. DHW has to be prpperly considered and designed.

I consider and find that a smaller and hotter tank with a mixer is far more efficient than a big hot tank with a mixer.

Paul48

I Differ

I believe 10,000 btu/hr for home with a heat loss of 50-55k is a very big deal. Generally, a higher turn-down ratio offers much better control, and will keep the system condensing during more of the heating season.

CMadatMe

What Does

Modulation rate have to do with it?



Boiler "A" low end is 20,000 which means I'm there at about 60 degree outdoor temp and Boiler "B" low end is 30,000... I'll bet my house that both are oversized under these conditions...



Should be more concerned with which boiler will produce the most condensate, without condensate your at 87-88%...



What about boiler delta-t does that have some play in efficiency?

SWEI

∆T and efficiency

For a given supply temp (which is what the boiler controls are aiming for) a larger ∆T means a lower returning water temp to the HX (which implies more condensation and greater efficiency.)

Paul48

Thank You

I couldn't have argued it any better myself. Under those conditions, the boiler with the ability fire at a lower rate, would be a better choice.

CMadatMe

No

Firing rate does not dictate whether a boiler is in condensing mode nor is it going to be the determining factor to which has better efficiency..



Which boiler will produce more condensate a boiler running a 25 degree delta-t or one running a 40? The one running a 40 because it will see the coolest return water temps and will be in the condensing mode more often and that 10K is equal to about 1-1/4 gallons of condensate..Over a season that 30 low end boiler could be significantly more efficient then the boiler with a 20 low end...

Paul48

Hypothetical

Same boiler manufacturer.....100,000 btu with a 5 to1 turn-down ratio and 100,000btu with a 3 to 1 turn-down ratio....which do you buy?

Gordy

Hypothetical

Depends on the climate where the heating plant is. Also depends on the integrity of the envelope where said plant will reside.



If the climate is such that there is not extreme fluctuation from highs to lows daily, or the envelope is up to insulation standards of present day.



With that being said are we being anal about 2:1,3:1,5:1 even 10:1 turn downs? The vitodens 200 bosts a 2:1 From 31kbtus to 93kbtus yet is as efficient as they come. The Canadian IBC VFC 15-150 bosts a 10:1 turndown with 15 kbtus up to 150 kbtus. So which is better?



I could see where the VFC 15-150 cold give some breathing room in the dhw department with some down right low load matching modulation for shoulder season. But is it more efficient to run at 10kbtus for say 60 percent of the day, or 31kbitus for 60 percent of the day in a shoulder situation. I think controls of the emitter end comes into play more than the boiler to a point.



Gordy

Zman

I'm with Robert

On the Domestic, say you have some load "spikes" of 6gpm but generally you average 2gpm. You could use a boiler capable of 2 gpm or less then use a larger and/or hotter DHW tank to take care of the spikes.



Condensing boilers are most efficient when modulated the fire all the way down with the lowest possible return water temp. The limit to this is a boiler that is "short cycling". I think it is important to look at the boiler and system water volume as well as the modulation rate when looking at boilers. I have heard but not confirmed that some of the manufactures claiming ultra low turndown ratios are suffering efficiency at low fire.



Carl

Zman

SWEI

How in the world do you type that little "delta" symbol?

I have to know!

Carl 

CMadatMe

Boiler Choice

I would choose the boiler that gives me the best control over of the system and the one I know will produce the most condensate over the entire heating season. For me that would be as Gordy has pointed out, a Vitodens 200.



Boiler pumps sized for a 40 delta which every of mfg shys away from. They seem to want a 25 or 30. The use of a temp supply sensor for the system side gives much better control in making sure the system will see the water temp it needs based on outdoor temp rather then just reading a sensor inside the boiler.



I also like the lifetime warranty on the hx and knowing that this boiler was developed for a 40yr lifespan...

clammy

mod con sizing

Not to throw more info out there or a angle some may not have thought of but i have seen it first hand and that is over sizing of a mod con .The real issue is not the mimiun firing rate to load  but the firing rate which the boiler fires at which is usually 50 % of hi fire and the fact that some mod con run at 50 % on light off and maintain it till the flame is stabilized (1 to 2 minutes on some manafactures )which when a system is mirco zoned will cause the boiler to short cycle . Some may think it is a non issue but when you have a larger boiler with a 50 % light off and alot of mirco zones you will have short cycling .As for domistic hot water use a good indirect pipe it as the boiler  manafacture states and use there sensor and checks on boilers supply and return off your heating system ,install a good domestic thermostatis mixing valve set your tank at 140 mix it down to 120 unless you have a jacuzzi or hi draw shower (10 to 15 gpm) you should be fine .I have a HTP munchkin 80 wall hung with a buderus st150 tank pipe as described and have had relatives over and done like 6 showes back to back and have not had a issues .I have been usingmy set up for about 7.5 years and no issues .I have installed this type of set for over 15 years and have never had a custermer call about lack of hot water .Size your boiler to the heat lose give or take 10 to 15 % .My 80 is slightly over sized but i am not mirco zones at all and i have  done this set up with 50 mbtu munchkins  set ups and also have had no complants even with larger draw shower set up like 8.5 gpm .Only down side is recoverer time is a little bit longer .If you do have a very large domestic draw maybe a on demand unit like a noritz would fit the bill better then over sizing your heating plant.I always fiqure if your shower is doing over 12 gpm then you should be able to afford a on demand unit like a noritz plus my personal feeling is the noritz will be cheaper on operational costs being it has no storage and only fires when needed .Hope this helps and does not confse peace and good luck clammy

Paul Rohrs_14

Character Map

Carl,



You can find it on the Character Map which every PC is equipped with, from there, you "Select", "Copy", Paste.



Another way to do it is if you have a keyboard with the 10-key keypad, you can Press/Hold the "Alt" key and the numbers 0, 9, 1, 6.



Best,



Paul

SWEI

Delta ∆

I'm on a Mac, so I type

[option] J

Apple has simple shortcuts for the most commonly-used scientific symbols and a few accented letters.



But yes, an English keyboard with WIndoze requires you to wrangle a few number keys.

 

TonyS

How can you change

the Delta T of existing system?

SWEI

changing ∆T

Change the firing rate of the heat source, change the flow rate of the water, or change the load.

Paul48

Yes

In part, the thing I was being anal about. One of the primary features of a mod/con boiler.....modulation.The ability of the boiler to match firing rate to load.When the boiler is not condensing, and none of them will condense 100% of the time, the boiler that can better match the load is more efficient. If that happens to be a Viessman, then so be it.

CMadatMe

100 Percent

Sorry not to post the last couple of days was at Triangle playing with their new Trimax Control...



I have boilers that condense 100 percent of the time and even if they are not condensing they still run in the 90% world. Percentage of modulation will still allow a boiler that is not in the condensing mode to provide 90% or better efficiencies.

TonyS

Why would I do any of those things

Changing the firing rate shouldn't change the delta t through the system. Heat lost through the terminal units remains the same.

Changing the flow rate to get higher delta t would come at the cost of discomfort and the last terminal units not being able to satisfy the load.

Changing the Load, such as installing enough board in a new addition to heat that addition with cooler return water, I can see that working but most times that is not the case.

I am usually retrofitting a new boiler to an existing system that was designed to operat at a fixed delta T (USUALLY 20 DEGREES)

Modulating boilers change firing rate constantly but that doesnt effect delta t, It does as far as the board actually giving up less heat as its temperature gets closer to the room temperature but that is keeping the delta t closer not further.

Yet I hear some guys talk about changing delta t  as easy as throttling a ball valve with no ill effects. Maybe im just missing something here!

CMadatMe

You Are

We are not talking about system delta we are talking about boiler/primary side delta.. Still could do the system side easily with a Taco VDT circ and in Sept with the ecm version Bumble Bee..

Paul48

Flow

Quote...."Changing the flow rate to get higher delta t would come at the cost of discomfort and the last terminal units not being able to satisfy the load."

Higher delta-t implies more heat being used by the system. The problem you describe would exist regardless of the delta-t. Uneven heat is caused by poor design.

CMadatMe

No

A btu is a btu.. no matter if I'm piping pri/sec or using a LLH. I can have a 40 degree delta-t on the boiler or primary side and run a 20 on the system side..For instance on a Viessmann Vitodens WB1B10-35 the max flow rate across the hx is 6.2gpm, the boiler has a btu/hr output of 109,000 btu/hr..How would I move those btu's on a 20, its impossible...Has to be a 40.



109,000/(40x500) = 5.5gpm



SoI just moved 109,000 into the secondary side, how would you like to take them?

Paul48

Chris

No arguments about that.......It was not your quote, and he was referring to delta-t on the system side.

TonyS

Higher delta t implies more heat

being used by the system if and only IF the flow remains the same. That wasnt being implied.

So let me get this right, If I have 60 feet of baseboard that was installed in a loop through 5 rooms each room getting the proper amount of board based on a 20 degree delta t entering and exiting that loop.

There would be nothing wrong with just throttling back to a thirty degree delta t?

Mark Eatherton

I'm in Tony's camp...

In existing systems, that were designed for a 20 degree DT, slowing the flow to get a higher DT is NOT going to be conducive to good comfort. That last emitters are going to suffer, and consequently, the occupants will too.



Even on systems that were designed to see a "20 degree DT" when I show up at design condition, and I see a system DT of 8 to 12 degrees F, it confirms my suspicions about the holy grail of 20 degrees. If you can't hit it at design condition, what makes you think you can hit a 40 degree DT without causing comfort issues.



Can a system be designed for that large of a DT? Absolutely. But it causes one to have to expend a lot more money for emitters just to make it work.



Why not go with a tighter system delta T, say 10 degrees F with a middle of the road approach temperature? The only trade off I can see with that is increased wattage of consumption and possibly hydraulic erosion potentials. I'm thinking the same rule may apply to the source side of the system as well. Having a heat exchanger see a lower average temperature across its exchanger makes better sense than having a large differential just to get the return water temperatures below 140 degrees F.



I just don't see how anyone can get a higher DT on an existing system without creating discomfort. In a system that is parallel direct return, using a variable speed pump based on temperature DT, those loops closest to the pump will probably be OK, but those loops furthest from the pump will definitely suffer...



I always try and keep an open mind, but my field experience goes counter to ideas expressed here. I've tested the flow rate across conventional boilers, and it does make a difference. Higher flow = higher efficiencies. Lower flows = lower efficiencies, and more heat up the stack. Can someone convince me that the newer mod cons are an exception to this rule?





ME

CMadatMe

Did I

Design my baseboard to overcome each rooms heat loss with 180 degree water? Very important fact you left out. Also left out whether or not a heat loss was done. The boiler side delta has no bearing on your system delta anyway.

CMadatMe

Not Arguing

The fact that is they were "designed" for a 20 and the conversation was concerning boiler delta on the pri side not system delta on the secondary side. That's why no mention of s heat loss in any of the post..

TonyS

It doesnt matter if the water is 180

All that matters is the output of the baseboard is sufficient to heat the room on a design day and that all the rooms on that single zone are heated evenly. Ideally, the more board you get in the lower the temp you can use.

So how is increasing the delta t on the boiler side going to influence your condensing time? Your incoming water temp to your boiler is still only as low as the return temp from the system.

Paul48

Delta-T (I wrote this on another thread)

It has always been my understanding, that the various delta-t options provided by mod/con manufacturers were provided to ensure minimum circulation through the boiler at various system delta-t's, and not that you could maintain a certain delta-t through the boiler.Some of the I&O manuals are not clear on this, and others clearly state it for selection of primary circulators.If you are shooting for this delta-t, based on system design, use this primary circulator.

Zman

Hot topic

I am squarely with Tony and Mark on this one. You can't make the system take more BTU's than it was designed for. It is also not a good idea to feed it a different delta T than it is designed for. It should be designed for a certain temp drop between radiators (I am assuming radiators in series). The radiators should have been designed to match the load of the area that they heat. The reduced water temp between the rads should have been figured in.Changing to this changes the system balance.

You can set the primary pump speed to optimize boiler performance under a certain load. At full load you may have a 10 GPM 20 Delta T system load. You could set your primary to 5 gpm 40 delta t and that might be great. How often are you at full load? It will change as soon as the load changes(zones stop calling for heat).

I would love to see a boiler with built in primary secondary and an onboard variable speed primary circ controller. That way the boiler could constantly peg the delta T to an optimal target.

Carl

Paul48

SWEI

Do you feel like you just got mugged? You answered the question correctly, as it was asked. He only asked how to "change" the delta-t in an existing system.

Zman

Hijack

I just read the original post. I think it was more a Hijacking than a Mugging.

Sorry all,

Carl

TonyS

Paul48

I was not trying to mug anybody, I understand the fact that I am talking to professionals. So when I ask a question like"how do you change a delta T" I expect an answer worthy of a professional And I expect the answer to be within the context of a properly operating system. Of course I know how to randomly change a delta t.

This is not the first time this subject has been brought up. Many people have a misconception about flow and delta t.

Siggy once wrote an article about BTUs jumping off the train because so many people have a misunderstanding of this concept.

So anyway.... I'm not trying to mug anybody, just trying to enter the conversation with a few questions.

As far as my opinion, I think the lower the firing rate we can obtain the more efficient the system will become. I would love to see a boiler that could modulate to 0.

That would mean that even on a 65 degree day we would have the right size boiler, just replacing what were losing.

Paul48

TonyS

I re-posted something I wrote on another thread above. It speaks to running a 40 delta-t on the primary side. No one commented on the other thread, or this one so far. Right or wrong, I'd like to know.

Zman

Why not?

I don't know why you shouldn't target a 40 degree boiler delta t. As long as you are within the minimum flow rate of the boiler at you are delivering enough btu's to the secondary. I think you could size new systems for this and be very successful. As far as replacement jobs, I think you are likely to hit limitations on  the system side.

Carl

Paul48

OK

The way I wrote it must be confusing. I am saying, I believe the manufacturers only provide different delta-t options like 40*, if you were designing the secondary side for 40*.

TonyS

leeds question

The simple answer is...the unit that can downfire to 20000. The return water coming back from the system will be whatever it needs to be to heat the house on that given day.

The system doesnt care whats heating the water, The water will have to be the correct  temperature to get the proper heat out of the emitters. Condensing is caused by the temperature of the return water. Since both boilers condense on the same return temp, they will condense the same amount of time, it doesn't matter how far you spread your delta t in the boiler...if you bring 140 degree water into the return of the boiler and one boiler increases the delta t by 20 and the other by 40, the boiler with the 40 delta t will be less efficient as Mark explained. So with this condensing nonsense out of the way lets take a look at your heat load,bring it back up on your screen and keep increasing the design day temp until you hit 30000 btus...that temp is the last day your boiler is properly sized, any day warmer it will just short cycle more. Now keep increasing your design day until you reach 20000 btus, your boiler will not be over sized nearly as much of the year

Stay away from boilers with tight exchangers that require high delta ts and pri sec pumping. Fire tubeexchangers now used on many brands offers low pressure drop and no need for a second pump. A small circulator requires 90 watts, thats 7- 13 watt bulbs... more than I use to light my whole second floor.

Hope this helps!