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# Don't size a hot-water boiler by measuring the radiators

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edited August 2017

Don't size a hot-water boiler by measuring the radiators

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On a semi-related note: Why are pipe sizing calculations based on a 20 degree differential when in over 20 years I've NEVER seen the return water less than 5 degrees that of the supply temp (if even that)?
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Saves the strain on your back as well , with not lugging in more cast iron then you have too....

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On a semi-related note: Why are pipe sizing calculations based on a 20 degree differential when in over 20 years I've NEVER seen the return water less than 5 degrees that of the supply temp (if even that)?

I think 20° was an arbitrary number selected, possibly to make the math easy. It could have been 19 or 21 with no adverse effect, I suppose.

Heat exchange in hydronic systems is a dynamic condition, as loads change in the space from sun, cooking, doors opening, etc the heat transfer conditions also change.

Sharp designers know when to design a system based on a 10° differential, and when a 40° differential makes better sense. Some systems may never operate exactly at the designed for differential.

The goal should be a comfortable, efficient system, not so much an imposed ∆.

http://www.pmmag.com/articles/95877-nature-vs-math
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
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An oil company I used to work for did not have a sales department for equipment replacement, only new accounts. The service techs would go out, check, and show the customer the price in our book. Whether you were an A, B or C tech. If you were in the area, you went. We had 1 page Survey sheets to get info to replace a boiler. Existing equipment, firing rate, pipe sizes, line or low volt controls, access in and out, carpet, tile, etc. At the bottom of the page was the dreaded, "Recommended new boiler".

Now of course, most guys went by what was there. 4 section out, 4 section in. I got called in one day to see the service manager. He told me I was taking too long on surveys. I told him I was measuring rooms, windows, calculating heat loss factors, getting into from the customer about hot water consumption, any future renovations or additions in the works?

He told me to just measure the baseboard and be done with it. I explained that was not proper. He said, "OK, if Mrs. Jones has a Peerless JOT-3 firing a Beckett AF at .75 GPH, then an "upgrade" to a Peerless WBV-3 with a package Riello will be fine. I say "But 'Joe', that's not the proper way to do it. AND, the installers won't change the factory installed .85 nozzle, and with the pump pressure, it will fire at 1.10." He says "all the better to keep up domestic hot water demand." I said "Don't send me to any more surveys."

No I know why I'm almost bald, and its not genetics.
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I have been teaching for over 35 years and have always taught Heat Loss Calculations. It is correct that there is usually too much baseboard in an older system and no one did a proper calculation when it was originally installed. Also, buildings have been upgraded with energy saving additions. When installing a new replacement boiler you MUST do a calculation but it involves doing a little math, oh God adding and subtracting and multiplying and if you have a computer program you don't have to any math. Guys that just measure radiation and replace a boiler that way either don't know how to do a heat loss calculation, or just too lazy to do it right.Mostly all of the boilers that were installed without a calculation are grossly oversized and the customer suffers.
The reason for the 20 degree differential is because flowing 1 gallon per minute for 1 full hour with a 20 degree drop will provide 10,000 Btu. The reason the differential is usually never 20 degrees is because most circulators will provide more the 1 gallon per minute so the drop is not as great but you still get the full amount of heat out of the baseboard. Because of this factor, we say that every gallon that flows per hour delivers 10,000 Btu, so, to determine the flow rate needed for a circulator, divide the total Btu that the circulator has to deliver by 10,000. If you have a total Btu loss for an entire house of 45,000 Btu and there is only 1 circulator, your circulator will have to flow at least 4.5 gallons per minute to supply enough heat. It is also used to size the zone piping and the supply and return trunks.
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I have sized many replacement h.w. boilers in old homes by making a list of the free standing cast iron rads.Number of sections, columns & height. Does this mean that the old boys, (dead men, to coin a phrase) were oversizing, back in the day?
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> @barnabe said:
> I have sized many replacement h.w. boilers in old homes by making a list of the free standing cast iron rads.Number of sections, columns & height. Does this mean that the old boys, (dead men, to coin a phrase) were oversizing, back in the day?
>
>
>> That, along with the connected load is how to size a steam system, never hot water. Steam doesn't care about heat loss. It only wants to get to the last rad before condensing.
A heat loss calculation must be done for hot water, forced hot air and A/C.
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While sizing the boiler to the emitters should not be done. I believe that doing an existing radiation survey is prudent, especially when installing a mod/con to get the most benefit through the lowest possible AWT to meet the load. Doing the survey will also make the installer aware of over, and under radiated rooms.
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I luv when a prospective customer says "the other guy is quoting the next size up". Always fun when that pops up.
Gary Wilson
Wilson Services, Inc
Northampton, MA
gary@wilsonph.com
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GW said:

I luv when a prospective customer says "the other guy is quoting the next size up". Always fun when that pops up.

Sure was...To bad most home owners no nothing about a simple heat loss or EDR calc...and as well why should they....That's why they hire the person they do...There appears to be a few savoy home owners who post here quite a bit...they kinda seem to be well read....With the cost of everything today it really pays off to be educated
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Yes true! I think it's a little delicate though; if a homeowner pretends he's as well versed as I am I'm not going to pursue that relationship. I like when people know the basics. I just don't like the "know it all" types. It's extremely rare that I bump into these people.

Years ago I got a package in the mail, some little residential job that was self-speced I tossed it, no engineer knows what we do about residential heating (unless they spent 10 plus years in the field)
Gary Wilson
Wilson Services, Inc
Northampton, MA
gary@wilsonph.com
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You can't blame the homeowners. Well, not completely.
They don't know anything about it and are getting told 10 different things. It's not only confusing, but can also be very frustrating.

Those who argue pricing need to put on the pro's shoes for a bit, then they might realize it's not so fun.

Personally I feel steamers shouldn't be sized by the radiation either, but I think I've made that more than clear enough over the years and the only one really agreeing with me is Dave Bunnell. That's ok, it's better than having no one on your side.

A lot of pros deserve a ton of respect, their jobs aren't easy and need to be done fast to be profitable. On top of it they have to put up with customers that are clueless and often argue prices etc.

Sizing a boiler properly is an art. Doing a proper heatloss or heatgain is also an art because as we all know it's far from perfect. I asked a pro on here for helping doing mine because I knew it was something that required a lot of experience, and that can't be learned from a book.

From my own experience, a boiler that is too big isn't just wasteful, it's also less comfortable. It causes faster than necessary temperature swings. At least this was the case with steam for me, I'm betting hot water is similar. This was by far the biggest benefit of shaving 25,000 btu/h off my boiler, it slowed how fast it heats the house.

The less people notice a system is even functioning, the better and slow is the key to this. Think about it, what's the problem with forced air, drafts aside? It's got zero mass to compensate when it's off and it's very noticeable when it's on.

I suppose, this is where a modulating boiler really shines.

When will someone come out with a modulating forced air furnace that can drop down to the set point? A forced air system running 24\7 blowing out air at, or close to the set point may work very well.
Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
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I think most homeowners put more thought, and research into many consumer products that mean far less than a comfort system in their home. A comfort system that will last much, much longer, and be a part of their daily lives.
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Gordy said:

I think most homeowners put more thought, and research into many consumer products that mean far less than a comfort system in their home. A comfort system that will last much, much longer, and be a part of their daily lives.

Agree
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Everyone has their priorities.

I guess that's to be expected.
Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
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edited August 2017
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On the service end, customers can Google any part of their equipment if they have a manual and get the price for it. Some don't understand labor, travel, overhead, etc.
"Can I get the part and you replace it?"

"Nope. You can get the part and you can replace it."
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ChrisJ said:

The less people notice a system is even functioning, the better and slow is the key to this. ....
I suppose, this is where a modulating boiler really shines.

It seems like a MOD-CON with cast iron radiators (either sdt or baseboard) would be a great, efficient and comfortable system...

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NY_Rob said:

ChrisJ said:

The less people notice a system is even functioning, the better and slow is the key to this. ....
I suppose, this is where a modulating boiler really shines.

It seems like a MOD-CON with cast iron radiators (either sdt or baseboard) would be a great, efficient and comfortable system...

Yes.
Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
• Member Posts: 1,801
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HVACNUT said:

On the service end, customers can Google any part of their equipment if they have a manual and get the price for it. Some don't understand labor, travel, overhead, etc.

"Can I get the part and you replace it?"

"Nope. You can get the part and you can replace it."

I could not agree more....Can I bring my own drink of choice and my own food. And can u cook it and serve it....I think not
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ChrisJ said:

Everyone has their priorities.

I guess that's to be expected.

Of course it is...