Indirect Sizing

Bill Riordan_3

I hope some of you can offer your advice concerning indirect hot water heaters.When performing a heat loss calculation, the boiler size required is relatively small. However, the indirect requires the boiler to be subtantial in order to provide first hour rating.This is where the problems start.
Indirect manufactuers all have different types of ratings and it seems impossible to compare the various brands.There are multiple styles and designs all claiming to be the best.I have a difficult time selecting the proper indirect, matched to domestic hot water load, and have been oversizing the indirect to cover my back.
I never want to oversize the boiler and feel priority DHW has some limitations. There must be a compromise.I wish to find that balance without leaving the customer short on hotwater. Thanks

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Brian (Tankless) Wood

Try one of these Bill.

Instead of the indirect.

The problem of small boilers with indirects, comes up all the time. So just separate the systems.

This is one of two that I installed recently for the same customer/freind. She liked the unit in her own home so much, that she had me install this one in a home she is remodeling for her daughter.

Both units are 165,000btu Takagi T-K1's. We also have 185 & 235mbtu models (and bigger if needed).

Let me know if I can help.

Brian.

Boilerpro_3

IMHO

I feel that most sizing charts for indirects are very conservative, in my rather limited experience. One of the first I put in was an Amtrol 40 gallon with a 80,000 out boiler with priority. Due to poor consturction techniques the home had a calculated heating load of about 56,000, but an actual of about 80,000. Heating system was about half heatway radiant staple up and half baseboard. The unit supplied hot water for all regular uses plus a 75 gallon jacuzzi. No problems. Just put a WEIL Mclain/ Triangle tube 40 gal in with a 110,000 out CI boiler and 75 gallon jacuzzi. I looked at sizing this way. I figure I want 115F water in the tub. The tank holds water at least this temp, so I have 40 gallons here to start with. I also usually use standing pilot boilers, so the water in the water in the boiler is also at this temp year round, so I have this additional stored heat. Then I look at the amount of time it will take to fill the tub, probably about 15 minutes. With 110,000 boiler output, this makes about 3.5 gpm of 115F water. So I also have 15 minutes times 3.5 gpm which equals another 52 gallons of hot water. All together I have at least 92 gallons of hot water available in the 15 minutes it takes to fill that 75 gallon tub. When you figure you got to leave some water out so someone can get in the tub without it overflowing, you don't even need that 75 gallons. The sizing lit that I have seen says you should size the tank storage capacity to the size of the tub, however, as you can see this is unnecessary.


Boilerpro

heatboy

Or........

sixe the indirect for the load of the tub. If it's a 75 gallon tub, my minimum indirect size would be 90 gallons. While the tank holds 90 gallons, not all of that will be readily available for hot water due to the efficiency of draw down mixing. Even Viessmann tanks, which are by far, the most efficient tanks I have used to date, will only have approximately 85% capacity in hot water. So the 90 gallons is actually only 76 gallons before it starts to cool off. All of the othe tanks, Mega Stor, Triangle Tube, etc., are nowhere near that 85% drawn down rate, so the issue of sizing is more involved.

My point, I think, is always add more storage than needed. While there is an increase in energy consupmtion due to storing more hot water, priority circuits are not needed and the boiler can recover on it's own after the dump without fear of having the home cool off due to priority circuits. Besides, if they have a 75 gallon tub or one of the drive through showers with 5 or 6 heads, are they really concerned about fuel efficiency? I think not, they just want plenty of hot water.

hb

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Bill Riordan_3

Would you match the 90 gallon indirect with a smaller than recommended boiler? Is the goal to have some storage volume and not to be so concerned about recovery time? Will this situation be greatly improved if a mixing valve is used to allow a warmer indirect temperature? Thanks for the advice.

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heatboy

With smaller boilers.......

it would seem to be even more important to have adequate storage since the recovery rate is not there. Adding a mixing valve to the tank and maintaining higher tank temperatures is one way of helping the first dump capability, but slows even further the recovery of the indirect due to it's higher temperature demands. No free lunch here, eh? Sizing the tank storage for the biggest possible demand then let recover as it will after the load diminishes would seem to be the best option.

hb

92 gallon indirect with a 80MBH boiler

kevin

it depends...

on what your heat loss is... if you have only 60,000 btu of heat loss you will need more boiler to match the output that a 120K btu water heater would need...there are other variables too.Do you have some specific numbers? The mixing valve definately helps...i have installed 3/4" sparcos on 45 gal. Superstor ultras and dare the owners to run out.No complaints yet... kpc

kevin

Bill...

what fuel is being used here by the boiler???kpc

Big Ed

Oh there it is.....

..... I have been looking for my AC tester everywhere

ScottMP

BoilerPro ?

I have come across a number of jobs sized just like you stated and none of them can fill the tub all the way.

I think the number that is off in your calculations is the gpm for the faucet. Unless your using a lav faucet to fill the tub the gpm should be closer to 14 gpms for a tub filler. I found Kolhers coralias line is 10 gpm, and some of their faucets are as high as 25 GPM. At 10 gpm its not going to take long to chew up your 40 gallon Amtrol its coil can only put out so much no matter what size the boiler.

Most people don't have the time to slowly fill the tub, besides the water cools off so fast the tub is cold if it sits too long. The norm is to open the hot and cold faucets and let her rip.

I size the indirect to the volume of water needed. Allthough we do install Sparco valves when the volume is close. 50 gallon indirect with a temperature of 160 and a Sparco set to 125 can do wonders.

Just my thoughts

Scott

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Bob Bona_4

in my

experience, sizing the indirect to 75% of the tub size, and using a mix valve (140 down to 115) is a comfortable realm for me. The biggest problem I have seen in the field is undersized boiler piping and pumps to the indirect. You need the flow to take advantage of all the boiler's available BTU's. Overheard at one job site: "Why the heck do they make the tappings 1 1/4 on this indirect?? I need to bring a 1 1/4x 3/4 bushing every time!!"

Oversized indirects sometimes hide all sins like that due to their sheer volume..

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Boilerpro_3

Thanks for the numbers on the faucets

I was thinking about how long it takes to fill our big old 1906 cast iron 5-6" tub in my own home with 60psi supply. Guess those tub fillers put out alot more water. That said though, that Amtrol install has been in for 7 years and there have been no complaints, maybe the homeowner's are more patient than some. We'll see how the 110 mbh 40 gallon WM works in a few weeks and this coming winter. So with a 10 gpm faucet you can figure about 7 minutes for a fill... about 40 gallons from tank @115F (assuming tank temp is up around 125F) and seven minutes of boiler firing gives you about another 24 (assuming a warm boiler with standing pilot and damper or a low water content boiler like many newer CI, condensing, or copper tube). Probably right on the line, especially when you take out some water to leave room for people in the tub. Maybe higher tank temp and mix valve will be needed...we'll see. Glad to see this thread up here. I've seen what appear to be some grossly oversized water heating systems. My local high school, built for a max of 500 students, has at least 600 gallons of storage (the tank is the size of a school bus) and tied to the main 14 million btu boiler (building load about 2.5 million) and another 1.2 million boiler for summer operation with a second boiler apparently the same size already removed.

Boilerpro

kevin

I hear you Bob...

when I use the Vaughn top performers(heavy things) the coil tapping is 3/4"..I use a 1x3/4" cma to get plenty of flow to the heater and back asap...kpc

munchkin-man

We have finised the new design for two wall hung Munchkins

The smallest has a ratio of 5:1 it has a range of high fire up to 50,000 btu's and will modulate down as low as 10,000 btu's. when the indirect calls the Vision one controler will deliver 80,000 btu's using priority technology.

The next sise up has the same 5;1 ratio a high fire of 80,000 btu's will modulate down as low as 15'000 btu's. When the indirect calls the Vision one controler will deliver 110,000 btu's to the indirect.

There is your answer you can use one of our high quality stainless steel life time warranty indirects and a Munchkin gas fired 92% AEUE fully modulating condensing 0 clearance to a combusable vented in PVC 40 top of the line boiler to do it all. For your conveinence we have a sising quides specificly for the use of Munchkin in combination with a SSU Ultra Superstor indirect.

These new wall hung unit's are coming to our suppliers in late summer to early fall.

Bob Bona_4

now

how cool is that!

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