How many btu's does an INDIRECT hot water tank require...

jeanRR

...to supply an average household's hot water needs? Is there a definitive answer or does it depend on other things? My understanding is that the number of btu's needed by a 40-gal hot water tank dictates the size of the boiler (we are talking Burnham MPO here) to be used. I would greatly appreciate your expert advice on this question. Thanks again for all your help.

Jean

John@Reliable_21

This will help

Jean, after reading your other post I find your contractors way of doing a heat-loss different, I will leave it at that. With a proper heat-loss done and with proper controls there should be no reason to over size the boiler for an in-direct. What part of MA are you in? John

WH–7, WH-41 SERIES
BOILER FIRST–HOUR CONTINUOUS
OUTPUT RATING FLOW
(5 gpm input– (90ºF rise) (70ºF rise
to BoilerMate) steady state)
BTU/HR GPH GPH
40,000 89 69
60,000 116 104
80,000 143 139
100,000 170 173
120,000 197 208
140,000 224 242

JasonB

There are two correct answers here.
1. Use the manufacturers info as they all supply it
2. As long as the boiler recovers demand. Many tanks are over sized therefore calling for a larger boiler. Must only recover the water used or demand. At that point a smaller boiler can be used or if you have a little longer recovery time a smaller boiler can be used.

Robert O'Brien

Dave Yates sums it up

http://contractormag.com/columns/yates/size-matters-indirect-water-0509/

Rudnae

average?

Seems like a loaded question to me. We prefer to go off of specifics, how many people in the household, how many appliances using the hot water, ie washing machine, dishwasher, extravagent shower system, etc. Figure out all the actual loads and then size storage and recovery to match the load. Most Mod/cons are sized to handle most domestic loads. Average changes for every person, kind of like "normal". With a standard indirect 75 USG a 150MBTU boiler can recover in 20 mins from stone cold. In my city this is a standard tank and standard boiler. If you require less boiler but more hot water then maybe adding storage is the way to go. If space is limited in the mech room then perhaps superior recovery, it really depends on the situation. I hope that helps. Rod.

jeanRR

Hi,

I'm situated in Billerica, on Nuttings Lake. Where are you?

Jean

Bob Gagnon plumbing and heating

I Like To Figure

At least 50,000 BTU'S for a domestic, that is about the same as a 40 gallon gas water heater. I know a lot of good plumbers that don't add anything for an indirect tank, they just use priority. I am a big proponent of setting your thermostat back while you are at work or sleeping, and if everyone jumps in the shower and turns the heat up with priority, the heat may not come up for a while. A SuperStor can take a lot of BTU'S from the boiler quickly, it just makes sense to have some ready and waiting. How's things down in Billerica, Jean? I'm up the street in Lowell. They are going to hold off on that big energy plant for now, huh?

Thanks, Bob Gagnon

jeanRR

Less is more

Hi,

I thought I recognized your name from somewhere, Service Magic, maybe? You know, 50,000 btu's sounds a lot more reasonable than 100,000 for a little 1700 sq ft house on Nuttings Lake. I hope I can reason with this contractor. We set our thermostats down at night, too. Did they kill the Home Depot project, too, or just knock it unconscious again for a while? Good to hear from a countryman for a change.

Jean

jeanRR

Hi,

Maybe I threw that word "average" in myself. Sorry if I confused the issue.

Jeanne

Unknown

I don't think it ever makes sense to upsize a boiler for a domestic load. Your domestic load is a small fraction of your heating energy use, and upsizing the boiler for domestic is making it more efficient for hot water but less efficient for heating. You want to prioritize your heating load efficiency.

This means smallest possible boiler for the tank required to meet load with that boiler. This is a BTU math problem only, assuming you are using an indirect large enough to accept the BTU output of your boiler.

Your maximum load is the only one that matters, not all the loads. That is, how many fixtures do you have to run at once, and for how long? Typically, this is a count of the shower nozzles in the home. A typical shower is 8 minutes. Regular flow head is 2.5 GPM, low flow is 1.5 GPM.. use low flow.

GPM x minutes x 8.4 x temp rise ground to fixture temp = btus needed.

Volume of tank is storing (140-fixture temp if you store at 140) x 8.4 x gallon rating.

The difference in btus must be made up in the time you are using it. So if you still need 5,000 BTUs over ten minutes, then you need a boiler capable of 30kBTUs/hr (and a tank able to take that output) to keep up through your demand.

finis. To Bob's issue... have your setback recovery start earlier if you don't have hot water at the same time as you are recovering from setback, don't upsize the boiler.

Charlie from wmass

Adding a little for the indirect is not such a bad idea as water temps in Mass are lower than many areas of the country. about 42 degrees F here in western Ma. The other trick is a nice thermostatic valve after the indirect heater allows for a constant 124 at the tap with 140 to 160 in the tank. I avoid priority control on the indirect.

Charlie from wmass

Jean I do not think the price difference is in the boiler but in a better installation on at least a complete installation of your boiler. I think going with the "larger" boiler will be best as it is simply the fire rate that differs not the actual cast iron part of the boiler. The price diff between the 84 and 115 is minimal. The stainless steel liner and related work would account for more of a difference, well worth it for the added saftey.

hot_rod

what type of performance are you

looking for? More BTU, or horsepower, more recovery.

Any tank could perform with 35,000 or in some cases 200,000. The more energy the faster the response.

What are you used to now? If you have a 37,000 input 50 gallon gas fired, then a 50 gallon indirect with that same input would perform about the same. Keep in mind the gas fired WH has a derate factor for altitude and efficiency. Could be as low as 65% efficient, plus the altitude derate.

The boiler sizes to the largest load. In many small, efficient homes the DHW load can exceed the heating load.

Don't forget to consider the solar DHW system

hr

jeanRR

Too technical...?!

Hi Charlie,

I hope this won't run too long. My house has a sunroom facing NE and overlooking a lake. For our first 10 years here, the room was too cold to use in winter. I kept the french doors closed and sealed off. When I had my present boiler installed 22 years ago, I put the sunroom on a separate zone. We still use the room conservatively, though, because our oil bill spikes noticeably even with recaulking storm windows, and so on. Here's the question: does a heat-loss analysis take into account any environmental factors such as lake-effect wind and northern/southern exposure? I understand the swe programs make some allowance for what isn't included in the analysis, but is a heat-loss calc more or less a guide or starting-point to help you decide which boiler size you need? My contractor and another contractor both arrived at around 64,000 btu/hr.

Many replies on this site have cautioned, rightly, I think, about oversizing a boiler. My contractor, however, advises me to install an MPO 115 for my 1700 sq ft living area -- for fear of too little hot water being supplied. He is now saying that there is hardly any difference between the MPO 84 and 115 -- just the firing rate, baffles, and nozzle size. He thinks I am getting too technical about an oil boiler. Then why all this discussion? One reason is that the label on my present boiler shows "144 btu" and I prefer not to repeat past mistakes.

But it's time to make a decision. So if you can downsize the 115, but NOT upsize the 84 if needed, then all things considered, is the 115 a reasonable choice after all, even if it will cost more over the long run in oil, maintenance, and longevity?

If you've come this far, thanks so much for listening.

Jean

jeanRR

Too technical?!

Hi,

I hope this won't run too long. My house has a sunroom facing NE and overlooking a lake. For our first 10 years here, the room was too cold to use in winter. I kept the french doors closed and sealed off. When I had my present boiler installed 22 years ago, I put the sunroom on a separate zone. We still use the room conservatively, though, because our oil bill spikes noticeably even with recaulking storm windows, and so on. Here's the question: does a heat-loss analysis take into account any environmental factors such as lake-effect wind and northern/southern exposure? I understand the swe programs make some allowance for what isn't included in the analysis, but is a heat-loss calc more or less a guide or starting-point to help you decide which boiler size you need? My contractor and another contractor both arrived at around 64,000 btu/hr.

Many replies on this site have cautioned, rightly, I think, about oversizing a boiler. My contractor, however, advises me to install an MPO 115 for my 1700 sq ft living area -- for fear of too little hot water being supplied. He is now saying that there is hardly any difference between the MPO 84 and 115 -- just the firing rate, baffles, and nozzle size. He thinks I am getting too technical about an oil boiler. Then why all this discussion? One reason is that the label on my present boiler shows "144 btu" and I prefer not to repeat past mistakes.

But it's time to make a decision. So if you can downsize the 115, but NOT upsize the 84 if needed, then all things considered, is the 115 a reasonable choice after all, even if it will cost more over the long run in oil, maintenance, and longevity.

If you've come this far, thanks so much for listening.

Jean

First posted to charlie in w/mass

Maine Ken

Do you have enough radiation to handle the MPO 115? If not, are you going to add some? I would never oversize by choice. The heatloss is the heatloss. Adding extra is just a waste. With a proper control strategy there is no need to size for the indirect.

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Bob Gagnon plumbing and heating

No Jean

The heat loss I use does not take into account southern exposure or lake effect wind, nor does it take into account when the weather stripping on the doors start to leak worse. A heat loss is a guess without a blower door test. I disagree with Rob and I feel you should add some btu's for domestic, what happens on the coldest of days when everyone jumps into the shower one after the other? The house gets cold that's what happens. Average 8 minute showers? Where, in the military maybe, it takes me 8 minutes just to scrub my hands, what happens when teenagers take showers for an hour, no heat that's what happens, I add for domestic. That being said, like I said before many good plumbers I know don't add anything for domestic, it's your call. Jean, I'm not on service magic, maybe you know me from my renewable energy systems, I get a lot of press for that, or maybe from battling the Hydro plant in town that has flooded us twice, that is why I am concerned with your energy plant if they treat you the same way they treated us.

Thanks, Bob Gagnon

Unknown

All you have to do is define the max load and do the math Bob. If the max load is "everyone jumping in the shower one after another" and everyone takes half hour showers (and you want to pay for that), then it's just a different math problem. In the end, the BTUs are in the water when you want them, or they are not. I completely ignore recovery time and I've never seen a house get cold as a result.

Often, when people see what those kinds of super high loads mean, they realize they might not mind if they have to tell junior to wait 15 minutes for a shower on a design day January 15th. or if maybe he needs to take 10 minutes showers or stand in cold water. It's about expectation. People knowing what they can and cannot do is not a bad thing.

Upsizing a boiler is never the right choice, IMHO. Unless you figure out how to use this domestic tank as a buffer tank or you're adding a buffer tank, in which case, upsize all you like with my blessing. But just plain upsizing without regard to cycle time is not as good as it can be.

And hey, if the weather stripping gets bad and the boiler stops keeping up in five years... maybe that's what's needed to remind people to fix the weather stripping, eh?

Heat loads are a "guess without a blower door"... agreed. But you can make pretty good guesses on infiltration if you narrow down the factors. Windy lake? bump it up an extra 0.2 ACH or so. Open flue fireplaces? Leaky windows? etc. Knowing when and how to be conservative appropriately makes a big difference.

Unknown

Why not just

install a Navien tankless. Nothing to figure out, it just produces exactly what you need, when you need it for exactly as long as you need it(at 98% efficiency)When you shut off the hot water, it shuts down and doesn't come back on until you need it again. It is the epitome of fossil fuel water heating. Anything else is just a compromise.

jeanRR

\"Adding a little for the indirect\"

-- do you mean by that, selecting the MPO 115 over the 084? So if I do select the 115, and if my particular heat load happens to fall somewhere in *between* the capacity of the 084 and 115, have I heard right that the firing rate of the MPO 115 can then be reduced appropriately??? (My boiler man really believes I need the 115 boiler to meet the 100,000 btu prescription for the 35- to 40-gal water tank. I think he goes by the book. And he says, you can downsize the MPO 115, but you cannot upsize the MPO 84.)

Jean

CC.Rob_15

my math

gpm minutes constant dT btu/8min btu/hr

2.5 8 8.4 70 11760 84000

Am I doing this right? 2.5 gpm shower for 8 minutes, 70F rise, needs an 84k BTU/hr boiler.

Charlie from wmass

The math would be right if you were heating it in a tankless manner. But I think it does show why the higher fire rating would suit Jeanne better IMHO. As Bob said what happens in january when the set back needs brought up and the family is lined up for showers? Also a 70 degree rise is optimistic as 90 degree may be closer to what she will need in the winter just to get to 124 F out of the heater. This is why I prefer to stay far from tankless water heaters. I also think if the teen can shower for a full hour with out running out of hot water they will simply do it and there goes the fuel bill, but I will save that rant for another day. Sometimes a bit of cold water it a good thing for those who are time challenged in the shower.

CC.Rob_15

so what's more realistic?

What is a more realistic or better way to figure the indirect requirement? Thanks. Indirect input has puzzled me for a long time.

Unknown

Why would you need 124 out of the heater? 110 starts to hurt at the fixture and you shouldn't lose 14 degrees from the heater to the shower.

2.5 gpm x 8.4 x a typical 70 dt = 1470 BTUs/minute, = 88200 BTUs/hr if you want continuous. 12k to 15kBTUs per regular shower head shower.

however continuous is not necessary. Your stored volume of heat counts too. If you keep the indirect at 140, it won't drop below fixture temp (110) for 30 x 8.4 x your gallons of storage. A 60 gallon tank would cruise through a typical shower without calling for heat at all. At some point it would, and even if you had a 60k output boiler it would only be 15 to 20 minutes of priority. Not remotely enough for a house to get cold.

Or you can use a smaller tank and count on the boiler output per minute to make up the shortfall.

I don't see any problems with this or setting your setback recovery a half hour earlier so you can recovery and then do all the showers. Not a big deal.

jeanRR

Not enough radiation...

...but 78,000 btu's is close for handling the MPO 115?? Also, the early bidder who came up with that figure may have missed some baseboards. I am going to measure them myself. What is the formula to determine the total btu's? (I might need help.) In any case, I'm told the 115 and the 84 are the same boiler, differing mainly in nozzle size and firing rate. Thanks for your input -- any further comments are welcome.

Jean

Paul Pollets

My 2 Cents

We've always asked the customer what do they need for DHW load, and that determines the size of the storage tank. I've always made sure that the boiler had at least 80-100K BTUs for priority recovery. While the modulating wall-hungs make selection and efficiency easier, in Seattle, the heat load was almost always lower (usually by 1/2) than the smallest boiler net output.

Never had a customer complain that he ran out of HW. I'm not a fan of on-demand water heaters; indirects, sized properly for both load and BTU input, provide a better product that's more efficient, and has lower life cycle maintenence costs.

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Charlie from wmass

Have you ever done dishes with 110 water? or tried to run a dish washer with 110 water? As the debate goes on Jeanne still has a leaking boiler. We are debating back and forth over an oil nozzle and what 2- 4 percent possible savings between the sizes? Before her head starts aching why do we not all agree to disagree as to specific water temps at the faucet and agree that either boiler will work if she has the best qualified installer put it in. since the larger one is the smaller one in all ways besides nozzle size I am guessing the price difference between the 2 installers is due to piping strategies and thing such as one quoting a stainless steel liner and the other quoting an aluminum one( which you can not do in mass for oil as far as I understand no matter if it would work or not in other regions of the globe) The leaking is costing her dollars and we will still be here debating this long after she is saving fuel with her nice new boiler.

klaus

Not going to preach, but I have a Super Stor SS40 40 gallon, and a very small oil boiler, Biasi B10-3. The heatloss on the house is 45k BTU at 10 deg outside, and we saw plenty of those days this winter and I had no issues running 2 showers in the morning, with night setback on the heat. I do run a mixing valve on the tank because my control post purges heat to the tank, so the tank temps do reach into the 140-145 range at times.

I think unless you have a bunch of kids around, and the demamd is very high, there is no need to upsize the boiler for DHW production. Another option might be to upsize the tank a bit to provide enough for the peak demand assuming you are not overly concerned with recovery time. Some of the more advanced controls like mine have the ability to realize the priority call for DHW is going to long and its time to bring the heat circ back on.

Bob Gagnon plumbing and heating

What Is

The heat output of you boiler?

Thanks, Bob Gagnon

RobK

inderect water heater

Size the water heater to the number of bathrooms, sinks, dishwashers and washing machines, then add the heat loss of the living space. Remember,if you have to sell the house it will have to meet the requirements of the new owners which cannot exceed the current size of the house.

jeanRR

My present boiler outputs...

D.O.E. 144 btu according to the label attached to the boiler.

Jean

Unknown

I'm with

the people that recommend sizing the boiler to the house heatloss . The heatloss and size of the house typically goes hand in hand with the hot water needs . Anyone ever see a 1500 sq ft house with 4 showers ?

If needed , upsize the indirect or add a storage tank to get the homeowner past the increased demand .

FYI , I have a 1500 sq ft home with a boiler firing around 60,000 btus and a 30 gal indirect . We haven't run out of hot water or had the house temp drop significantly on the coldest mornings after 3 or 4 showers in a row ( with 2 women taking at least 40 min showers ) . And the boiler is on priority .

Unknown

most dish washers can bump up their own temps. studies also show that raising water temp isn't the answer. Better soap is. You can do your math on whatever temps you like, I'm just saying I've never had a complaint on DHW using those numbers though. You sure as heck do not need 124 though. 115 would be a maximum for safety.

I think 2 to 4 percent savings is worth reaching for. Why throw it out?

edit: I also think you're talking about a heck of a lot more than 2 to 4 percent loss by upsizing boilers, and you lose it on a load that is easily at least 5x the yearly energy usage of domestic.