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I commented

Wayco Wayne_2
Wayco Wayne_2 Member Posts: 2,472
on another thread about sizing a boiler for the heating load of the house. I got some E-mails asking questions about it. I always sinze to the heat load. I feel it's especially important for a mod con boiler. I often find myself at odds with supply house designers who recommend to their clients the contractors, to size it for the domestic hot water load. I want to throw this open for discussion. 

My reason for sizing to the heating load is that during the Winter the boiler is mostly heating. That's it's job 99% of the time. With a mod con it is especially nice to target the capacity of the boiler so it can modulate up and down rather than shutting on and off. You can acheive the highest efficiency that way. If your boiler has a nice tight throttling range that matches the house's heat loss you can maximise your efficiency. It's just like the difference between stop and go city driving  vs. Higway mileage in a car. Low firing rates and long cycles saves the most money on your fuel bill.

Domestic hot water is another animal altogether. Usually in the morning everyone wants a shower. You want a lot of hot water all at once in a short time period. This is totally at odds with your heating needs.

What do you want? Lots of power and fast recovery, or long steady minimal gas firing? You can't have both with one appliance.

I run into this conflict a lot. A lot of row houses downtown have very small heat losses. I often see a need for only 40,00 btuh on my heat load calcs. The row houses are often being renovated by young uban professionals who want, along with a shorter commute to work, a ginormaous soaking tub with a gazillion jets to unwind in after a hard days work. I am also seeing multi head shower stalls that would make a car wash envious. You can't support that with a 60,00 mod con boiler. No way!

I always go for more storage. A larger indirect tank. A lot of my customers like the option and I make sure they inderstand why it is needed. Sometimes to save space We put in a small indirect and run the hot water through a tankless water heater. That way, if you start running out of hot water you have a back up. You get the high efficiency boiler water from the mod con boiler and the longevity of the tankless. There are now condensing tankless water heaters so you can do away with the storage tank altogether if you want, and not lose the high efficiency you crave.

Another way to go is to up the boiler size and install a buffer tank. That's waht I have in my house. In my case though the buffer tank also doubles as my solar thermal storage tank. It helps my boiler to keep from short cycling a lot. In the Spring and Fall the solar collectors fills the tank with hot water and the boiler gets to rest. Yaah!

I understand the supply house designers are doing their best to sell what they think is right but I think they should be required to get some field experience too. How can they be put in a position of resonsibility as important as design and engineering and not be tempted to just throw a big capacity boiler at a job to CTA?

Sorry I ran on so long. I hope I didnt bore you. Please share how you appraoch this matter. WW 


  • Rich_L
    Rich_L Member Posts: 75
    Heat Load of the Structure

    I very much agree with your points Wayne. I size the boiler to the heat loss of the structure and if that wont satisfy the domestic load, go with a gas tank type or tankless water heater. I haven't done a tankless condensing water heater yet but am looking forward to those.
  • CMadatMe
    CMadatMe Member Posts: 3,085

    I find myself torn in the same way. Low heat loss but high domestic demand. I;m finding myself going the storage route plus trying to get away from the 3/4" smaller coils in most indirects. Just found a new indirect with a 1" coil that is pretty competatively priced that we are trying out this week. It has some nice features such as 2 annodes that are mounted in the front for easier service as well as all the taps. DHT was more than happy to give us one at no cost to try out.

    They are also coming out with a wall mount 20,30,40 gallon indirect that compliments the mod/con.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,837
    You have to cover the worst case scenario...

    be it DHW or SH. The SH calcs you already know. What you DON'T know is what kind if load diversity you have on the DHW side of the system.

    Here in Colorado, in the ski resorts, people are paying upwards of $1,000 per day for a 4 to 6 bedroom lodging facility. When the folks slide off the slope and into their cabin, they ALL want to jump into the shower at the same time. There is NO load diversity in that case, and if they run out of hot water, you can bet they will be on the phone to management demanding a partial refund or worse. Compounded DHW loads can be 4 to 5 times great than the SH load. Excellent application for multiple mod con boilers with inter communication abilities.

    The DHW load is calculatable, but load diversity is the one question that never gets asked and answered, and is going to raise its ugly head.

    If you had the conversation with the consumer and they agreed to give the system a little rest between loads, and you have an adequate storage tank, then it is not an issue. They expect to run out of hot water, but know that it will recover in short (relatively so) order. This scenario will NOT cut it in the resort setting, but will in a single family dwelling. Also, see how many teenage women they have in their family, or will have in their family in the near future. These little women use MORE hot water than an NFL football team...

    The other thing that has to be taken into consideration in sizing the tank is the largest individual DUMP load (1 man, 4 woman hot tub) and their expectations on that. And this is the one area that gets overlooked, and then when the Mrs of the house has warm water up to the middle of her hips, and runs out of hot water, then it gets REAL ugly.

    Cause, if momma ain't happy, ain't NOBODY happy.

    Do the math, talk to the end users, and give them what THEY want, not what YOU want. It is the Golden Rule after all. They have the GOLD, and they make the RULES...

    Do it right the first time, by asking the questions and giving the consumer what they expect, because it gets REAL expensive to have to fix it on your own ticket.

    Also, quit messing around with storage tank heat exchanger combinations that suffer from extreme thermal constipation. Drop a reverse indirect in place and let the boiler give its FULL potential to the load IMMEDIATELY, not over an hour. It is worth the extra buckage if you know how to sell it.

    My $0.02 worth.

    Keep the change :-)

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • icesailor
    icesailor Member Posts: 7,265
    You comented/I comment:


    You understand the problem better than most. I'll give you my experience with an account I have.

    A 44 bed nursing home. It has three W-M WGO-7's I replaced in 2000. They are fired at 2.25 GPH each. The original boilers were three Hydrotherms fired at 3.00 GPH. It is primary, secondary with outdoor reset. A heat recovery system for outside fresh air. In downsizing the boilers, I have had no problems with the boilers heating the building and as far as I know, the system temp. has never been over 170 degrees. That's the heating. Now, the domestic.

    They have two Bock 73E 70 gallon oil fired hot water heaters. They recover 220 gallons per hour. The burners (Now EZ-2's from the original Waynes) are fired at 1.75 GPH (adjusted). That's a recovery of 440 gallons per hour. We all know that things are over fired so I decided to drop the nozzles down a notch to save them some cash. I dropped them to 1.50 GPH. The next day, they were running out of hot water. I replaced the nozzles and life was and continues to be good. You have no idea how much hot water you are using until you start to run out. There are three users of the hot water. The commercial kitchen that makes the food to feed the clients, the laundry where they wash the clothing, and the clients personal needs. They have two huge whirlpool tubs they can put the clients in to bath them. Every morning, they bath selected clients from 9:00 AM to 11:30 AM. They can suck the hot water system cold in an instant. Same with the laundry, and same with the kitchen. I have them on a schedule of who and when can use the hot water. I put the tub rooms on Noritz instant gas water heaters to feed the tubs only. Taking that load off. 100% improvement. The kitchen fills their triple bay pot sink before 6:30 AM. Everyone gets a shot. Everyone is happy.

    I have another account. A large private club with a restaurant. When I started working there in 1991, they had a 100 gallon oil fired water heater to provide hot water to the club. It was fired at 1.25 GPH. I think it recovered less than 100 GPH. It ran out of hot water but it wasn't my problem. There was an oil guy that dealt with it. The heater needed to be moved and I did the moving. I recommended a Bock 73E and a 120 gallon storage tank, piped in my way that everyone says doesn't work but it does. The oil fired unit is the heater and the storage tank is the supply with water circulated through the tank and hot water circulation through the building. It works great except when the Chef calls me and tells me they are out of hot water. I ask him if the kitchen help is outside washing the rubber floor mats again with hot water again as it is 8:00 PM and they aren't supposed to do that until 10 O'clock after the dishes are done and they have finished serving. Sorry. That's what is wrong.

    My point is that the hot water load is so much greater than the heating loads, that it will bite you in the butt.

    The supply house guys know what they are talking about when it comes to hot water. The plumbers who under size hot water heaters are in deep do-do.

    I was once asked by someone who wanted to grow shellfish, how he could heat a large amount of water. I asked how much water. 20,000 gallons of sea water. I asked him how much time did he need to heat the water? An hour, a day, two days? Because if he had to heat 20,000 gallons of water in an hour, do the math. Water weighs 8.33# per gallon, times 20,000 gallons times the temperature of the water you want to heat and to what temperature you want the water to go to, in an hour. One huge boiler to make the hot water. On the other hand, if you can do it in a day, divide that amount by 24 for 24 hours. That cuts the boiler size down drastically. He said in two or three days. After telling him how to do it, he found someone to do it for labor. It was for a good cause I guess.

    Making hot water is like driving a car. You need far more power than you need. Except when you need more and there is not enough there for you and it gets dangerous. I had a 1986 Volvo 240DL wagon. It had plenty of power for a 4 Cylinder. Except when I would drive my daughter up to U-Mass Amherst with my wife, daughter and her "stuff" in the car. Then, it became downright dangerous to enter a highway because it had no acceleration nor would it go up hills. Down wasn't a problem. It just didn't have enough power.

    That's what happens with an undersized boiler. It is like putting the wife, daughter and "stuff" in your heating system in the winter and trying to maintain speed. You must give up something.

    Remember, when a customer calls up and starts crabbing about running out of hot water, that is the final call. They have already crabbed to at least 10 others about running out of hot water, how much they paid for the new high tech system, how they are running out of hot water, and what a lousy job you did in not giving them enough hot water. There goes your reputation.

    Sorry for being so long. I'm never brief.
  • icesailor
    icesailor Member Posts: 7,265
    I comment again:

    What will be the ideal situation is like the computer systems on cars.

    A computer that knows what the loads are. Heating, and/or domestic. Run through a temperature a computer actuated valve where the exhaust temperature is set for maximum efficiency and pollution control, the water, one part going to the heating load at whatever temperature needed and the other for the DHW load. Priority DHW sucks because if you get up to a cold house and use setback thermostats, the bathroom and house doesn't get warm while you are making hot water for the shower. If my wife got in the shower and the room wasn't as warm as she likes it and the water isn't as hot as she wants it, life as I know it is very uncomfortable.

    And funny how I find most priority switches, off.

    A boiler is just a heat engine extracting heat and power from a fuel. Like a car engine or a reciprocating engine in an airplane. Except that the car has total computerized engine controls. All adjustments are done automatically by an on board computer. The FAA will not allow at this time, computerized engine controls on reciprocating engine aircraft. Even ones that are turbocharged and fuel injected. The pilot sets the prop speed, fuel flow and fuel mixture. The mixture is determined by the exhaust gas temperature and manifold pressure. With EGT, too hot is bad, cold is inefficient but in the middle, hot is just right and the most economical. And a cold engine is warmed up until it is in the green. Millions of cars are manufactured every year with these controls on them. Why not for heating equipment? There's a breakthrough out there. This cold stuff goes against what is understood. If cold is so good, why are cars supposed to warm up as fast as possible with exhaust gas recycling to get the intake mixture as hot as possible for the cleanest burn?

    You tell me.
  • icesailor
    icesailor Member Posts: 7,265
    ME, Well said:


    Very well said.

    The best way I have found to get the most out of hot water is by the use of hot water thermostatic mixers. In MA, we are supposed to keep domestic at 125 or 120 degrees at the outlet. That's not a lot of stored hot water in a say 40 gallon tank. And thermostatic mixing shower valves do not work well with incoming water at 120 degrees. The hotter the incoming water, they better they work due to the ratio of hot to cold water for mixing. Hotter water requires more cold to give you the same temperature. But if you raise the tank temperature to a higher storage level, theoretically, you have increased the size of the tank because you have more available water to mix. Use a thermostatic valve and you can overcome a smaller sized tank. You can run 150 degree water if you need to and not worry about scalding.

    So you'll understand, do the math. In heating, you deal with 20 degrees. 150 degrees going out and 130 degrees coming back, The boiler has to deliver enough to recover the water back to 150. A gallon per minute, times 8.33# X 60 times 20 degrees= BTU's per hour. With domestic water heating the water is 50 degrees, X 75 degrees, times 2 or 3 GPM X 8.33# X 60 + how many BTU's per hour to heat the water?

    And that load is 24/7/365. And telling them to wait while the hot water is getting hot doesn't cut it because someone will run to the tap and run it for 5 minutes to see if it is hot yet. And waste the hot water you just made.

    The same men and women who drag race across an intersection when the light turns green so they can get ahead of the person beside them, will be the one crabbing about saving money while turning their boiler down to save money. I guess to save up enough money to pay for the extra gas they just used.
  • icesailor
    icesailor Member Posts: 7,265
    Indirect feeds/ I commented

    Another thing. it doesn't matter the size piping of the coil into an indirect. If it is 1/2", 3/4" or 1". If you use a 60,000 BTU boiler on it, whether it is a 30 gallon indirect or a 100 gallon indirect, it only recovers as much hot water as the boiler is capable of delivering.  Look at the specs of SuperStor. To get the high recoveries you need a high output boiler.

    It's either long and slow or fast and furious. Not both. One or the other. If you want hot water, you must provide a means of making it.
  • Brad White
    Brad White Member Posts: 2,393

    Wayne- good topic!

    Sort of an Elephant in the Room type of thing.

    Personally, if there is a large discrepancy between the building heat loss and the DHW load, I recommend that the DHW load be dealt with separately.

    In my experience, where a ModCon/Indirect setup works best is when the design heat loss is within 80% of the gross boiler output and the DHW load is "conventional", something a regular 40 gallon separate heater would handle, one or two baths, no car wash showers, no hot tub.

    Not that I want a margin for DHW necessarily, but that the boiler has lots of turn-down room and matching the heat loss is sheer luck sometimes.  I might also time the DHW morning warm-up to occur an hour before heating on-time, but I also acknowledge that 100% of my boiler input will go to DHW production, so it will not be long. If the house has cast iron radiators, best of all.

    If the house is hydro-air, it gets dicey. A 20 minute deprivation to take a shower stops the heating side, literally cold on a cold day.

    The worst is where the DHW drives the bus. I was brought in to consult on a house, brand new deep energy green LEED retrofit. Boiler was a 200 MBH NTI for instantaneous domestic hot water, no storage. Design heat loss is about 27 MBH on a five degree day. "How Not To Do It".

    The NTI cycles like Lance Armstrong going back to Sheryl Crow for make-up sex.

    I recommended a separate DHW  tank and a dedicated 50 MBH boiler to heat the house. Having just spent a fortune, the owner declined.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • icesailor
    icesailor Member Posts: 7,265
    I commented/FWIW Redux

    Brad, I don't disagree with you. Especially about having the DHW driving the bus But I prefer to have the DHW as a passenger on the bus. And the Bus is a good analogy of the discussion. The heating system is like the bus. It only uses as much energy as it needs to move the passengers. When it starts the route, it needs only enough power to move the bus. As it picks up passengers, it needs more power. The driver opens the throttle. When the bus is full, it needs maximum power. After dropping off passengers along the route, the load goes down. At the end of the route, there is no load. Like a heating system. And a DHW system.

    Your story about the 200 MBH instantaneous water heater is why I don't like them. They are always over sized and are often like killing flies with hand grenades. There is another way you could have solved the problem. And I do it all the time. And it works.

    A late good friend used to always say, "When they call you on the phone, they want to spend money." It's my job to make them want to spend that money with me. That 200 MBH heater probably won't come on at the flow rate to fill a glass of hot water and the owner is PO'ed because this 200MBH heater isn't making hot water. 30, 40 and 50 gal gas water heaters are rated at 35,000 BTU input. 70 Gallon gas heaters are rated at 75,000 BTU input. Why do you need 200 MBH to heat water that a 50 Gal storage tank will do at 35 MBH? I don't think that you do. Such a problem with these instantaneous gas water heaters doing this, they now "suggest" using tanks by whatever cute name they want to use.  Here's a better way to have solved solved the customers problem. Take a 30, 40 or 50 gallon electric water heater and use it as strictly a storage tank. It is heavily insulated and is the cheapest tank you can buy. It is also pre-wired for you.  Connect it as it is a water heater. Connect the instantaneous with the inlet connected to the bottom of the tank with a circulator pumping to the cold inlet of the instantaneous. The hot outlet connected to the cold inlet of the water heater. Connect the circulator electrically to the bottom thermostat of the water heater and set it for whatever you want. 125 degrees? Now, when the water heater is cool and calls for heat, the pump starts, satisfying the flow switch. Set the instant heater to a higher temperature than the pump. As long as the thermostat and the pump are calling, the burner will run and modulate. When the pump stops, the flow switch opens and stops the burner. You are not junking a $4000 instantaneous water heater, the customer is thrilled because you solved his problem,. You have full flow through the water heater and avoid the restriction of the tank less. All for the cost of a water heater tank. The customer will be so happy. They might even find some other fun project for you. Your approach was too complicated and expensive for them to grasp. And they did nothing. Nothing to solve their problem. Perhaps they would call me. That is what I would have done. I do it all the time. It works.

    The indirect on the 50,000 BTU system. You need to think of it as being on the bus, being driven by the boiler, on the route. The passengers are the heating load.  The boiler/bus is driving around, picking up and letting off passengers. It stops by a school at 3:00 PM 50 students get on the bus. The load goes way up. As the students get off, the load goes down. When all the students are off the bus, the excess load is off. I make the DHW a passenger on the bus, driven by the boiler. You heat DHW every day. You don't room heat in July and August. The bus drives 365 days a year. The DHW is 365. The room heat is less than 280 and has a peak with drop off on either side to nothing.

    I can always keep the house warm. I'll pay hell if the shower water is cold.  
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