Boiler sizing for DHW

leed

I have gotten a couple of estimates for new modcon installation with indirect dhw. I understand that sizing the boiler is important and having a heat loss calculation for the house. My question is how do you determine an increase the size of boiler when adding dhw production to the mix? 

Jean-David Beyer

I am just a homeowner.

When I replaced an old oil fired GE boiler and a separate electric hot water tank with a mod-con gas fired boiler and attached indirect, I calculated some of this.



The smallest mod-con was about double the size I needed to heat the house even when it was 0F outside, and design temperature is 14F. And my domestic hot water needs are modest (just me; no teenage girls). It was the smallest boiler in the product line. I wish there had been one about half that size.



The manufacturer (W-M) sell their boilers, and rebrand a series of indirect tank-within-a-tank indirects presumably made by Triangle Tube. W-M have a worksheet to calculate what size indirect to get. The contractor recommend one one size bigger than I calculated. They also recommended a boiler one size bigger than I got, even though that one was already twice the size I needed.



I answered all the questions in the worksheet, and came up with 70 gallons for a first-hour rating. Then I multiplied this by 1.5  for "heavy usage". When all was said and done, I got their 40 gallon indirect, and is has always been enough. It should have been, because that is enough for two people, and I am only one. That is enough if I have a washing machine (I do), but I run it cold most of the year, and just barely warm (at least 60F) in the winter. That setup on their smallest boiler will supply me 124 gallons first hour at 140F. Now the indirect has priority, so if it calls for heat, it can surely get it from that boiler. Recovery time (I do not run the hot water continuously) is of the order of 10 minutes, so the interruption of the home heating is too short to notice.



If I were running a small hotel or commercial laundry, where most of the heating load went to the domestic hot water, and very little to heating the building, the results would have come out the other way. I would size the boiler to run the indirect (that would have been much bigger), and check to be sure there was enough left-over heat capacity to also heat the building. I imagine most real situations are in between.



But if the system is priority operated, and the boiler is anywhere near large enough to heat the house, my guess is that you would not need to have too much extra capacity to run the domestic hot water. If the boiler is just barely large enough to heat the house on design day, and there are a couple with 6 teenage daughters, you might wish to go up one size on the boiler, but if my experience with just one boiler in one house is realistic, the boiler sizes are so far apart that unless you are right on the border of boiler size, it will not matter much.



I assume a professional here can fill in the stuff I do not know.

Mark Eatherton

Size it to the greater of the two loads...

Which, if the house is not overly large, and the loads relatively reasonable, the DHW will be the greater of the two loads.



The real time load depends upon your intended usage. Back to back to back showers, or two or three showers in parallel? Large dump load (filling a bath tub). Load diversity (do you want to wash clothes while you are showering?)



You would NEVER want to add the loads together. The boiler would be grossly oversized at that point.



Figure out what your loads are and size accordingly. The boiler will prioritize its output to the DHW load during simultaneous loading.



ME

Jean-David Beyer

if the house is not overly large,

"if the house is not overly large, and the loads relatively reasonable, the DHW will be the greater of the two loads."



Since I am just me, I have no experience with other houses and other loads. But your comment surprises me. I know that in my house, and the usage I make of heat and hot water, the heating (in winter) is by far the greatest heat load.



My only use of gas is the one boiler that runs all year (it does not fire all the time), because it supplies the indirect. My May-June gas bill was for 4.22 therms, which is surely hot water only. The maximum this (warm) February was almost 80 therms. In colder years, I have used 120 therms. So I use about 18x to 25x more heat for the house than for hot water.



I am prepared to believe I am not typical, but to be completely reversed from your experience is a surprise to me. My house is not overly large (1150 square feet Cape Cod).

leed

usage

I don't this of our house as heavy usage overall. We will have 2 shower going at once, or possible a shower along with a washer or dishwasher. The estimate is around 50K btu's would be adequate for heat, and sizing the boiler around 90-110K  with the dhw demand. 

Mark Eatherton

JD's DHW math...

OK, so its just you. And you shower once a day for 7.5 minutes (national average) and you are using a water conserving (2.5 GPM) shower head.



So, 8.33 (pounds per gallon) times 2.5 (GPM flow rate of shower head) times 100 (degree F rise, worse case) times 7.5 minutes = 15,600 btu's. Boilers are rated in hourly capacity. At that rate, you'd need a boiler output capacity of 125,000 btuH to cover the 7.5 minute load. Assuming that 60% of the draw is hot water, and the balance is mixed cold, then you could downsize to 76,000 btuH.



Compare this to your calculated space heating load at design, and I think you will find it is in the scenario.



Granted, you don't need all of that hourly capacity, but you do need the capacity per minute, and boilers are not rated in btu/minute, but btu/hour.



Make sense?



ME

Mark Eatherton

Leeds DHW math...

Would be 2 times JD's DHW math, or around 150,000 btuH capacity.



Can you get by with less? Absolutely.



Can you expect to run out of hot water? Possibly, depending upon variables like how much stored hot water is on hand.



Is there a better solution? Yes, stagger your showers, and never start a machine while you are in the shower, then your numbers could be halved.



Oversizing ANY heat source is not a good idea. It causes short cycling, which is not a good scenario.



The other thing to watch out for is the ability of your indirect to put OUT what you are putting IN. In certain (most) North American indirects, the coil size WILL limit the ability to transfer the btu's to the water. I prefer the use of a reverse indirect like Turbomax. It can transfer 99% of the energy generated to the DHW load, avoiding short cycling.



Make sure you are getting what YOU need, and not what they want to sell you.



ME

Jean-David Beyer

JD's DHW math...

"OK, so its just you. And you shower once a day for 7.5 minutes (national

average) and you are using a water conserving (2.5 GPM) shower head."



This is probably all true. I have a water conserving shower head, and it has a sliding valve that allows me to lower the flow rate all the way down to a dribble, and I run it part way off. But I do run it kind-of hot (well under 120F, though). And I shower once a day most of the year, and more often in hot muggy summer days. But sometimes I shower at the YMCA gym instead of at home, so it is on their bill.



"So, 8.33 (pounds per gallon) times 2.5 (GPM flow rate of shower head)

times 100 (degree F rise, worse case) times 7.5 minutes = 15,600 btu's.

Boilers are rated in hourly capacity. At that rate, you'd need a boiler

output capacity of 125,000 btuH to cover the 7.5 minute load. Assuming

that 60% of the draw is hot water, and the balance is mixed cold, then

you could downsize to 76,000 btuH."



I think I would need 15,600 BTU/hour to cover that load only if I were running the boiler as a tankless hot water heater. But mine has a 36 (or so) gallon tank.



I did not check your math, but you may well be right. Now my indirect tank holds 36 gallons of water at about 125F on the domestic side and 6 gallons in the outer tank. I do not know if this should be added together to 42 gallons or not. If I use 2 gallons per minute of hot and 1/2 gallon per minute of cold for 7.5 minutes, I will get all I want from the tank without ever firing the boiler. Now if I take multiple showers, it would be different.



But is my math wrong? I did not really do much. I use about 4.5 therms per month for domestic and 100 therms in very cold months, so is it wrong to say I need 20x more BTU to heat the house than to heat the hot water? This presumes I always get enough of each, and I do. I admit my boiler is oversized, but it is the smallest one in the product line.



My boiler input can be up to 80,000 BTU/hour. I do not know how much of that is converted to useful heat when running the indirect (less condensing than when heating the house), but say I get 70,000 BTU/hour out of it. Then the question is how much of that gets to the domestic side of the indirect. I do not have a coil, but a tank within a tank indirect (Triangle Tube). For that model, and that boiler, W-M say I need to run 6.6 gpm from the boiler to the indirect, and if piped as directed, a Taco 007 will do it. And if I do that, I can get a first-hour rating of 124 GPH delivered at 140F, or 160 GPH at 115F -- assuming 190F supply temperature (their default setting). I use 175F because it recovers fast enough that way, and perhaps it condenses a little more that way. But I do not care, since it runs perhaps 20 to 30 minutes a day making hot water.



The thing is that I do not have to be able to do this all day. Sometimes I run the washing machine and take a shower at the same time, and it does not seem to matter much. I do notice a temperature increase and flow decrease when the washing machine draws water, but not enough to jump out of the shower or anything. And I do not usually do that anyway.



"Granted, you don't need all of that hourly capacity, but you do need the

capacity per minute, and boilers are not rated in btu/minute, but

btu/hour.

Make sense?"



I guess, but with a large enough indirect, its recovery rate is less important. If I can get an hour's worth of domestic from the size I have, then BTU/hour would be the correct measurement, rather than the BTU/minute or BTU/second.



It may be that for some usage patterns, it would require a ridiculously large indirect, but not for me.

Paul48

J-DB

Now if you could figure a way to add 15 gallons of cold water to the indirect without trigger a call for hot water, you'd be very wealthy :-P

Jean-David Beyer

Perpetual motion machines?

"Now if you could figure a way to add 15 gallons of cold water to the

indirect without trigger a call for hot water, you'd be very wealthy :-P"



I suppose my indirect is pretty much like other tank within a tank indirects. The cold water comes in through a dip tube that goes pretty near the bottom, and the hot water comes out the top. Ideally, the cold water remains at the bottom and slowly displaces the hotter water. Until the cold water gets up to the height of the aquastat, in a well at the center of the tank (hangs down from the top), it would not call for heat until half the water in the tank was used.



Realistically, some mixing probably takes place. What I should do is set the boiler so it does not respond to a call from the indirect and then take a shower and see what happens. My guess is that it would be OK. I acknowledge that I would not get 36 gallons of full temperature water from it with the call for heat disconnected.



I do know that if I set the dishwasher to run (it is connected only to the hot water line and a drain), that it does not usually result in a call for heat from the indirect. It may be that it uses less than 15 gallons.