sizing storage tank

rick in Alaska

I have a new customer who is putting in Warmboard heat along with a new bathroom remodel. I have run the numbers and the heat loss is around 24,000 btu's. He has a two person Jacuzzi style tub that is rated at 86 gallons capacity without anyone in it. What I want to do is use the Lochinvar whn051 for the heat source along with the smart 80 indirect. The indirect gives about 70gallons of usable output. I am figuring on running the indirect at 140 degrees and mixing it down, but my brain is having a real hard time doing anything right now, and could use some opinions if this the best way to go.
I know the boiler is still over sized and am still coordinating with the owner on his zone needs. I think we are going to just have one master zone with the other ones balanced to keep things even throughout. Hopefully this will keep cycling down.
Thanks in advance for any thoughts.
Rick

hot_rod

The 80,000 turns down close to the 50,000 as it has a higher turndown rate, it may help recover that indirect quicker if that is a concern.

With the ramp delay and other functions it should behave fairly well in a single zone system.

140F tank setpoint would be good for capacity and also bacteria potential.

Lochinvar has a nice simulator program at Lochinvar U, it's free.


http://www.lochinvaru.com/resource/calculator/storagetank_mixingvalve_cal.swf

rick in Alaska

The calculator at Lochinvar U is nice! Good to know how much extra capacity you have at higher storage rates.
I will have to look at the KBN a little more. Not sure if we have issues with that heat exchanger up here or not, but don't think so. Water conditions are usually pretty good, and if I add treatment, so much the better. Because of the location, I did want to stay with a wall mount boiler, but I will look at the location again to see if it would work.
Good point about the time for full recovery also! Wasn't thinking about that part yet.
Thanks, Rick

Mark Eatherton

You need to ask the consumer their expectations. How many people will be showering at one time? Will there be any machines using hot water at the same time? (Clothes and dish washing machines). How often will the tub be filled?

Once these parameters are established, then you can go to the task of sizing the source and the storage tank. Tanks are generally sized based on largest individual dump load plus the load of any parallel loads. 86 gallons is the dump load, but will there be any parallel showering occurring during that period of time? If so, it needs to be added to the storage, or the recovery portion needs to account for the additional simultaneous loads.

Most storage tanks are capable of delivering 80% of their stored capacity before dilution kicks in and destroys the hot water output. So, in your case, 86 gallons divided by .8 = 108 gallons. If you can't or don't have adequate storage, then here is where recovery has to kick in. So if you only have 100 gallons of storage, then that is 80 gallons of output from storage. You then need to do the math for the balance of the need, which is 6 X 8.33 X 100 = 5,000 btuH, which in your case, is obviously not a problem.

As it pertains to tank design, I prefer the use of a reverse indirect. They are capable of transferring 99% of their energy input into the water. You also only need to maintain a boiler supply temperature roughly 10 degrees hotter than your targeted tank water temperature. This means that when needing 130 F tank water, your boiler only has to produce 140 F water, which in your case means your heat source stays in condensing mode, even during DHW production.

I love Lochinvar and their product line, and have no familiarity with their indirect design, but knowing the people I know there, I suspect that their heat exchangers are very well sized.

You have some questions to ask and some answers to obtain.

ME

rick in Alaska

The big tub is the main issue. The wife and husband are both bath people. They do have a walk in shower upstairs and downstairs, but I don't see them being used at the same time as the tub. The tank he specified, (did I mention he and his wife are major ocd, and he is a geek who researches too much of this), and he likes the smart tank. However, he is also up to changes if it is for the best, so maybe we can go with a reverse indirect.
The reason I am "thinking" of the KBN is because of space. Since the exhaust comes out the back, it puts the boiler off the wall farther and in to the room. The room is about 5' x 7 and has to hold everything along with the HRV, and still be able to work on it. I stayed awake last night running it through my head and believe I might be able to make it work, but will have to look at the space later and lay things out.
I still want to know why they can't make a variable turndown boiler clear on down to zero. What is the real issue?
Rick

rick in Alaska

Ok, after checking units out and talking to my supplier, I was informed of the Lochinvar KHN. This boiler is a floor mount, but the exhaust comes out the top instead of the back, so it can be put up against the wall, thus saving 14 inches of floor space.
This boiler is same heat exchanger as the wall mount, has the con-x-us capability ( important to homeowner), and best of all is 10/1 turn down. So, that is what we have decided to go with. Hopefully it will do what I need it to do.
Thanks for all the advice.
Rick