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Water Demand High - Heating Load Low - Ok to oversize Boiler?

Antitorque
Antitorque Member Posts: 7

I am going to do a boiler in a buddies ski townhouse this spring. This is an atypical install for me. The few I have done have been at low altitude and never with a DHW attached. I am going to satisfy the 55K btu heat loss for this ski townhouse in Vail with a Lochinvar Knight KHB085 (85,000 Input) I figure my altitude derated btu's will be about 65K. Will this not be enough boiler for the Lochinvar SIT80 (80 gal DHW) and will the city allow you to oversize if it is going to be for hot water? The owner is expecting plenty of hot water for his 4 baths, two with big soaking tubs. I am willing to put in a buffer tank for the oversizing. Thanks for your thoughts on this.

Brad

Comments

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,710

    Ofter we are required to size a system based on the DHW load and install the system accordingly. As far as Code Requiring equipment to match Load Calculations of the building, If that is a requirement for permitting, then you need to show the load of the DHW demand along with the building load.

    If you are turned down, then install the system as a Hot Water Only desigh and a second smaller boiler for the heat demand to get the permit. Once you are are finished the system (without the second boiler), get that inspection approved so you can start using the hot water. If they call you in a few years for the second boiler inspection, Just tell them you decided against installing a new heating system. The old one is working just fine!

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • hot_rod
    hot_rod Member Posts: 20,762
    edited March 28

    Yeah, tough call, but you need to size to the larger load, in your case DHW.

    Two options, a large indirect to store a dump load, maybe an 80 running 140F with mixing valve. That should get you close to 100 gallon useable down to 110.

    Handy calculators at Lochinvar U

    Or enough boiler BTU to generate constant DHW

    Assume 35°F incoming water heated to 120F, 5 gpm flow

    500 (f) ∆T.

    500 X 5 X 85= 212,500 BTU/ hr boiler

    Use the same math to calculate the recovery time for the 80 indirect if you pull it to say 100°F

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Antitorque
    Antitorque Member Posts: 7

    Great Ideas! Thanks for the quick responses. I guess I'll have to consider the 199K or 150K Loch I didn't think of the colder tap water making for a larger deltaT for the DHW. If the boiler delivers 180˚ in Priority mode to heat the DHW, what is the highest storage tank water that can or should be achieved?

  • hot_rod
    hot_rod Member Posts: 20,762

    With a proper listed ASSE 1017 mixing valve you could run 160, that is my comfort number for glass lined tanks. Stainless maybe a higher temperature, check with the tank manufacturer first.

    But know the tank life is shortened at higher operating temperature, mineral precipitation increases.

    Is the heating system zoned? If so what is the smallest load? A 150K could turndown to 15,000 on low fire. If you have small micro loads might need a buffer or a high mass mod con to prevent short cycling.

    I've never tried this but some installers swear by the reverse indirect to buffer heating loads and give you DHW. I'd use the TurboMax sizing program to assure you have the boiler output to get you what you expect. The tanks need to run hot to get max. performance.

    Its a BTU game no matter how you cut it.

    Your first assignment is narrowing down the actual DHW load.

    Or another way to approch it is make your buddy aware of how much DHW you plan on providing.

    Ski homes, especially rentals are a tough DHW load to predict. Large tubs are the biggest challange. If you want to generate 5 gpm continously, for fast filling a tub, that going to take some serious horsepower. If someone want to shower while the tub is filling…, you get the idea.

    What is in the home for DHW now? If that is what they are use to, match it with an indirect.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Antitorque
    Antitorque Member Posts: 7

    This is new construction, the smallest load is 3,000 btus for a interior room. I was planning on using the buffer tank. So it looks like I should up the boiler side to account for the possible high hot water usage. Thanks for your guy's advice!

  • hot_rod
    hot_rod Member Posts: 20,762

    The reverse indirect option I showed above sort of behaves like a tankless water heater. It heats water as it flows thru the coils. The water volume in the tank covers the HX instead of a direct burner.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream