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Thread: HWH and Boiler?

  1. #1

    Default HWH and Boiler?

    Hello.

    I am new here, Thanks for being around!

    I have this plan for a Brooklyn, NY Four-story Condo housing seven dwelling units.

    Attached is a section of the basement floor.
    My question is, Why is there a HWH (hot water heater?) and a bl (boiler?) per each dwelling unit? Aren't they the same thing, water heating devices?

    Please see attached.

    Thanks a lot.
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  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    A boiler typically is a closed loop and runs at a higher temperature than a WH. Codes typically restrict the max temp of a WH for domestic use, and safety means you rarely need more than say 120-degrees. A boiler often runs at closer to 200-degrees.

    There are a few common ways to heat domestic hot water...a stand-alone tank, either electric or gas, a demand system that uses a heat exchanger in the boiler itself, or an indirect tank that is essentially a separate zone off of the space heating boiler. The fuel used could vary from electric, natural gas, propane, to fuel oil.

    My guess is they have separate boilers and tanks so that the utilities can be billed separately...this keeps someone who wants to economize from subsidizing somebody that likes it tropical (although they get some parasitic heat from their neighbors).

    So, not sure if this answered your question.
    Jim DeBruycker
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  3. #3
    Jack of all trades frenchie's Avatar
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    They also might be indirects - the water heater gets its heat from the boiler.

    Or it might be so that you'll still have hot water in the summer, without having to run the boiler, more economical.

    What part of Brooklyn?
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  4. #4

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    Thanks.

    Frenchie. Thinking about the possibilities you suggest, is it possible that one of the two (HWH, BL) can be switched on and off for quick heating and the other is only economically used when running non stop for the entire winter?

    Prospect Avenue, Brooklyn.

    Thanks.

  5. #5
    Like an engineer alternety's Avatar
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    If you look at the fresh air calculation and the HWH manufacturer you can see that both appliances are gas. If this is going to be built, a condensing boiler and indirect tank for each apartment might be a good way to go. Better efficiency, which can make the apartments more attractive because of lower fuel bills.

    If this is an existing installation in good condition you would most likely not want to fool with it.

    What is your objective in asking about shutting things off?

  6. #6

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    A builder has asked me to check plans for him designed by others. I just wanted to know if it makes sense to have the water of each apartment heated by two different water heating devices.

  7. #7
    Like an engineer alternety's Avatar
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    If it is not built I would use a condensing boiler and an indirect tank for each apartment. Simplest to pipe, control and bill. It is also possible to get equipment that can measure the amount of energy sent to each user using the water flow and temperature. I have no idea of the cost. But then you could use two boilers and two indirect heaters (for redundancy). If the boilers were each capable of heating the building then they can be alternated to even out wear. Used together they could make twice as much DHW on demand.

    You need to have someone work the math (cost of alternatives, equipment, installation, maintenance, space, etc.). Load will determine sizes and quantities of things. Two boilers should be less maintenance; which is probably paid by the owner. Likely to take up less space. I assume they are going to do radiant heat. If you do it in the floor you can use lower water temp which will allow the boilers to operate most efficiently. Indirect water heaters will last much longer than an individual gas water heater (owner longer term cost savings again). Higher efficiency most likely. The tenants will be happier.

    These sort of system need to be engineered by someone competent in this sort of work. It is actually not very simple. There are fewer people that know how than there are those that say they are.

  8. #8
    Computer Programmer Bill Arden's Avatar
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    Why wouldn't it be easier to use one big water heater and use water meters to measure the amount of hot water used per place?

    At the end of the month total the number of gallons of hot water used and divide by the electric used then multiply to find out who owes what.
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  9. #9
    Like an engineer alternety's Avatar
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    The reason for not using one big tank is redundancy. The owner has two choices. Build a system where if a major component fails it is a crises because the building is without heat or hot water. Or, it can be a system where the other equipment can keep things fully operating (or on reduced basis if the boilers are not equally sized) until repairs can be made. If I were the owner I would consider this benefit carefully. Particularly if I was the one they called at 0200 Sunday morning while I was vacationing on a tropical island on the profits from owning apartment buildings.

    Short answer to flow metering - yes. And it is workable. You just need meters of the right type. Owner now has to read meters, find out what the fuel costs are (since the gas/electric company reads those meters and bills the owner), calculate apartment share, and bill/collect the money, pay the utility company. This, of course, happens with any design that does not give the apartments their own system. Just one of the factors the owner has to use to choose.

    You could go by the amount of water used for DHW. It will probably be a decent approximation. Water temp will vary, so it is not an exact measure. One example - how much cold water comes out before usable hot water in the apt closest to the boiler vs furthest. Keep hot water moving in the circuit to prevent this and it raises everyones costs. It is all about being to say - you are paying your energy costs vs we are sharing and I am going to decide how to do that.

    Heating is trickier. Water may flow but the amount of heat extracted in each user may not correlate with the fact that there is water flow. If one apartment has a window open because they like the feel of hot floors it won't allocate properly. If there are zones within an apartment, it is worse. Depending on design with the apartments (size of apartment same of different, multi/single zone, where bypasses are vs variable speed pumps, etc.) It is not necessarily simple. Heating cost is a function of how much heat is removed from the water; not necessarily the amount of water flowing.

  10. #10
    Retired Defense Industry Engineer jadnashua's Avatar
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    There is much less chance of abuse if each unit is separately controlled and billed.

    I think there is a requirement in commercial buildings of hot water recirculation if the run was beyond a certain distance. With 4 units, you may exceed that limit and have to supply it. It can be a pain for anyone, depending on the size and length of the pipes. It's about 40' from my heat source to the shower, and recirculation prevents me from wasting a lot of water at a fairly small electrical cost. My gas bill doesn't seem to be affected much, either. I've got my indirect set to allow the tank temp to fall more than normal to minimize the on/off cycles.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  11. #11

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    The apartments are to be sold. So that explains why there should be separate heating systems for each apartment. Just to sum up my understanding of my original question- each unit has its own boiler AND its own HWH... for redundancy?

    Thanks.

  12. #12
    DIY Junior Member therinnaiguy's Avatar
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    Default save money

    Check out the boiler made by Luna called the Baxi. It is a combo boiler and will give you heat and hot water. Saves you a grand each unit. Make sure the rate of hot water is enough at 3.5 gpm.

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