does domestic hot water come into play when sizing new boiler?


Old 10-06-08, 09:27 AM
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does domestic hot water come into play when sizing new boiler?

Just wondering since in all my talks here i never really asked anyone if you need to add anything to the calculations for the new low mass boiler?
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Old 10-09-08, 06:15 PM
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I was told that my new boiler size was more dependant on my current hot water needs (2.5 baths, 4 people = WM- Plus 40) was more significant than the heating needs of the house (3K sq. Ft colonial in CT) - not sure if I buy that yet though
Old 10-09-08, 06:51 PM
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You don't add anything to the home heat loss to size an indirect. Broadly speaking, you size to the greater of the two loads: house or indirect.

An "average" house has a heat load that is one-half to one-third of what an "average" indirect needs to achieve rated specs, e.g., a house with a heat load of 30k BTU/hr and an "average" indirect that the manufacturer states needs 120k BTU/hr in order to achieve its rated performance of xxx gallons in the first hour.

With a fixed-firing rate standard boiler, you would seemingly be stuck sizing to the higher of the two loads, and thus have a boiler that is 2-3 times larger than needed for the space heating load.

What's a person to do?

Here's some options:

1) add an outdoor reset control to lower to the space heating supply temperature and stretch out the firing cycles. This is often accompanied by...

2) some form of boiler protection that allows even lower, more efficient water temperatures to be used while protecting the boiler from internal condensation that will literally corrode it away. These include standard boiler bypass, thermic bypass, or mixing (by motorized valve, or injection pump). The latter is often done in what's known as primary/secondary (p/s) piping, which isolates the boiler in its own little loop that runs hot, while the rest of the system can be whatever water temperature is needed to match the heat loss, which is principally a function of the outdoor temperature, be it 50F or -10F.

3) using a boiler that is somewhat smaller-sized than the indirect manufacturer calls for, but adding an anti-scald tempering valve on the domestic output of the indirect tank (i.e., the pipe that goes to the faucets) and have the tank set to 140F. The anti-scald valve will temper the water temp to the ~110-115F you want at the faucets. This effectively decreases the rate at which the hot water is drawn down (i.e., makes a 45 gal tank act more like a 55-60 gal tank). It also allows the tank to call for boiler heat before the tank temperature gets too low.

Any of these options are often accompanied by...

4) a control strategy called "domestic hot water priority" or DHW priority or just priority. This allows the boiler to turn off the space heating for a few minutes to devote full power to making hot water. Then it goes back to space heating.

Many outdoor reset controls (the tekmar 260 leaps to mind...) include all of these functions in one small electronic package.

And now, enter the gas-fired modulating/condensing boiler. It constantly adjusts its firing rate based on the building heat loss (again, a function of the current outdoor temperature), or whether the indirect is calling for heat to make domestic hot water. These boilers typically come with the reset and priority control already built-in. Great stuff.
Old 10-10-08, 05:51 AM
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just got off the phone with my 1st contractor, the one that sized the boiler by measuring the pipe length. He did tell me that all installs include boiler bypass (score one pt for them) but he couldnt tell me the ideal temp for the return water and how they would balance it. It's very apparent this guy really is just a salesman and thankfully i'll probably have 'real' installers do the job. So i told him that at least we resolved the problem of cold water coming back to the boiler but he STILL insisted on sizing the boiler to the piping. I just ignored him and said i'm leaning towards the 128K unit, not his 150K.

He said he adds 10% for domestic. Now i told him i redid my heat loss at 10 degrees outside (and corrected my faulty room measurements) and it's about 105K. So even if i add in his 10%, which from what you all say here is BS, i'm well below the 128K DOE unit i feel comfortable getting.

also, very funny that even with his 191ft pipe calc he did NOT estimate anything for domestic a month ago. He did 105K for the pipe length, 25K for the radiator, and 20 k for pipe loss. Unless inherent in the pipe loss figure was his indirect cost. But somehow i dont think so. I think he has no clue and just goes with the topic of the day.

edit: also he told me when we met that the domestic would have a different temp then the heating water. I told him i hate burning myself wiht 160 degree water in winter and he said oh you can keep your domestic at 110 or 120 or whatever. So based on what i've read above, i guess he puts some sort of antiscald device on there too.

also i have only myself right now in the domestic needs are virtually zero
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