sizing a hot water replacement boiler

RMA

existing oil fired being replaced with gas fired hot water boiler.

134 ft existing base board @ 580 btu's per ft.
52 ft existing high output base board @ 1000 btu's per ft.

1 indirect hot water heater

recomended Boiler size???

Alan(CaliforniaRadiant)Forbes

Sizing

The math is:

134' x 580 BTU/ft. = 77,720 BTU

plus

52' x 1,000 BTU/ft. = 55,000 BTU

equals 129,720 BTU.

You should choose a boiler with a net IBR (output) of at least 129,720. You don't need to add for the indirect.

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Jerry_15

As Alan said that's pretty much it. I used 600 for a little margin, and easier to figger and came up with 132,000, OUTPUT remember. DHW doesn't count, as it is usually prioritized anyway, and with that kind of btu, the recovery will be screamin fast. I would probably put in a 150 to 175 input, but it depends on the efficiency of the boiler, and the incremental sizing of the brand. Also while I bet that there are several zones here that will seldom be on at the same time, I prefer to err on the side of caution. Saving money can be very costly. Bigger is not always better, and the existing flue size alone has been known to nurture some very creative solutions. Then of course, we are out by SF. Let the calculators click.

Joe Brix

What about a heat loss?

Anyone ask do you realy need all those BTU's? How often will you need to run water @ 180°, if ever?

Brad White_9

Absolutely. Joe is 100% Correct. Start with a heat loss.

For all you know, your heat loss might be 50,000 or 75,000 BTUH. Radiation is not heat loss. Your boiler just may be grossly over-sized if you perform a "Long Island Heat Loss". (Ask Dan H. what that is.)

You may well be blessed to have an abundance which means you can run a lower water temperature even on the coldest day (saves $). You may also find that you need every blessed foot you have installed.

But until you perform as detailed a heat loss calculation as you can, you will never know.

Size the boiler for the heat loss. If using an indirect for normal usage (not a hot tub for example) you should not need to add anything for DHW production. If you have a high DHW load that may exceed your heat loss but that is a rare instance.

CC.Rob

Yes yes yes on the heat loss. I'm involved in a similar exercise right now, and it is obvious that simply sizing the boiler to match the radiation is quite shortsighted. Do the heat loss, see where you end up, then think about boiler and control options to get the optimal efficiency and comfort out of the system.

Jerry_15

I knew the calculators would start clicking. As I mentioned, the key is the type of boiler. A down-sized boiler is dandy, based on heat loss calcs, if it's condensing. But if it's conventional atmospheric, it must come up to speed quickly, or it will be raining in the firebox, flue, etc. On cold start-up, it must be able to get to operating temperature quickly. Heat loss calcs be damned. I'd love to see your boiler sizing based on heat loss calcs from our resident geniuses. This is an existing system; you dance with the one you came with.

John Ketterman

John Ketterman

I cannot believe that anyone who spends time here would suggest sizing a hot water boiler to match the radiation. It is here on the Wall that I first learned that this was a big no-no.

John Ketterman

Bypass.

Jerry_15

Ooh I knew I'd be in trouble again here. I can hardly wait for to find out what the real boiler sizing should be, and for what kind of boiler.

Alan(CaliforniaRadiant)Forbes

I'm sorry

Sometimes I just get lazy. You're absolutley correct, i.e. the boiler should be sized to the heat loss and not to the connected radiation when talking about hot water heating.

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John Ketterman

Do you understand that one cannot do a heat loss calculation with the information provided? And do you know what bypass is?

Bob_36

From what I read here on the wall, I learned to get a heat loss and size my boiler to that. But I recently read on HTP's website, if installing a Superstor indirect, a T50 was not reccomended. The smallest boiler to use was the T80. So do I ignore the indirect and match the T50 to my heatloss, or do I "upgrade" to the T80 to supply the indirect with the needed supply.

Brad White_75

Munckins and SuperStors

This is one of those times when your domestic HW load exceeds your heat loss. You always size for the greater of the loads and here it is your DHW load.

Indirects thrive on the notion of "high input for a short time". Less than the recommended input means long recovery times or lower water temperatures.

They recommend the T80 combination for that reason. Modulation is your friend.

Jerry_15

Of course Jiminey, that's why I didn't bother to try. In my experience a by-pass is a cheap way to under-boiler and over-pump a system. That's why they use it on pool boilers. A properly designed system with a by-pass has a primary and secondary loop, and all the wonderful controls to match. Not happening on a single family. Most of the time it is just used to rob the flow from the heating system to keep the boiler happy. Think about it, and check the delta T on the heating system next time you dial in that wonderful happy boiler. I'm still waiting to hear the real proper size for this job. This is a replacement issue. not a design issue. Still waiting for the real correct boiler sizing........

Jerry_15

Get lazy Alan - this is an existing baseboard system - no improvements, up-grades, yikes. I'm afraid I might have to get a special tattoo to sign in here.

Wayco Wayne_2

If you install

a "vision 1" control on the Munchkin T50 it will ramp up to 80,000 btu on a call for hot water. Then after the call for hot water is over it will go back to a 50,000 btu mod con boiler. I use these T50's a lot for that reason. You can get closer on some heat loads and still have a big boost for the indirect. WW

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Guy_6

Indirect

I can't say for certain, now that I am no longer with HTP, but I believe that even the 80K boost output on the T50 is marginal on an indirect. It WILL work fairly well, it just may not provide the whole house performance that Super Stor is known for.

Brad White_9

Wayne- I had understood

that only the the M80 (maybe T80) would ramp to 110 MBH during DHW only. Did not know that the feature went down to the lower size. A good thing. Is this a new feature? And how do they do it?

Thanks

Brad