How much load should I add to account for an indirect hot water heater

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Old 12-05-12, 01:07 PM
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How much load should I add to account for an indirect hot water heater

I'm in the planning stages of replacing my boiler.

I used the Slant fin application and did a heat loss analysis on the house
The program states I have a 61,496 BTU / Hour heat loss which is about 1/2 of what the current boiler is sized for. The boiler used to have a tankless coil so some where I remember reading that adds about 55K btu to what is need. If this is true then the current boiler is sized correctly.

I currently have a Weil Mclain Gold plus 30 28 Gallon indirect fired HW heater that I intend to use with the new boiler it is only about 5 years old. I disconnected the tankless, the hot water heater is on it's own loop with a circulator.
The system was updated with a Honeywell outdoor reset panel which runs the show (3 zones and the HW ) and I intend to keep that system.

Based on what I've read and understand that the additional zone that the hot water heater is connected to is not nearly the demand of the 55k need for the tankless.
So how much additional BTU capacity should be added for the above hot water heater in sizing my boiler.

Thanks
 
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Old 12-05-12, 01:28 PM
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Hi,

The pros in that area will be on shortly.

They probably will ask what make boiler do you intend to purchase? Some higher end boilers come with space for ODR controls.

I dont think you add any btu to the boiler for the specs for that WM 30. Go by the specs I believe. Page 67 here shows 50k btu for a 90 gph rating. Set your control to have the indirect as priority over heat and you should not have an issue.

http://s3.pexsupply.com/manuals/1283..._PROD_FILE.pdf

Size boiler by DOE from what is preferred from the pros here.
 
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Old 12-05-12, 01:37 PM
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Thanks.
I was confused by the specs page.. I do see the volume of HW calls for about 70K with a 120 GPH first hour rating which is close to the calculation of 129 first hour based on our household demand.

Being oil fired I guess the smallest I could go is The Burnham MPO84 (DOE 74) or the Crown CT3 (DOE 92)

The current boiler is I believe 120 I=B=R I know they say the design size already has fudge in it I guess I should get comfortable with the numbers and use a smaller boiler.
 
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Old 12-05-12, 01:55 PM
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Here is one data point. I have a standard 40 gal gas fired water heater. Its input is rated at 40,000 Btu/hr, so its thermal output is probably around 35,000 Btu/hr. But, I can think of a couple of reasons not to add anything to your calculated heat loss for sizing your boiler.

First, the Manual J heat loss procedure (which the Slant/Fin program is based on) has a significant built-in safety margin. Secondly, for space heating at your design atmospheric conditions, it is conceivable that all zones could call for heat continuously. Your water heater shouldn't have to call continously during such time (unless you run the showers, dishwasher, and do laundry continuously on the coldest day of the year).
 
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Old 12-05-12, 02:07 PM
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I see your point... It is not unusual for all 3 zones to call for heat at the extreme which is below zero weather. We don't have that often but when we did have it for a week straight one year, the boiler was running long cycles to keep up. I do have the control set to HW priority.

So I guess the MPO84 is a closer match for my situation.
 
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Old 12-05-12, 02:18 PM
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One of our mods just installed one.

http://www.doityourself.com/forum/bo...n-finally.html
 
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Old 12-05-12, 03:15 PM
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You don't add anything for the indirect. You set it up to run prioority. That mreans when the indirect needs to be heated, it shuts down all heating zones until it is satisfied. Then the heating zones come back on.

As far as the specs for the indirect, those are for first hour ratings. So you have a smaller boiler, it means it will take longer to recover. It doesn't mean it won't work.
 
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Old 12-05-12, 03:18 PM
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Thanks guys for the info!

Any thoughts on my boiler choices?

Pro / Cons?
 
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Old 12-05-12, 04:12 PM
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You probably want a 3 pass design, and there are a few manufacturers that do that.

Whats your budget?

The oil guys will be on soon to better guide you.
 
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Old 12-05-12, 05:19 PM
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What have you got now? make/model

and why are you thinking about replacing it?
 
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Old 12-06-12, 06:40 AM
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Repco Turbo flame TOS130 133,000 inut net as per the tag 115,700 which states the net found by dividing the input by 1.15

The boiler is what I put in back in 1984 It was ahead of it's time... when set up properly it runs 86+% I can't complain about it, it has been virtually trouble free.

It is steel and getting old so I think that it may spring a leak at some point.
I've also heard that if a steel boiler is maintained properly it should last. I guess that said the system 2000 is steel and they claim steel is superior to cast iron.

So what I'm doing is preparing to replace it when it fails or when I get a good deal on a replacement come up. The Crown and Burnham mentioned were the two I was considering
The size was in question but my heat loss calcs have pointed me to CT3 and the MPO 84
Basically I have a working outdoor reset system ( Honeywell) and the indirect water heater so the only piece I need to replace is the boiler and it need to be efficient and simple.

I was trying to find a low mass steel boiler but it seems the cast Iron boiler are what is most popular, if they do just as good a job and are more familiar to the service people it probably makes sense to use a cast iron boiler.
 
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Old 12-08-12, 04:19 PM
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Just like droo said. Set the indirect up as priority and the only btuhs you need in priority is for the indirect. To get the most out of you indirect the boiler needs to be sized to that (probably around 100-110k btuh) but the rest of the time it will be oversized for heating. Thats when a nice modulating boiler comes into play. Best of both worlds.
 
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Old 12-08-12, 06:13 PM
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To get the most out of you indirect the boiler needs to be sized to that (probably around 100-110k btuh)
Agreed,

and would you agree that firing the indirect at say 64K will only take a hit to the recovery time?

(and recovery time would still be worlds better than an electric, and probably better than a gas stand alone) ...

Or would that hit the 'first hour' rating as well?
 
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