Sizing a new hot water oil boiler


Old 12-10-16, 04:34 AM
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Join Date: Dec 2014
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Sizing a new hot water oil boiler

Hi everyone,
When I had our boiler cleaned a few weeks ago, the tech mentioned that my boiler is nearing the end of it's run. He noted this because it is rusting around the coil area, and the fact that when the boiler is shut off for a period of time, it will weep some water. When it is at running between the high/low limit, there is no water weeping.
So with that in mind, I want to start the preliminary search for a replacement. I realize that a heat loss calculation should be done in my home, but I want a starting point based upon my current boiler.
The current boiler works perfectly and provides plenty of heat and hot water via the coil. Even on the coldest days we are never at a loss for heat. Based upon my satisfaction with the current set-up, I assume that the boiler is sized properly or a touch over sized.
My home is a ranch that is roughly 1,200 sq ft with one zone and the coil for h/w. In a replacement boiler I would do a indirect h/w heater, so the new boiler would actually be running two zones.
Based upon the data plate of my current boiler, what size (range) of new boilers should I be looking at?

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Old 12-10-16, 08:36 AM
Join Date: Oct 2013
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I happen to have to have the size house and the same boiler as you have with an indirect hot water heater that I installed 32 years ago and is still going strong. If the only reason you are looking for a new boiler is the rusting and cool water leaking you can replace the coil gasket to stop that leaking for pennies compared to the cost of a new boiler.

Without knowing I was in the business I've had people look at mine which does the same thing as yours and tell me for years That it should be replaced until I ask them a couple of questions and they realize what I did for a living and they pack up and with different excuses find a reason to quickly move on.

There is no shortage of people out there willing to sell you something if you need it or not. Now I'm not saying that these boilers last forever but I would get a couple of opinions and estimates before jumping into anything.

It used to be you could trust companies a little more. By that I mean a tech would have suggested replacing the gasket or even the plate if it needed it but today they just want to replace instead of repair. It's all about the bottom line.

As far as your boiler sizing goes, that is a 3 section cast iron boiler and a very good one. They actually do make a 2 section but I definitely would not go smaller. Depending on what you plan to add or what the boiler is heating would determine if you need to go up a size or stay the same.

Hope this helps.
Old 12-10-16, 05:38 PM
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Get the smallest oil boiler they make.. Thats about 85 K btu..

Get an indirect water heater and have the new boiler a cold start.

No chance of natural gas in your location???

IMO you also can replace the coil in your boiler.. Some warm start boilers like your weep when they go cold... normal...

Coil like your s is the worst way to make hot water..

May even be better off with an electric HWH
Old 12-11-16, 05:10 AM
Join Date: Oct 2008
Location: New England
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Having just been through this for my daughter I can strongly recommend going through the entire process, heat loss, energy improvements, indirect water heater considerations, choosing your installer/service, and have it ALL planned out. The worst case scenario is a major failure in the dead of winter when there is zero time for planning. Then you end up taking what THEY want to sell you, size, brand, and package, because there is no time to shop around.

Then you can more comfortable stay with what you have for longer. But when it is replaced you will probably go much smaller and enjoy some savings on heating costs.

Old 12-11-16, 08:08 AM
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Old cast iron hydronic boilers do not have fixed life span. They really have not been around long enough provide meaningful data. A little rust on cast iron is usually nothing to worry about.

The “weeping” during summer shut down is probably not an issue but more a more detailed description with pictures would help.

In the 30 years since installation there have probably been improvements to the house that reduced heat load/loss. Boiler probably has excess capacity that would allow a lower nozzle size to increase efficiency.

An accurate way to find building heat loss is clocking run time on current boiler for period of degree days and gallons used. Oil delivery companies use a home degree days per gallon history to schedule deliveries. With run time hours the actual nozzle GPH and degree day per hour can be found.
Old 12-13-16, 05:22 PM
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Join Date: Dec 2014
Location: United States
Posts: 102
Unfortunately we are stuck with oil. When looking at the Supplyhouse website, there does not seem to be a 114,000 BTU boiler, so that is why I was looking for a range.
The current boiler is fired with a .85 gph nozzle. It is nice to know that I can safely downsize.
Old 12-13-16, 06:01 PM
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Hi again, I just want to make a couple of points. One, adding an indirect w/h doesn't actually add an additional load as a second zone might. When the water heater calls for heat it operates with a priority and discontinues the heat for the other zone. When the call is complete the boiler goes back to heating the house. For actual sizing an indirect w/h doesn't increase the boiler size. Pros can correct me if I'm missing something on this.

Second, the boiler is basically way oversized for 90% of its operating life. It only needs to be some predetermined size on the coldest of the cold days and for those few times it will run continuously. In most cases the boiler never ends up being sized for the exact minimum so the occasions of continuous running never really occur.

My guess would be the heat loss for your house will come in somewhere below 50,000 and if not, I'm an energy auditor and can point out some simple improvements to get there. To be considering a 115,000 btu boiler when 50,000 is already oversized is just wasting oil. A smaller boiler with longer run times is just more efficient. And, since they don't make small boilers to fit our needs we end up installing one in the 75K to 85K range anyway.

There are simple programs that can tell you what youar heat loss is and point out where your energy improvement s will be most effective.


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