Boiler oversizing factor--Siegenthaler recommends max 10% (!!)

jesmed1 · December 27

@fentonc has very informative thread going on his efforts to optimize efficiency of a very oversized hot water boiler. We're in a similar situation where we have cold-start cast iron boilers that are 3x oversized for our heat load.

I'm just starting to read Siegenthaler's "Modern Hydronics" bible from which some of the boiler efficiency curves in @fentonc's thread were taken (from Chapter 3.9 Efficiency of Boilers). One thing I just came across in that chapter is Siegenthaler's statement on the bottom of pg 77 (in 4th edition of Modern Hydronics Heating & Cooling) where he says that:

" to keep the boiler cycle efficiency as high as possible, do not needlessly oversize the boiler. The author suggests a maximum oversizing factor of 10% of the (properly calculated) heating load. Minimum oversizing allows the boiler to run for longer periods, and thus attain higher efficiency."

I've never heard anyone here quote Sigenthaler's recommendation of only 10% oversize, so I'm mentioning that as an interesting data point in the ongoing discussion/debate about how to properly size hydronic boilers. For now I'm stuck with the boilers I have, but in future I do hope to replace with a properly-sized boiler (or two), and the recommended sizing factors I'm aware of have varied from the 40% oversize recommended by ASHRAE to the various other numbers recommended by the pros here. But I can't recall anyone recommending only 10%.

Obviously you want to be very sure of your design heat loss number if you're only going to oversize by 10%, and I doubt any pro is going to want to cut it that close with a customer. As an engineer who has run the numbers many times, many ways for our 4-unit condo building, I'd be willing to trust my numbers and go only 10% oversize, but if I were a heating contractor I wouldn't feel comfortable installing less than, say, a 25% oversized boiler, and 50% over would make me sleep better. And I doubt many pro's here would want to go less than 40-50% oversize either unless they were very confident that the design heat loss number was very accurate. Usually that can only happen if the homeowner has reliable energy consumption data for several years, and if the building envelope has remained unchanged over that time.

mattmia2 · December 27

there is no reason to oversize hot water boiler at all, they should match the heat loss of the building. the only reason to maybe oversize it a few percent would be to account for errors in both your calculations and the boiler operation as installed vs as tested by the manufacturer.

hot_rod · December 27

I suspect most load calcs have some fudge factor also. Do an accurate load calc with actual data if you can develop it, like infiltration numbers.

Personally I would go with a mod con regardless of the type of heat emitters, then the oversize factor goes away.

Depending on the total load, two smaller mod cons get you redundancy and 20-1 turndown!

Hot_water_fan · December 27

that’s aggressive but mostly moot. Cast irons don’t come in enough sizes to get within 10% that often and modulating boilers can be oversized easily.

pecmsg · December 27

undetsize

For the 1-3% of the time it’s colder blankets!

fentonc · December 27

Thanks @jesmed1 !

I think the oversizing thing is just 'structural' in the HVAC industry, in that the customer bears the cost of inefficiency (and is extremely unlikely to notice or be able to quantify it, unless they hang out here a lot), and there is a huge aversion to being called back because the customer is too cold. The net result is that boilers are chronically way oversized, but they 'heat well' (and probably burn 20-30% more fuel than a well designed system would).

Ironman · December 27

The only time that we consider upsizing a hot water boiler from the calculated load is when it’s being connected to an old gravity flow system where the mass is so high that the response time would be slow.

pecmsg · December 27

not every home has the same air changer as such you cannot use one size fits all

Generalize for leakage is the best we can do.

ethicalpaul · 1:35AM

Is it the case that for hot water boilers, steam boilers, or forced air furnaces, that truly correctly sized means that they should run 24 hours straight on a design temperature day? Or is that incorrect thinking? I have heard that before for furnaces, but I have never once seen any type of system that actually ran 24 hours on a design day.

I would welcome such a thing myself, but it would probably freak a lot of people out. And I would welcome a system that on the coldest day couldn't quite meet the thermostat, because then I would be sure I wasn't oversized, but I am weird.

DCContrarian · 1:56AM

A Manual J is going to use a 99th percentile design temperature, which means that 1% of the time the temperature is colder than that. That's about 88 hours a year, and while those hours don't all come at once that's enough time that there is a significant chance of not being able to hold interior temperature.

I like to look at the mean annual minimum temperature — MAMT. That's the average lowest temperature recorded in a year. If you use the sizing tool on the NEEP.org website you can see it. You don't have to size to meet MAMT, the house has some heat capacity so it can have the heat delivery be below the heating load for short periods without the interior temperature changing.

Interestingly, California is moving toward requiring that heat pumps be sized for between the 99th percentile and the MAMT. They want to cut down on people using heat strips during cold snaps, if a lot of people use them at once it can create problems with the grid.

EBEBRATT-Ed · 2:19AM

HW boilers have a 1.15 piping & PU factor built in so that is already a 15% safety factor over and above its actual heating capacity.

I am comfortable with that.

Also as @Hot_water_fan alluded to you do a heat loss there is always going to be some "guestimating" so that builds in a little fudge.

Most customers are not going to want to hear "put on another blanket" or "wear a sweater" when in the rare instance the ot slips below design. So the contractor put in a little fudge.

When you do a heat loss no credit is given to your refrigerator condenser output, your stove or dryer or water heater or lighting or people load or anything so here comes some more fudge.

In fact, probably the only time of max output would be when the building is vacant. And if it is vacant, you don't need 70 degrees.

Of course, you have to be a believer that the equipment MFG and the testing labs are doing their job, and the equipment will put out its rated btus.

All the MFGs know how to "fake it" so take it with a grain of salt. I have seen equipment in the MFGs own labs that wouldn't produce.

Eastman · 3:01AM

If it's a low mass hydronic system, I'd rather have it undersized. I wanna see it run around the clock occasionally.

mattmia2 · 3:57AM

https://forum.heatinghelp.com/discussion/comment/1879486#Comment_1879486

even when they aren't fudging the test the conditions in your house aren't the same as in the lab so the output isn't going to be exactly the same as in the lab test

hot_rod · 4:55AM

there are a number of different ratings on boilers IBR Gross, IBR Net, CSA/DOE, and AFUE. Different calculation methods are used to come up with these numbers.

I suspect even the lowest of the numbers still has some fudge factor built in?

Plus altitude derate above 4000’ elevation.

A good explanation here

https://www.hpacmag.com/features/making-sense-boiler-ratings/

Boiler oversizing factor--Siegenthaler recommends max 10% (!!)

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