Choosing boiler size


Old 01-04-18, 07:40 AM
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Choosing boiler size

Hello. I need to replace an oil fired hot water boiler. Discussion here:

I believe the boiler I currently have is oversized a lot. It's an old Weil-McLain 68 at 151,000 BTU. My house is about 1400 sq ft. The Slant Fin heat loss calculator gives me a requirement of 46,100 BTU heat loss. I have about 110 feet of baseboard, single story home. Some of the pipes supplying and returning the baseboard run through an unheated attic. I'd also like to use a model with a tankless coil (to replace old water heater).

So it looks like the minimum boiler size I would need is about 50k BTU, but that doesn't account for tankless coil or exposed pipes. The Slant Fin calculator brings me to the Intrepid TR-20 (100K BTU). Would this be appropriate or is that still way too big (got a quote where they were just gonna swap in another 150K BTU unit)? How do I determine what additional BTU level I might need to account for tankless coil and exposed pipes?
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Old 01-04-18, 09:47 AM
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There are different schools of thought on this subject.

Personally I'm not a big fan of cutting way down on a boiler to match exactly what your heat loss is.

There is more to consider than just heat loss. For instance design temp, what temp do want to maintain at 0 deg., Will the small boiler keep up in weather like we're having now.

How much baseboard has to be heated. For example, you have 110 ft. of element and baseboard is figured at roughly 600 btu's a foot at 180 deg supply water, which comes to 66,000 btu's p.h. your boiler must produce.

You want a tankless. Again, nothing is carved in stone on this figure but when I started we used to figure an additional 30,000 btu's for the coil. Some people used to figure 15,000, others didn't figure anything because they said the boiler was running anyway.

I can only tell you what I did and it served me well with never a complaint.

The problem with the tankless today is the low water content in the boilers. The old boilers had a high water content which the coil sat in and didn't cool down as fast as they do today.

The old boilers like the one I had in my house had 50 gal. of water. The typical 3 section boiler today has about 13 gals. so the minute the pump comes on, the boiler empties out and the coil is sitting in tub of cold return water until it heats back up again.

Now there are controls that prioritize the hot water but it's not perfect.

Anyway, all that being said, sight unseen 151,000 seems way oversize for what you're heating, but 50,000 in my opinion is too small.

To me, and I'm sure there will be varying opinions, the 100,000 would be my choice.

Just my thoughts.
Old 01-04-18, 02:27 PM
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Why do you want to install a boiler with a tankless coil? The tankless (or thankless as one member calls it) coil is the second worst method of heating domestic water known. The absolute worst is an open kettle on a wood-burning kitchen range. The tankless coil worked, sort of, kind of, in a somewhat acceptable manner when boilers had a large amount of contained water. With the much smaller boilers of today the limitations of the tankless coil become even more obvious.

Personally, I prefer a stand-alone water heater, if for no other reason than if the heating system goes down you still have hot water. Even if you decide you want an indirect water heater (it uses boiler water to heat a separate storage tank) it makes more sense than a tankless.

You state you have 110 feet of baseboard convectors, is that the total length of the housings or actually the finned elements. Many homes have the housings running the full length of walls but have much shorter elements inside. Assuming that the 110 feet IS all elements you have the ability to emit about 66,000 BTUs/hr. of heat into the house. It doesn't matter how big a boiler is installed you will only get that 66,000 BTUs/hr. into the house. Using the fudge figure of 30 BTUs per square foot your 1400 square foot house would use about 42,000 BTUs/hr. That 30 BTUs/hr. figure is for a fairly leaky house and one that has had any amount of energy saving upgrades such as air sealing and full insulation should come in significantly lower.

In most cases boilers are NOT upsized to allow for also heating domestic water. They usually DO have priority control for indirect heaters that will shut off the heating for a short period of time to "recharge" the storage tank.

I am not up on what is available but size-wise I don't think I would consider any boiler over about 75,000 BTUs/hr. output and even that might be 50% oversize.
Old 01-04-18, 03:34 PM
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I agree with Furd's suggestion to use a separate standalone domestic hot water heater, even if, in your case, it has to be electric. In addition to Furd's reasons, during summer months, the boiler can be shut down. Also, when the boiler is down for maintenance or repairs.

I also substantially agree with Spott's suggeston not to size the boiler to exactly match the calculated heat loss - even though a significant design margin is built into Manual J calculations, which are used by most computer-based programs. For one thing, a slightly oversized boiler will provide a bit quicker recovery timeswhen the thermostat is set up in the morning.
Old 01-04-18, 04:02 PM
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"...a slightly oversized boiler will provide a bit quicker recovery times when the thermostat is set up in the morning."

Not really. The amount of heat emitters in the house is the limiting factor. If you have only 66,000 BTUs/hour worth of emitters it doesn't matter how big your boiler, it will simply shut down on high limit until the temperature drops, at the rate of heat release from the installed emitters.

On the other hand, having MORE emitter capacity is a very good thing as it will allow the boiler to operate at a lower temperature and still supply all the heat needed. 100,000 BTUs/hr of heat emitter coupled to a 80,000 BTUs/hr. boiler would allow the boiler to run flat-out forever.
Old 01-04-18, 07:06 PM
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Furd: I want the tankless coil because the area the boiler and water heater are in is very small. I could use the space savings. The house was poorly designed in this regard but that's what I have to work with. I had a tankless coil in my last house, which had two bathrooms, and it was never an issue. This house has only one bathroom. Although it may be less efficient the tradeoff for space is one I'm willing to make.
Old 01-07-18, 07:31 AM
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Mike Speed 30 "...a slightly oversized boiler will provide a bit quicker recovery times when the thermostat is set up in the morning."

Furd: "Not really. The amount of heat emitters in the house is the limiting factor. "

Mike Speed 30 is right and Furd wrong.

In the morning, when thermostats step up to day time temps circulator sends lots of cold water to boiler creating the “recovery rate” issue. It also can pull water below condensation temp.

Burner BTU firing rate and return water temperature are major factors in recovery of boiler water temp. It makes no difference if return water temp results from 1 or 20 emitters. What counts is the BTU's lost, not how.

I use a low temp control to turn off circulator below 137 F during the boiler recovery time. Usually a couple of minutes until it recovers to 139 F. Typically happens once at 5 AM on cold mornings and is not noticeable to occupants.

Last edited by doughess; 01-07-18 at 09:27 AM.
Old 01-07-18, 04:04 PM
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Actually they are both correct. the boiler will heat up faster if a it oversized and the more radiation you have more btu's are put into the living space at any given water temperature. A boiler a bit oversized is not the end of the world efficiency wise. A slightly oversized boiler will shut down more often and a boiler not running does not heat.

In this past cold snap I had some contractors de-rate the oversized mod/con boilers to heat the home better. Yes, due to oversized boilers they were shutting down and cycling back on. y de-rating the boiler stayed on and held the set point temperature.

With todays low water volume cast iron boilers I would never suggest circulator reverse on a circulator unless you have a domestic hot water coil in a boiler, but than I would never suggest a domestic hot water coiler in a boiler.

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