Boiler Size - Am I asking for trouble?

Domark68

This summers project, oil to LP changeover. Pulling my 550 gal fuel oil tank out of the ground, have a 500 gal LP inground that only services my pool, planning on having this now supply an LP boiler as well. Small house, 2 bed/1bath 1000 sq ft single story over 1000 sq ft finished walkout basement, 1/2 of which is below grade. Just completed a detailed heat loss calc which yielded a figure of 82,000 BTUH loss at a design temp of 2*f here in the northeast.

26 years ago I installed a Weil Mclean GO3 rated with an I=B=R rating at 100k btu/hr output, Absolutely no issues heating and running an Amtrol 40gal indirect. The heating system consists of a fan coil in the attic heating the main floor along with a towel warmer, one side of the basement is baseboard and the other side finished at a later date has a radiant loop under engineered hardwood over Warmboard (an amazing product). Controlled by a Taco SR506-EXP w/outdoor reset, temps to my fan coil and baseboard hover around 150*f.

My question: I have the opportunity to use a new Lochinvar KHB155 Knight series mod con purchased for pennies, this boiler has a min input of 15.5MBH and a max of 155MBH (15,500/155,000 btu/hr), it's heating output is listed at 144,000 btu/hr. Is this unit to large to consider using? My main concern is short cycling. I have no experience at all with mod cons and their abilities to self modulate, been trying educate myself programs available in this unit, I read about spreading temp differentials and utilizing an anti-cycling mode as ways to combat short cycling. The install would be exactly as directed by the I&O manual with a PRI/SEC loop, a system return temperature sensor, outdoor reset and a Lochinvar Squire SIT-040 indirect whose domestic temp would be controlled directly by the boiler, I always utilized priority w/o issue so don't believe any upsizing is necessary for the indirect.

I'm aware a buffer tank would/could solve this issue but I don't have the room where this equipment lives along with the extra expense involved. Using this boiler is only a very cheap option, not set in stone.

What do ya think?

jesmed1

82,000 BTUH is a massive heat loss for a house that size. For comparison, that is equal to our well-documented actual heat loss in a circa 1924, 4800 square foot 4-unit condo building (not counting the unheated 2400 sq ft basement) at zero degrees outside temp in the Boston area. We have upgraded windows but not much wall insulation, so this is not a particularly efficient envelope. Your envelope would have to be grossly worse to lose that much heat in less than half the square footage.

For a sanity check, was your WGO-3 running 100% of the time at design temperature? Because if not, even the WGO-3 is oversized. I'm guessing it's running more like 50% duty cycle at design temperature, unless your house is massively air-leaky.

It's not as important to have an accurate heat loss number when your boiler can modulate, but as you know, any oversizing will tend to make short-cycling problems worse when you're trying to heat smaller zones.

Jamie Hall

Oversizing is nowhere near the problem with a mod/con that it is with a conventional boiler. That said, the trick is in setting up the programming in the unit to take advantage of that capability. It is larger than I would like to see for your application, but not enough so to overcome a significant cost break.

Now… that said, who is going to install and set this boiler up? While not rocket science — quite — both of those steps really need a knowledgeable technician.

EBEBRATT-Ed

As @jesmed1 correctly pointed out you heat loss calculation is way too high. With that size house you should be around 50K. I had a 1950s ranch and the walls were masonry with a 1" air space and plaster. leaky 1955 construction with only a minimum of insulation in the attic and it heated with 90 feet of baseboard which will output about 50,000 btus. The original boiler was 120,000 input and the replacement was 55 input. House was 1170 sq. feet.

mattmia2

does that model lochinvar allow you to do different het calls at different setpoints for the high temp and radiant zones? keep in mind that boiler will get something like mid to upper 80% efficiency in the high temp baseboard and hydroair zones. if you have to make high temp water and mix it down because of the way the boiler is controlled (might not have to depending on boiler) you will gewt similar efficiency on the radiant. you need to be able to call at the radiant temp to get good eficency out of the boiler.

hot_rod

that boiler can be limited to any btu output between 15,000- 150,000 so it would not be oversized even if the load is more like 50,000

The modulation us good to have with a zoned system

You will need two temperatures, high 140-160 perhaps for the fan coil, maybe as liw as 100 for the radiant

pecmsg

I have the opportunity to use a new Lochinvar KHB155 Knight series mod con purchased for pennies,

How?

That's a lot of extra capacity that will never be used. Mod cons are only highly efficient when operating below 140°F. Above that and there similar to cast iron. One repair a year can wipe out all that savings.

hot_rod

If you can get a great deal on it I would buy it. While it is oversized it does actually run a few % higher efficiency when locked down to a lower firing rate.

See this graph to explain how 25% firing increases efficiency. At any supply temperature actually.

You expose a larger surface area to a smaller flame, so more surface area.

Mad Dog_2

You could always mount a buffer tank later, off the ceiling, tucked away in a corner. Mad Dog

ChrisJ

I agree with others.

I have a 1600sqft plus basement 1860s 2 story house with little insulation and something like 22 original windows with terrible storm windows and my actual heatloss is something like 68,000 @ 0F. I think I saw 74,000 @ -8f. This was going by boiler run times assuming the DOE output rating is close to correct.

I'm not sure how bad a structure would need to be to pull off 80K in 1000sqft.

GGross

https://forum.heatinghelp.com/discussion/comment/1890326#Comment_1890326

screwing around in loopcad, 1000 sqft with a 1000sqft walk out basement. I had to tell the program there was R-11 in the main level walls, block basement walls, no insulation on the walkout, none at all in the ceiling, with 10 4x4 windows main level, big sliding glass door and 2 windows in the basement walkout, and label the construction as "loose" which is described as "There has been no effort to seal the structural panels, the associated corners, cracks, joints, and penetrations, window and door assemblies are not rated, or rated at more than 0.50CFM per running foot of crack at 25mph wind speed" comes out to 81,487 btu/hr at +2f

so a bit like a 2 level pole barn with some rolls of batt insulation on the walls and a bunch of windows.