Proper sizing for combi boiler

suee

Greetings. Switching my old house from oil to gas. 2.5 story colonial, circa 1910, house footprint 25’x25’, no insulation, original single pane windows plus top of the line new storms and doors. Unheated walkup attic, heated basement. Total of 9 radiators, 2 full baths. Most walls are orig.plaster. Lowest outdoor temp here on LI NY rarely falls below -10F.

What size combi boiler would you recommend? (None of the plumbers I interviewed offered to do the calculation).

HVACNUT

A Combi is usually sized for the domestic load. The space heating usually just goes along for the ride. Your place wouldn't need more than 40K BTU's on a design day. But what's the maximum domestic hot water GPM at a given time? If you were given models of boilers, see which modulate the lowest for space heating.

And don't call back any of those "plumbers". Get quotes from recommend HVAC contractors. Have you checked the "Find a Contractor" link at the top of the page?

SuperTech

Just my opinion but a boiler properly sized to the heating load by a manual J load calculation and an indirect water heater is a much better option than a combi. Most combis tend to be massively oversized for the heating load because they have to make hot water on demand. This leads to excess wear and tear from short cycling and premature failure. The quality of your water should also be a consideration. Combis and tankless water heaters are pretty finicky when it comes to water quality. They don't last very long where I work, tanks usually last much longer and need much less maintenance. How do you make hot water currently? If you are using a tankless coil in an oil fired boiler pretty much anything will be an upgrade.

hot_rod

Nine cast radiators all on one zone?

If so a 120 combi would probably handle your DHW needs. It turns down to 12,000 for heating, so it will cycle much less than a fixed 50 or 60,000 cast boiler with indirect.

Many mod con combis have ramp delay settings also which will help mild season cycling. So use ODR, ramp delay and firing limit settings to dial in a combi mod con to small heating loads.

This maximizes efficiency and all but eliminates short cycling. It takes some time and patience to get the settings dialed in for your exact application. It is good for the owner to know how to make adjustments and fine tune a mod con, using all the control features.

Properly adjusted you may run 90% efficiencie for part of the heating season with a mod con. You will not come close to that with a conventional fixed output cast boiler.

A mod con will be less standby loss, more efficient, and no combustion air into the space. More venting options. Wall hung units take up very little space

What size boiler do you have now? How is it vented

suee

Current oil boiler is very old, pls see 3 pics attached: Back, Front, and MFGR label. Not known when it was converted from coal and steam to oil and water. Had an old mechanical aquastat, high setting was 185F, when circulator started the water pipes shook violently. New aquastat installed, circulator oiled (?), hi setting was eventually lowered to 165F, pipe shacking almost stopped, now radiators never get very hot, but on cold days the boiler works almost nonstop, when it overheats, lets out a drip of water for which occasion I keep a bucket under the valve. Being mindful of its age and hi oil price, my (single) thermostat is set to 62-64F in winter. I’d prefer 70-72F after conversion.

Hot water household use: 1 shower daily ab.4G hot water use, 1 dishwasher (Maytag 2024) run once every 3-5 days, 1 clothes washer load/week, sinks use 2-3G daily. These figures are my crude estimates of course. No hot water tank, all water is supplied by the boiler.

The reason I’m looking at combis is my low use of hot water. A hot water tank would just sit there for days with min.use. A hot 40-50G tank in my basement would dissipate enough heat to keep the basement hotter in an already hot summer, and send unwanted heat upstairs. The house has no central air. So on-demand hot water seems a good idea. Am I wrong? My water seems pretty soft, many years of living in this place and minimal scaling on faucets noticed. A small puddle of clean water on clean surface dries with very little residue visible on the perimeter of puddle.

One of the 6 plumbers proposed Laars 150 BTU combi, another one Rinnai IP120199C 120,000BTU plus descaler Halo Ion 6. Having read your replies, looks that either would be a huge overkill and cycle all the time? Even with very low household water use?

Once I speak to manufacturers I will post info re modulating the lowest for space heating, per question by HVACNUT.

Ultimately, would like to get reasonably priced installation and the longest-lasting, problem-free equipment. Any questions, just ask. Many thanks.

P.S. current boiler vented through brick chimney. The plumber who proposed Laars would build a separate exhaust for it.

As to my average GPM, if that means Gallons per Minute, its close to zero. The highest might be 5. In a very rare case 2 showers running simultaneously it might be 15.

All radiators on one zone, 5 of them cast iron, 2 baseboard type.

I’m totally not versed in heating terminology so don’t understand “Many mod con combis have ramp delay settings also which will help mild season cycling. So use ODR, ramp delay and firing limit settings to dial in a combi mod con to small heating loads.” If not too troublesome, please explain in layman’s terms?

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suee

typo corrected: All radiators on one zone, 7 of them cast iron, 2 baseboard type.

suee

another bright idea of mine is that in summer i could keep the combi boiler off and turn it on only when hot water is needed. Could I ?

EBEBRATT-Ed

I never knew Kohler made boilers.

I would PM @JohnNY or @Mad Dog_2 . If the job is not in his area one of them may be able to recommend someone.

You don't need your plumbers coming back if they won't take the time to size the equipment.

I would stay away from combi's.

Your much better off with a separate water tank of an indirect water heater.

HVACNUT

That is old. I haven't seen a Kohler boiler in almost forever. What town are you in? Check the contractor link. There's plenty of good HVAC companies on the Island.

Is that asbestos?

The Hydrostat looks like a Rockstar next to the rest of it.

Is there an outside entrance to the basement? Otherwise it's probably gotta be split to get it up the stairs. Thats going to be a messy one. Get all your good stuff away from the area and pathway.

Kaos

Combi only makes sense if you are tight on space or you need a lot of hot water for filling a large tub.

Doesn't sound like that is the case, so I would stick to a stand alone hot water heater.

Boiler guys like to install indirect water heaters but they are expensive and nowhere near as efficient and most think.

Your lowest operating and install cost is a heat pump water heater. Next is a regular power vented gas tank.

Modcon boilers with a standalone gas water heater are pretty much the norm around me.

If you really must have the combi, I would go for the smallest unit they make and install a resistance tank water heater down stream of it. The idea is that as long as the combi can keep up all the hot water will come from the combi, once it starts loosing steam if multiple showers are going, the tank makes up for the difference. Bonus is the resistance tank also acts as a buffer so there are no delays on hot water.