Oversized Boiler Issue - Replace or Not

Brad White_9

I do know the houses I referred to are essentially identical because I live in both of them and monitor their systems.

While the OP is not planning necessarily to use condensing, I was advocating it as an option. My boiler, a Monitor MZ, while condensing does not modulate. It is about as efficient as my Burnham Series 2 with a 4-way mixing valve based on my historical data going back to 1986. The principle point, that of having a boiler capacity resemble the heat loss was lost apparently.

The other point lost from my original posting is the cost of fuel. Used to be at one time that at 80 cents to a dollar a therm things did not pay back. Now that gas is 50-75% more, certain options become viable. In our case, it is palpable.

I agree with you that buffer tanks, setting changes and so-on are relatively cheap options but we are beyond that at this point. 67 MBH design heat loss home with an average outside temperature of 31 degrees really means a heat loss of about half that number, call it 35 MBH, tied to a 164 MBH input on-off boiler. Jet engine on a canary. The run fraction was observed between 15 and 20 percent. Yeesh!

Reset will save significant money when it is tracked accurately and with adjustable differential as I have proven. The key is having a boiler capacity proximate to the heat loss and certainly not double. The ramping up and down, short cycling and so-on is eating gas like you read about.

I know because we are living it and am taking steps to resolve it, starting tomorrow. The 164 MBH boiler, barely 9 years old, will be retired or donated to a much larger house.

GWR

Oversized Boiler - To Replace or Not To Replace?

My home, built in 1925, is approximately 3000 square feet, located near Philadelphia. The heating system is gas with cast iron radiators and a total EDR of 1055. A 40 year old Bryant 300,000 BTUH water boiler was replaced with a 305,000 BTUH Weil-Mclain PFG-6 model (80% combustion efficiency) about seven years ago. This boiler has no damper or any other controls but does have electronic ignition. AFUE values are not provided since it is above 300K-BTUH. Since then about half of the house walls have been insulated to R11 and the attic upgraded to R30. In the spring the steel casement single pane windows will be replaced with Marvin double pane argon Low-E types. The chimney has been lined and an efficient damping system installed. Even the garage door is now insulated to R9. Based on a calculation using HVACalc 4.0 software (0 outside 74 inside) the resultant new heat loss will be about net 100,000 BTUH. This would translate into a mid-efficiency unit with input rated at about 150,000BTUH.

The existing boiler will be tremendously oversized!

Is is worth replacing a seven year old system, over 100% oversized, to achieve efficiency or is it money wasted? It should be noted that the replacement seven years ago was relatively cheap at basically material cost only (long story). Will the addition of outdoor reset controls, repiping and a damper be enough or are the greatest savings realized with all these items and a new boiler?

Note that since 12/01/04, gas prices in my area have been raised by 42%!!! Before the increases my Winter gas bills were in the $600/month range. I am looking for cost effective savings wherever possible.

I have been considering a Burnham non-condensing unit with 164MBH input, 118MBH net (IBR) and 84% AFUE. This would include an indirect heater to replace an existing 9 year old standard model, a Tekmar 260 controller with outdoor reset, constant circulation and boiler bypass piping to maintain proper return temperatures. I considered a condensing type but considering their proprietary controls, greater complexity, limited warranties, venting issues, steam plumes and +$2,000 cost it is eliminated.

I have asked this question on a few other forums in search of a consensus. Thanks for any replies!!

chapchap70_2

Unless you feel like heating the neighborhood, replace it.

"Is is worth replacing a seven year old system, over 100% oversized"




Yep

Jim Pompetti

`Is your system steam or hot water.EDR of the system is only of use in a steam system. A heat cal is necessary if the system is hot water. In either case would you wear a shoe that was to larger just because its there.The proper size and properly installed system will save you money and run better,Find a professional in your area [names are aviable on this site] and get what you need done.

J. Cricket

Oversizing is bad, but remember, even a correctly sized boiler is oversized every day except a design day.

You will save money operating a smaller boiler, but in its lifetime this saving will not make up for the cost of buying the boiler. Not even close.

Some contractors will tell you different but they have a financial motive.

Bruce M.

I-B-R rating is lower.

Keep in mind that the net I-B-R rating for the Weil-Mclain PFG-6 is 215,000 BTU/Hr. Since you do not have the automatic vent damper it would be good to add it now. It was originally an option on that model. How many zones are on this boiler? Is the circulator pumping away? There are a few things you could do for not much money and continue using this boiler.

Brad White_9

Depends on cost of fuel and other factors

As a design-side person (HVAC engineer) it all gets down to payback and the cost of fuel is the wildcard of course. The engine that drives the decision in the first place.

I have seen paybacks in as little as three years and as long as ten. So long as they are less than the mid lifespan of the boiler, I say go for it.

Naturally, if the system even if over-sized works well enough or has supplemental controls to mitigate cycling, this makes the replacement decision more difficult.

Some other threads on this site have some really solid experience about replacment boiler performance. With agressive re-set, fuel consumption has been reduced by one third to one half or more, even from boilers that were originally close to their design heat losses. Let alone boilers that are 50 to 100 percent over-size.

Example: My Susan's house has a boiler easily twice the size that the radiators can emit. (164 MBH input versus 67 MBH radiation output with 180F water). House is 1800 SF, well insulated and she used about $500 worth of gas in December, 300-350 therms I think it was. My house is similar but with a non-modulating 94 MBH input condensing boiler which makes domestic HW too. It used less than 160 therms for the same period, a $220 gas bill if memory serves.

Over a year's time this is some serious cost, perhaps a thousand dollars all things being equal? And the cost seems to be continuing. If we spend several thousand dollars on a replacement, that is the number of years to payback. And as fuel cost goes up, the cost will be less.

Our analysis to replace an oversized boiler installed only in 1997....

R. Kalia_8

The OP is asking whether it is worthwhile to replace a 7-year-old boiler with a new smaller boiler. All other things being the same, this change will not save much money and will not ever pay for the new boiler. (You cannot compare gas consumption in two different houses; how do you know they are identical? You don't. Plus your home has a condensing boiler, which the OP is not planning to add.)

Short cycles can be reduced by using a buffer tank, and by changing the stat settings to allow a somewhat wider temperature swing. (Select 1 cycle/hr, or "steam" setting, and see if there is an unacceptable temp swing.) Those are cheap options. Reset is great for comfort, but will not reduce the gas bill dramatically; it can be added to the existing boiler using bypass. There is no reason to throw away a 7-year-old boiler.

GWR

Followup

Just to toss something else in the discussion. Yes - The IBR net for the PFG-6 is 215MBH. The resultant loss is 90MBH. My water heater is a 40 MBH model. Assuming 50% efficiency, there is another 20MBH loss. The total heat loss going up the chimney, 110MBH, is higher than the design load heat loss of my house at 0 Degrees. I am already over-radiated so adding additional system mass will not help.

It seems that I am siding with the replacement folks. When I look at the expense of adding an automatic damper, outdoor reset and bypass piping to a 305MBH boiler the economics do not seem to be there. The upfront cost ($7800) is hurtful but I begin to believe the savings derived from the new system, including the indirect heater, make sense. It is hard to be exact but I don't expect gas prices to start a steep decline.

I wish I had done this level of research and outreach before the last replacement. A lesson to all non-professionals. Do your homework. These forums are increadibly useful tools. The quick and cheapest deal (7 years ago) may cost you in the long run.

Mike T., Swampeast MO

If you really want to save on fuel, use a condensing/modulating boiler. Judging by the size of the house, the EDR of the radiation, the outdoor design temp and the increased insulation/weatherization your radiators will be MASSIVELY oversized requiring quite low temperatures to heat the house in the coldest weather.

A condensing/modulating boiler will utterly thrive on this sort of system and your fuel consumption is nearly guaranteed to plummet. >50% fuel reduction compared to the current boiler and prior to insulation/weatherization can be expected. An ideally sized, perfectly installed and highly controlled conventional boiler is incapable of achieving this sort of fuel reduction.

Install thermostatic radiator valves (TRVs) on the radiators to maximize comfort, adaptability and efficiency.

Because some condensing/modulating boilers are literally "everything in one case" and the rest almost always have sophisticated controls built-in, the cost difference between conventional and condensing/modulating is not as high as you may think.

Yes, this will be an expensive update. If you opt for TRVs and have muscle, a helper, a couple weekends, simple tools and basic skills you can save significant $$$ by installing the TRVs yourself--and the contractor shouldn't begrudge your doing such.

If you don't replace the current boiler (and I must agree it's somewhat "iffy") a properly sized buffer will help on the efficiency side. But combined with outdoor reset, automatic damper, piping changes and likely a large buffer tank the cost of "just" this won't be particularly inexpensive.

p.s. You are absolutely, totally, 100% correct in suggesting "research before replacement"!!! This is probably the best place to start and verify your research!

Ron Schroeder

GWR,

Are you steam or hot water?