Mod/Cons and boiler oversizing

G-rott

Can't mike determine his actual usage by using a gas usage per heating degree day calculation at a specific heater effiency?

[Deleted User]

mod/cons and oversizing boilers

I am a HO looking to replace my 20+ year old boiler next fall. I have been doing some research and have a question about modulating boilers and oversizing.

I know my current boiler is oversized. I can keep the house at 85 when it's below 0 outside and the boiler is just getting warmed up. It would come at a huge cost, but possible.

All my research warns heavily against oversizing a boiler because that reduces efficiency, and I mostly understand the concept. My question is, would a modulating boiler negate the inefficiencies of a (moderately) oversized boiler? That is, if an oversized modulating boiler only ran at 80 or 85% of full capacity, would you still reap the benefits of the system?

The reason I ask is that my current Pro is having trouble with my heat loss calculations. I have an old house with single pane windows/storms, and decent attic insulation. The problem is that there is no wall insulation. The house is brick (two courses with @ 1" of plaster). I don't think he has come across that often, and I want to be sure we don't undersize. I was hoping that an oversized mod/con would solve the issue.

Am I way off base here? Thanks for the help.

Constantin

Hi Mike!

Congrats on finding the site and a tip of the hat from a fellow homeowner who has had many questions answered here. In my mind, the best way to go about sizing boilers is to reduce the heat loss as much as possible and then to chose a boiler that best fits my needs. Since your current boiler is still alive, you can use it to get a heat loss calculation that is quite accurate, easy to accomplish, etc.

Wait for a cold evening, as close to the design-day conditions for your location. Ideally, the wind will be blowing 15MPH and the sun will have been down for a few hours. Now go into the basement, settle down with a good book, then meter how much the old girl is firing.

If its an oil boiler, the combination of pump pressure and nozzle size will give you an idea of how much oil she's using. For gas, the ratings are usually on the boiler or can be observed via the gas meter. Now multiply the AFUE of that boiler times the input rating and you have a pretty darn good idea of what the heat loss is for that exterior temperature.

For example, let's say you normally keep the house at 70°F and you happen to have some free time on a night where it's a balmy 35°F outside. You note that over the course of an hour that your 100kBTU/hr input, 80% AFUE boiler was on for 20 minutes. Thus, for a 35°F ΔT, your heat loss was approximately (100x20/60) x 80% = 26 KBTU/hr.

Through research via either the wunderground site, NOAA, or the local weather station, you discover that the design-day conditions in your area are approximately 0°F. Since a design-day is twice as cold as your testing conditions, your heat loss would double on such a day, i.e. become 55kBTU/hr. The relationship is linear.

This 55kBTU/hr heat loss is what your new boiler will have to be able to handle with it's DOE output capacity at the specific water temperature your emitters will need to reach. This is one important variable with mod-cons:

If you use high-temp emitters in the home (baseboard sized to only cover the heat loss on a design day by running at 180°F, for example), then a mod-con must be sized carefully to ensure that even when its output is curbed somewhat by the high return water temperatures that its output will still cover your needs. Ideally, add emitters or insulation until the mod-con can condense year-round.

Manual-J and other professional publications use 65°F as a base interior temperature, though the difference is probably negligible. Even if you get a "reasonable"-looking heat loss calculation this way, I'd use these results as a yardstick, as a room-by-room heat loss calculation will disaggregate all your building loads, pinpoint improvement potential, etc.

Uni R_2

> Congrats on finding the site and a tip of the hat

> from a fellow homeowner who has had many

> questions answered here. In my mind, the best way

> to go about sizing boilers is to reduce the heat

> loss as much as possible and then to chose a

> boiler that best fits my needs. Since your

> current boiler is still alive, you can use it to

> get a heat loss calculation that is quite

> accurate, easy to accomplish, etc.

>

> Wait for a

> cold evening, as close to the design-day

> conditions for your location. Ideally, the wind

> will be blowing 15MPH and the sun will have been

> down for a few hours. Now go into the basement,

> settle down with a good book, then meter how much

> the old girl is firing.

>

> If its an oil boiler,

> the combination of pump pressure and nozzle size

> will give you an idea of how much oil she's

> using. For gas, the ratings are usually on the

> boiler or can be observed via the gas meter. Now

> multiply the AFUE of that boiler times the input

> rating and you have a pretty darn good idea of

> what the heat loss is for that exterior

> temperature.

>

> For example, let's say you

> normally keep the house at 70°F and you

> happen to have some free time on a night where

> it's a balmy 35°F outside. You note that over

> the course of an hour that your 100kBTU/hr input,

> 80% AFUE boiler was on for 20 minutes. Thus, for

> a 35°F ΔT, your heat loss was

> approximately (100x20/60) x 80% = 26 KBTU/hr.

> Through research via either the _a

> href="http://www.wunderground.com"_wunderground_/a

> _ site, _a href="http://www.noaa.gov"_NOAA_/a_,

> or the local weather station, you discover that

> the design-day conditions in your area are

> approximately 0°F. Since a design-day is

> twice as cold as your testing conditions, your

> heat loss would double on such a day, i.e. become

> 55kBTU/hr. The relationship is linear.

>

> This

> 55kBTU/hr heat loss is what your new boiler will

> have to be able to handle with it's DOE output

> capacity at the specific water temperature your

> emitters will need to reach. This is one

> important variable with mod-cons:

>

> If you use

> high-temp emitters in the home (baseboard sized

> to only cover the heat loss on a design day by

> running at 180°F, for example), then a

> mod-con must be sized carefully to ensure that

> even when its output is curbed somewhat by the

> high return water temperatures that its output

> will still cover your needs. Ideally, add

> emitters or insulation until the mod-con can

> condense year-round.

>

> Manual-J and other

> professional publications use 65°F as a base

> interior temperature, though the difference is

> probably negligible. Even if you get a

> "reasonable"-looking heat loss calculation this

> way, I'd use these results as a yardstick, as a

> room-by-room heat loss calculation will

> disaggregate all your building loads, pinpoint

> improvement potential, etc.

Uni R_2

Everything Constantin Said

But furthermore, without the leaking chimney and without the bang bang very hot boiler heating cycles and without sucking damp outside in through your walls (moisture is no friend of fiberglass insulation) you will need much lower temps than you had before... It's more than just the efficiency numbers, a direct vented mod con really helps the envelope work better.

Your flue temperatures won't be much above your return temps and they will be more constant and therefore you'll condense far easier than simple subcooler logic would say.

Plumb Bob

Modulating is good, and yes, it does help get increased efficiency when the boiler is oversized. But then you don't get the full benefit of modulation, because you lose some of the modulation range.

To do the calculation Constantin suggests, it is not necessary to wait for the coldest day. Just get some data for a few cold days and divide the total BTU consumed by the total degree-days. Then multiply it by the number of degree-days in a design day. This gives you the maximum BTU per day; divide by 24 to get BTU per hour.

Of course that's for your old boiler. Even if you reduce it by only 10% for the new boiler, to be safe, it'll be more accurate than a heat loss calculation, because the dirty secret few people here like to admit is that those calculations are very approximate. Scientists will tell you that an experimental measurement (if correctly done) measures actual physical reality, and so is always much more trustworthy than a theoretical prediction.

Xc8p2dC_2

Done deal

If you decide to go with a traditional cast iron bangbang, you pretty much have one shot of getting it right or wrong with the unknowns you have. With Mod/con you have a better shot of matching what your house actually needs

In my case, I used an Ultra80, my heat loss [new construction] is under 39K btu at 9 degF [and we hardly ever get that cold]... With a couple of pushes of the digi controls, I have it dialed down to 48,000 btu gas input, and still overshoots room temp.. Lots more room to work with if need be... Not sure if all ModCons have this easy adjustability... but i'm pleased, almost like a new toy

Mike T., Swampeast MO

Single pane windows (in good shape and weatherstripped) with good storms are at least as effective as modern multi-glazed windows. While solid masonary has little insulation value, good caulking around doors/windows generally keeps infiltration low compared to older frame homes.

Ceilings are the greatest area of heat loss so "decent" attic insulation helps as well.

Since you have some time before replacement with winter ahead...

Do you have iron radiators? If not what type of emitters?

If you do have iron rads there are some fairly simple experiments you can conduct to get a very good idea of not only your true heat loss, but the required boiler temperature and reset curve.

Brad White_9

Not much one can add

to what was said already. (Has that ever stopped me?

Knowing your emitter output versus heat loss is a great place to start right after your heat loss is known. Brick is as you say, what it is. Not much you can do. Infiltration becomes your single biggest wild-card at this point.

A direct-vent mod-con serving all your heating and DHW needs will enable you to seal the chimney (major exfiltrator 24/7) for at least what you are now using it for.

If you have regular wood burning fireplaces, a Lymance chimney top damper is a great thing we just had installed. What a difference.

As stated by so many, size it close to the heat loss to preserve modulation range.

[Deleted User]

The nice thing about brick walls....

is that you don't have to discern the difference in area of window versus wall, because they both have an R value of around 2, so it makes the math easy.

Now, infiltration is the BIG killer of heating requirements, and if you can avoid guessing, you are that much better off. Find a weatherization person in your area that can do a blower door test and eliminate the guess work.

Personally, I find ModCons do best, working up hill against a steady load. If sized to the theroetical load, in my experience, it typically results in a 50% overkill scenario, but I would never consider reducing the size of a non mod con boiler by 50%. Even in my own home :-) Too darned heavy to be dragging up and down the stairs.

Snowed in in Colorado

ME

[Deleted User]

THanks and more questions

Thanks for all the responses so far. Your responses did generate a few questions on my part:

1) What are emitters? They were referenced a few times.
2) I have been recording gas usage off my meter for the past year that I have been in the house...mostly just so the gas bill is not a complete shock. I guess I'll need to correlate those readings to the temperature going forward, correct?

Finally, there were a couple of questions asked to me...I have natural gas and cast iron radiators...no TRVs, but that's another discussion and I'm still researching it.

I will take the time to monitor the gas usage and temp after the holidays...no time before that. I will then run the formulas and get back to this thread.

Thanks for all the help!

Mike T., Swampeast MO

Your cast iron radiators are your emitters.

Constantin

... and usually really good news.

Many cast iron radiators were sized to meet heat losses with the windows open. If you keep the windows closed and have done any kind of insulation work, chances are that your standing iron will be able to be a great emitter for a mod-con. A room-by-room heat loss calculation will tell the whole story.

Guy_6

oversize

My feeling on the Mod/Con boilers has always been that they have a huge advantage in the oversizing dilemna: The boiler will only operate at a rate commeasurate with the load. If your existing boiler (conventional) is 110K, and you replace it with a 110K Mod/Con, the new boiler will automatically operate on the actual system load. If that is 77K, the boiler will run at 77K.
The other huge advantage is with indirect tanks: My heat load is a mere 78K, but with MY kids, if I want a hot shower, I need to supply my indirect with considerably more BTUs. A Mod/Con can flex like that, giving optimum performance for the HW, while offering optimum efficiency for my heating.
Another note on YOUR system: Those cast iron radiators are great with a Mod/Con boiler-they can handle a fairly wide range of supply temps, making outdoor reset very worthwhile, AND, because of the volume of water that they hold, they will make the boiler condense more, thus be more efficient. With many Mod/Cons, you also dont have to worry about thermally shocking the boiler with cold return water- they love it.

All that before coffee...

[Deleted User]

Broad averages...

are tought to track and estimate. The time weighted value takes a big toll on accuracy.

A simple over night test is more accurate, or if you want to invest some money in equipment, you can diagnose it to the Nth degree, like I do...

http://www.contractormag.com/articles/column.cfm?columnid=417

I'm getting ready to come out with a pump evalution program whereby the pump (existing) pressure differential, and consequently operating flow rates, can be recoreded in real time and compared to real time fuel consumption and real time degree hour heating demand exposure.

ME, Wilo, Bill Shady and Siggy will be putting our heads together to come up with the data collection/evaluation requirements. I have the PERFECT over engineered, underdesigned candidate in which to perform the eval process. I smell some new WILO light commercial pumps being installed in the very near future....Possibly VS.

Stand by.

ME

Mike T., Swampeast MO

If by "actual usage" you mean "actual loss", it's VERY difficult to do that in the real world because the "heater efficiency" of a conventional boiler is anything but constant--particularly when driving a system with high volume and fairly high mass like standing iron rads and [likely] gravity-sized piping.

I sincerely tried to do just that and the best I could come up with was an abysmal--dare I call it--true annual efficincy utilization of 47% of the energy consumed.

Granted this was with a conventional boiler driving a fully reset, constantly circulating system with TRVs (with constant system bypass adjusted for "efficiency") but without any consideration for low temperature protection. Not a particularly common sort of system in the U.S., but actually easier to make calculations/estimates as you can fairly assume that room temperature is constant as long as TRVs aren't adjusted. I also won't try to claim that this was the most efficient way to operate a conventional boiler but do know that the addition of TRVs, constant circulation and outdoor reset did reduce my fuel consumption considerably compared to the previous so-called "control" system where the single thermostat operated both the circulator and burner.

What I do know however is that with a mod-con (Vitodens) and a closely-adjusted heating curve that I'm getting darned close to 50% reduction in fuel consumption compared to the conventional boiler operating the exact same system in the exact same manner.

So...if my initial estimate of fuel utilization was anywhere near correct then I'm close to if not exceeding the official Vitodens AFUE of 95.2%. I suspect that any mod-con using the same heating curve would achieve similar but then, as now, I'd be VERY reluctant to use an aluminum heat exchanger in such a system.