Heat Loss Doesnt match Heat Load

Mark Eatherton

Is the infiltration factor confirmed with a blower door test, or just guesstimated?

If confirmed, sized the boiler to the calculated load, (am assuming a modulating/condensing heat source here) then its a good thing, because it means your heat source will be able to satisfy the heating needs at a significantly lower operating temperature, which means $AVING$!!

If non condensing, and you are comfortable with the numbers, then size it to the load. It will probably still short cycle during less than design conditions, but such is the nature of the beast.

Also, if it is an older home, the designers probably assumed most of the windows would be open to avoid :Vicious Air", a common malady in the good old days.

ME

Paul MacDougall

Heat Loss doesnt match Heat Load

Im currently working on a proposal for a gravity conversion boiler job. The heat loss for the home was just over 60,000 btu, but the radiation came in close to 130,000 btu. Does that make sense because it is a gravit system. Is there a correction I should take into consideration when sizing the boiler

The Steam Whisperer (Formerly Boilerpro)

Use the heat load....

It seems many higher end homes were overradiated to start out with, probably for the same reasons today, better efficiency. If the envelope has been tightened with insulation, windows weatherstripped or replaced, etc., The radiators will naturally become oversized. My own wood frame two story home at about 2800 sq ft and built in 1906 had about 120,000 btu/hr of radiation (at 150btu/sq ft EDR) and now has a peak heat load of about 45,000 btu/hr. 60,000 btu/hr is a pretty healthy heat load, so if your numbers are right go for the small boiler.

Boilerpro

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scrook_3

run cooler

use a condensing boiler and run the radiation cooler reducing the radiation's effective size. should be fantastic on fuel

Jim Erhardt_3

Perhaps a matter of semantics...

...but in a hot water system, heat loss = heat load. In other words, you can have 500 feet of baseboard in the system, but if the heat loss is only 40,000 BTUs, that is all of the "load" the boiler will ever see (once the pick-up load is covered). All that the over sized radiation will do is to allow meeting the heat load with a lower supply temperature.

Think of it this way - if the water temp flowing through the baseboard is 120 degrees, and the air temperature entering the bottom of the baseboard is 120 degrees, how many BTUs per foot will the baseboard be producing?

Modcons + oversized radiation = efficiency happiness.

mel rowe

How does heat loss calculation relate to actual gas consumption? For example, if I take the acual gas consumption for the coldest month we've had for 25 years, and calculate the gas usage/hour, it comes out to about 88,000 btu/hr. So the net heat output would be about 88,000 times the .82 efficiency,or about 72,000 btu/hr. So does this amount to the same as a heat loss calculation?

Jim Erhardt_3

"How does heat loss calculation relate to actual gas consumption?"

Theoretically, they should be the same - at any given time, the energy content of the fuel used X system efficiency = heat loss.

scrook_2

but...

that's 72,000 BTU/hr over a range of outdoor temperatures, perhaps 1/2 day or a couple 1/2 days in 30 of which may have been at/below design, (1/2's as it's usually warmer during the day than at night) so it understates design temperature heat loss somewhat, but it gives you an idea of the losses.

If you had data for coldest DAY or couple days in the season, vs. coldest month, (or percent run time during a coldest night) you'd be much closer.

Too, if you knew how much the average temperature for the coldest month was above the design temperature for each of the 25 years (the data should be available/extractable) you could probably extrapolate to design day conditions.

e.g. Boston MA (data taken @ Logan Int'l airport NWS observatory in recent decades (post 1930's or 1940's?), inland would be a little cooler), average daily *min* temp is 24.2°F (1971-2000) and 2009 daily min 24.9°F
mean (average) Feb temp was 31.5°F (1971-2000) and mean Feb 2009 temp was 32.9°F 2009
Lowest in Feb 2009 +8°F, Boston design temperature is 0°F, and minimum record since 1872 is -18°F in 1934. all data from National Weather Service

So 70° in - 0°F out = 70°, and 70° - 31.5° = 38.5° so your monthly AVERAGE heatloss is perhaps 38.5/70 or 51% of your design day heatloss! If you used coldest day only, then your loss that day perhaps would be (70-8)/70 or perhaps 89% of design day. These are by no means hard numbers, but give an idea.

Do a manual J heatloss calculation and compare to coldest single night data (means you have to read your gas meter or measure your oil or LP use for the single day or night period though) for a reality check.

mel rowe

This question of boiler capacity based on applied load vs. heat loss ( especially after installing thermal windows, foam in the walls, and 8 inches of foam in the attic of this old house), has interested me for some time, in prep. for eventual boiler replacement. With a calculated EDR of 612, and 82% efficiency, it calculated that I only needed about 180,000 BTU/ hr. gross capacity. If I added in a pickup factor of .3, then it increased to about 240,000 BTU.
Since my current boiler is 300,000 you can understand why I have such an interest.
This winter I picked up a good idea on the Wall of using a digital counter to keep track of boiler run time. So I just happen to have actual gas flow and boiler run time data for the coldest night we have had in 25 years. It went down to minus 13 degrees. I have previously downfired the boiler from 5.2 cubic feet/ hour to 3.3. With this gas flow and the recorded run time of 73% that night, it calculates out to 138,000 gross BTU/hour. So this is a good estimate of the actual heat loss during the coldest time I can imagine.

For this house, after all the improvements in insulation, that 138,000 heat loss indicates the need for a lot less boiler capacity than a calculation based on the 612 EDR, which would be 180,000 BTU, or with a .3 pickup factor, it would be about 240,000 BTU.

Now the real question is should I really plan on buying the 240,000 boiler or 180,000 or should I have the guts to buy a boiler to match the actual heat loss data shown above, which would indicate 140,000 should be adequate? Thank goodness I don't have to make that decision right now, so I will continue to ponder it, while I continue to look for opinions from the experts here.

Jamie Hall

Just keep in mind

that sizing the boiler to the heat loss works for hot water. If you are talking steam -- it isn't really clear -- you have to size the boiler to the EDR. Only way you can downsize steam is to remove some radiation either completely or by turning off -- and leaving off -- the radiator valves.

Jim Erhardt_3

A heat loss calculation...

...is just that - a caluclation! Still, for hot-water systems, it's far better than using some rule-of-thumb that will nearly always GROSSLY OVERSIZE the boiler.

Your method is as good as it gets - measuring the actual fuel (BTUs) used at design conditions. Heat loss calculations usually don't figure in things like heat gain from appliances, lighting, people, etc. Infiltration remains a guessitmate too.

So the answer is - 140,000 BTUs net IBR. Based on the starting point of a 300k BTU boiler and the improvements you have made, it sounds right too.

BTW, we just moved into a house in NH. The previous owners had a new boiler installed a few years ago. My heat loss calculation comes out to around 55,000 BTUs. This only confirms that most heating contractors don't know how to properly size a replacement boiler - they installed a 144,000 BTU Burham V8.

Yes, steam boilers are sized to the radiation (just like the fan/coil and condenser must be sized to each other in an AC system).

mel rowe

Sorry I forgot to mention that mine is a steam boiler.

Jim Erhardt_3

Boiler Sizing!

Hot water boilers are sized to heat loss.

Steam boilers are sized to radiation.

mel rowe

I understand that the conventional approach would be to size to the applied load. Are you saying that I should size a replacement to the rad EDR plus the pickup factor? In this case it would be either 240,000 if you include the pickup factor, or 180,000 if only the rad EDR is considered. But in this case with all the additional insulation I've added I obviously have more than required radiator capacity. I'm not going to go and change all my rads, but why do I need to size the capacity to the EDR of all the old rads? Why wouldn't a boiler capacity sized to the actual ( not just estimated, but actual) heat loss data for the coldest night in 25 years work for me and be much more efficient because of running much longer heating cycles? I could understand the need to be more conservative than that, if I only had available to me estimated heat loss calculations, because those estimations probably have a lot of inherent variability, and you wouldn't want a homeowner calling back in the middle of the night complaining of lack of heat. But in this case I have actual data from the coldest night we're likely to see, so why is it a seemingly inviolable rule to size to the applied load? I just don't yet understand the jeopardy to sizing to the actual heat loss data in cases like this.

Paul Pollets

Another Approach

I look both at the heat loss and the connected radiator load. If the connected load is 130K BTu's and the actual heat loss is 60K, the best approach is to put TRV's on each cast iron radiator (a must!!) and install a condensing boiler. I'm particular to Viessmann, and would use the smallest Vitodens, the 6/24, which modulates between 22K and 90K, and have room to spare with your loads. You would need a "low loss header" or hydraulic separator to make the flowrates work. Deleting the TRV's from the radiators creates difficult balancing issues. TRV's tame the beast.

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Larry C_13

Steam boiler sizing to heat load.

When the boiler starts with cold piping, the steam will be condensing in the piping and not even reach the radiators. Until the pipes heat up, the radiators will sit there cold. In a worst case scenario, the boiler will just be trying to heat up the piping and will never even reach the radiators. The next worst case will be running for a long time before steam begins to reach the radiators. Instead of a 2 or 3 minute runtime before the steam reaches the radiators, you might be looking at a 15-30 minutes before the rads start putting out heat.

It is analogous to sweating a 4 inch copper pipe with a small propane torch instead of using a larger torch to heat the pipe up quicker. Eventually you can make it work, but it will take a lot more time AND more fuel to do the same job.

Larry C

mel rowe

Thanks Larry. I can understand that it might take a bit longer for the system to heat up the first time. But with much longer heating cycles and shorter down cycles that you would have with lower capacity boiler, the pipes and rads don't cool down very much during the relatively short down cycle, and thus heat back up a lot quicker and with possibly less overall total BTU's than with a larger capacity boiler that inherently has longer down cycles.

From a cold start on design, that first cycle will take a while to get pipes up to temp. and get the whole system up to the temperature at which it is emitting and losing the same amount of BTU's as being provided by the boiler.

If the BTU capacity were the same as the actual heat loss of the house, then I'm not yet sure why it would be better or necessary for the boiler capacity to equal the original load, when the actual heat loss (and effective applied load)of the house has changed drastically due to all the changes in insulation. Under these conditions the radiators don't have to get completely hot to provide the radiation necessary to match the actual heat loss. Of course the system would have to be well balanced to accomodate the lower boiler output.

Jamie Hall

At some point

which depends on the system configuration, a steam boiler which is undersized relative to the connected load will be completely unable to supply steam; it will just simmer. All of the steam generated will be lost to the piping and, if you are really lucky, the closest few radiators.

The problem is that steam always comes to the radiators -- or whatever is condensing -- at the same temperature: about 215 or so. And the condenser (for instance, the radiator) will always produce the same amount of heat (within very broad limits) which depends solely on the size (the EDR) of the radiator and has absolutely nothing to do with the heat loss of the space within which it is located. Therefore, with a steam system, the boiler must be matched in size to the condensing capability of the system -- the EDR. The only control available is either turning off the boiler or the radiator. This is quite different from a hot water system, in which it is possible to vary the temperature of the water, and thus the amount of heat radiated by each radiator; ideally with a hot water system one could (and good ones do) vary the water temperature to match the load and run the boiler only long enough to keep the water temperature where you want it.

But steam, no. Wandering around on the Wall here will find you a number of examples of folks who have come to grief because of undersized boilers leading to some or all of the radiators simply not working at all.

Which is not to say a steam system can't be sized to match the heat loss of the house. It can. But remember: I said steam SYSTEM, not steam boiler. The system is just that: boiler, near boiler piping, supply mains, radiation, returns, and vents.

I hope this helps.

mel rowe

Thanks to all for your patience reading and responding to this discussion. Jamie, I think the key item in all this is the assumption that for a given radiator and its EDR, that it has to be fed the amount of steam that matches that EDR, and nothing less, or it won't work. Clearly the rad has to see that much steam if it is going to emit that much radiation. However, it seems to me that a radiator will emit basically whatever amount of heat energy it is fed. If you feed it less, it will emit less. If conditions have changed from the original, such that you don't need the full EDR amount, why feed it more than needed? Why not base capacity on actual system heat consumption data, if you have it?

I noticed a while back that BoilerPro seems to have similar thoughts. Here is a quote.


"In addition, and this goes against popular opinion and standard industry practice, I do not believe steam boilers need to be matched to the radiation. A properly balanced system that does not use setback, can operate very efficiently and evenly with a boiler smaller than the radiation. I have seen a number of systems, including one pipe steam systems, that have been operating for at least a decade this way very well. However, don't "undersize" the boiler unless balance the system properly."

Perhaps this accounts for the fact that I was able to successfully operate my steam boiler at 190,000 gross BTU/hr and running only 73% of the time on the coldest day we'll likely ever see, when my 612 total EDR and a .3 pickup factor would dictate 233,000 gross BTU. With the improved insulation and based on the actual system heat loss I calculated with actual gas flow and boiler run time, it suggests that I should be able to run my boiler system at an input level of 138,000 BTU, running 100%. It would be very interesting to see if this could be confirmed, but don't think I can further downfire my current boiler.

The Steam Whisperer (Formerly Boilerpro)

Some ???, thoughts and resources....

Do you have two pipe or one pipe steam. If two pipe it is very easy to change the condensing capacity of the radiators to the current heating conditions... either close down the valves until the room doesn't overheat or add inlet orifices and control the pressure tightly. Google Frank Gifford and you should come across his article on this topic. From what I rememeber, he has done hundreds of these upgrades in New York City for a number of years with huge success. You can also Tunstall Assoc., they have info and parts for this upgrade. There are all sorts of orifice vapor systems out there that run with less than full radiation capacity boilers.
One pipe steam is more of a challenge since you usually try to balance at the air vent (there are one pipe steam valves that can be throttled, however), so I tend to keep the boilers larger on these systems,often close to just the radiation and actual piping heating loss and not with a full pick up factor. Just think about how a typical steam system works, it almost never completely fills the radiators on a heating cycle. So if you balance the steam flow so that it favors the mains (big main vents AND SMALL RADIATOR VENTS), it will fill the mains before starting to provide much heat in the first radiators.

Boilerpro

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mel rowe

My system is one pipe steam. The mains are all very well vented. As I mentioned insulation has been seriously upgraded for the entire house. I have one inch of molded fiberglass on all the pipes, up to the risers for the rads. So, in view of all this, I have tried below to clearly show the alternatives as I see them for my eventual boiler replacement.


Conventional Approach
Option A Option B Option C Option D Option E

190,000BTU 138,000BTU 180,000 233,000 269,000

Based on Option A Sized to 612 EDR 612 EDR
actual use adjusted EDR of plus .3 plus .5
during minus to 100% 612 pickup pickup
13 nite. run time factor factor
Boiler on
73% of time.
Maintained
stat set pt.
of 70 with
no prob.


So, among these options, based on the conventional approach (options C,D, and E) and also based on actual results which were satisfactory (option A),which one would you recommend. Which approach might give you a competitive advantage when bidding on a replacement boiler in an old house, and assure the customer will be happy with his gas bills, and yet will never call and complain about the lack of heat on the coldest night?

Please give me your choice.

mel rowe

My system is one pipe steam. The mains are all very well vented and the system is pretty well balanced. As I mentioned, insulation has been seriously upgraded for the entire house. I also have one inch of molded fiberglass on all the pipes, up to the risers for the rads. So, in view of all this, I have tried below to clearly show the alternatives as I see them for my eventual boiler replacement.


Conventional Approach Option A--boiler sized to EDR of rads only (612) would be 179,000 BTU gross at .82 eff.

Conventional Approach Option B--Boiler sized to EDR of rads (612) plus .3 pickup factor would be 233,000 BTU gross

Conventional Approach Option C--boiler sized to EDR of rads (612) plus .5 pickup factor would be 269,000 BTU gross


Option D--boiler sized to same as current boiler, which has been downfired to achieve better overall result. The actual gas usage was determined from coldest night every likely in this area (minus 13 degrees). The boiler ran for 73% of total time (recorded with digital counter) and all rads were getting hot enough to achieve stat temperature of 70 degrees. This capacity would be 189,000 BTU.

Option E--boiler sized to actual results as in option D, but adjusted to assume letting the boiler ran 100% of the time during this coldest night. This capacity would be
138,000 BTU.





Clearly there is a huge range of options here, with a big impact on initial installation costs, as well as ongoing operating costs and customer satisfaction concerns.

So, among these options, based on the conventional approach (options A, B and C) and also based on actual results (options D and E),which one would you recommend? Which approach might give you a competitive advantage when bidding on a replacement boiler in an old house, and still assure the customer will be happy with his gas bills, and yet will never call and complain about the lack of heat on the coldest night?

Please give me your recommended option.

scrook_2

"Option D--boiler sized to same as current boiler, which has been downfired to achieve better overall result. The actual gas usage was determined from coldest night every likely in this area (minus 13 degrees). The boiler ran for 73% of total time (recorded with digital counter) and all rads were getting hot enough to achieve stat temperature of 70 degrees. This capacity would be 189,000 BTU."

If the boiler ran 73% of the time it only implies it is sized for the radiation plus at least a 1/0.73 = 1.37 pickup factor (it could be more as the radiation is probably at least somewhat oversized for design temp, so some of th at 1-0.73 was probably due to t-stat vs. the p-trol being satisfied.

Assuming they measured the radiation EDR and used the standard steam P/U factor of 1.33, then installed the next bigger available size boiler,(since they are only available in finite output steps -- well, if oil, you may get down to 5% steps or so by changing nozzles in the range of the particular boiler, a gas boiler the steps might be closer 20% -- The important thing isn't to match to the last BTU but to get as close as reasonably practical.

On replacement, size w/ 1.33 P/U, maybe a little less, unless the mains are unusually short or long. If the radiations grossly oversized you could replace some w/ smaller, but you don't want to get caught waiting for the mains to heat up.

Nicest would be a High-Low fired steam boiler w/ a pair a vaporstat (low-high control) & p-trol (high-off control) or pair of vaporstats, either gas or oil, in residential sizes -- 100% & 70% output (in other words high fire of 1/0.70 = 1.43X of low) would probably be enough turndown, so you'd heat water then mains quick then cycle high to low to maintain pressure, vs on-off.

Now, to get someone to build such a burner -- (scale down a commercial dual fire?)

clammy

i agree with scrook

Gotta agree with scrook the guys knows what he is talking about and i personally would go with sizing to edr plus pick up factor 1.33 but if alot of envouple improvments have been made i would drop down the pick up factor this also depends on the outputs of your replacment boiler .Also i would do a over sized drop header and keep the exiting velocity down real low between 10 to 15 fps and keep it dry .you can't argue with scrook he's been to belo horizonte maybe even central market peace and good luck clammy

Steamhead (in transit)

The pick-up factor of 33%

on a steam boiler is already figured into the Net rating. So unless you have a lot more piping than usual or something else that would increase the pickup factor, there's no need to add more.

Adding more most times just results in an oversized boiler.

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mel rowe

Steamhead, For the boilers I used all gross numbers, with no factor built in. I took the different pickup factors into account in options B and C. So which option would you recommend?

Steamhead (in transit)

I'd use Option A

with the boiler sized to the amount of radiation by its Net rating. Then down-fire from there if you want to fine-tune it.

Options B and C would result in a boiler that's way too big for the system.

Options D and E might work, but wouldn't give you any headroom to adjust upward if needed.

So I'd go with A.

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TK03

I always use the sq ft number and never btu's. If the sq ft of steam matches the connected load (radiation) and not do add anything else has always worked for me. I make sure all mains are insulated and also near boiler piping.
It has worked for me for the last 30 years.
EDR radiation = Boiler Sq Ft steam rating. Manufacturers deduct 33%.