calculate needed boiler size from actual usage?

R. Kalia

Questions about boiler sizing:

(a) I have four boiler quotes. Two ran around and counted windows and measured rooms. The other two ran around and counted radiators. Windows and room sizes I understand, but radiators??? If I installed more radiators in the same space, would my gas bill go up?

(b) The two radiator-counters came up with very large numbers around 200,000BTU, because we have an old house with lots of radiators. The two window-counters came up with somewhat smaller numbers, around 150,000BTU. But I come up with much less, using the current usage of my current boiler in this actual house rather than a theoretical house. I've been reading daily gas usage on the meter, and looking up the degree-days for that day from my gas company's web page; they are in lockstep with less than 10% variation. The other day it was close to 0F all day (which is the design temperature in my area), and the furnace consumed nearly 24 therms in a day. That's just under 1 therm per hour, or 100,000 BTU. Add some for record cold days and to cover my ****, subtract some for the increased efficiency of a new boiler, it's still around 100-120,000 BTU, not the 150-200,000BTU that the four contractors are recommending.

The boilers under consideration do come in sizes smaller than quoted but bigger than my own estimate. For example, instead of a Munchkin 199, I could get a Munchkin 140.

So who should I believe, myself or the professionals?

Uni R

Method C

If you know how much heat your boiler provides. Says it's 200,000BTUs and 70% efficient netting you 140,000BTUs, then you could also go by it's cycle time. If you keep your house at a normal indoor temperature and it is running 20 hours on the coldest day of the year, then your new boiler should also net that amount. If it runs for only 10 hours on the coldest day then you should probably get a boiler that nets half that amount. This method can provide a good sanity check for the other methods. In the perfect world, the perfect boiler should run 24 hours on the coldest day.


Method C assumes you don't already have a modulating boiler.

Greg Swob

I had a client who used his own heatloss calcs

by charting actual energy usage history. His figures were within 5%-6% of the calc's we came up with using Writesoft. We selected the model of equipment using what boiler sizes were available vs. what was needed. Modulating was not in the budget - too bad. Actual history will be a great guide - show the contractors this and ask how they arrived at their figures. Some will add a cushioning percentage 'just to make sure it's big enough'. As for the firms who measured radiators, if they have a copy of Dan's book EDR, they will be able to calculate the heat output of the radiators your home has at various water temps & add for uninsulated piping losses. As for the firms who did not care what radiation was available... ask why not? The radiation available is a major factor dictating replacement boiler size. We do both calc's. First, I want to know if there is enough radiation for the home's heat loss then I need to know if I've got too much/not enough boiler for the radiation available. Are rooms or zones uncomfortable - too hot or cooler than wanted? Are more baseboard or radiators needed in some places? Curious - albeit all were larger than you feel necessary, who was closer to your history? Those who measured radiators or those performing generic heat loss? Boilers have two ratings: AFUE (annual fuel utilization efficiency a.k.a. how many dollars do not float up the flue) and I=B=R rating or: "how much radiation can be served by this boiler?" Greg

R. Kalia

same info as gas consumption, right?

> If you keep your house at a normal

> indoor temperature and it is running 20 hours on

> the coldest day of the year, then your new boiler

> should also net that amount. If it runs for only

> 10 hours on the coldest day then you should

> probably get a boiler that nets half that amount.


As I said, we used 24 therms on what was essentially a design day. I didn't measure running time, but the current boiler has an input rating of 175,000BTU (I watched the meter dials for a few minutes, and indeed it consumes gas at roughly that rate) so it must have been running for 24/1.75 =~ 14 hrs that day.

However, of course I don't know how efficient the existing boiler is. The nameplate shows an output rating of 140,000 but I understand the AFUE is NOT 140/175 =~ 80%. It is thirty years old. So the efficiency is a wildcard in my calculation.

Cliff Brady

Same system here

Was a 175000/140000 Weil McLain, replaced it with a Peerless Pinnacle 140 and can't get it to eat over 80,000 btus/hr in a sub-design day. Your analysis is good but do you really want the 'extra just-in-case' capacity. Your current boiler efficiency is probably prety good on the coldest days. The shoulder seasons of Winter are where the efficiency of a new condensing boiler will pay off. I wouldn't go over 100,000 btus if I were you.

R. Kalia

yes, but I want a Munchkin

> I wouldn't go over 100,000 btus if I were you.


I bet you're right, but I want a Munchkin, and unfortunately an 80K would not be a good risk.

R. Kalia

f there is enough radiation, do I care how much?

>The radiation available is a major factor dictating
>replacement boiler size.

My question is, why? If there is excess radiation (the heat loss guys did ask about that, and there sure is), why should the boiler size be dictated by radiation available? Does it have to produce enough heat to satisfy the needs of the house, or the needs of the radiators?

>Curious - albeit all were larger than you feel necessary,
>who was closer to your history? Those who measured
>radiators or those performing generic heat loss?

Those who did heat loss. See original post, part (b), first 2 sentences.

Uni R

Yeah but simpler...

I'm a firm believer in KISS and Obbvious Adams for reality checks. And what if their therm numbers are off (who knows anything for sure) you have a secondary check.

Uni R

Excess Radiation?

No such thing... You have a better ability to take advantage of low temps and a condensing boiler.

R. Kalia

yes, of course

Yes, you're right. By 'excess' I meant there is more than needed to radiate the necessary heat with the circulator running continuously on a design day using 180F water.

How do I know this? Well, in my amateur way, I am using the usage reading of my thermostat. It doesn't tell me how long the boiler has been on, but it does tell me how long the circulator has been running. On a design day, it ran about 16 hours, i.e. less than 24.

Of course that is a good thing, and outdoor reset is a good thing, but I still want to know, why would one need a bigger boiler just because one has more radiators?

Uni R

You know too much now...

It may be too late for you... Be wary in restaurants. LOL

Doug_7

Gas use vs. degree days

I would forget the number of radiators and windows. If you have actual daily gas use vs. degree days information on an existing building you can calculate the boiler size requirement very accurately.

I may be a bit compulsive but I plotted a graph this winter using 100 daily points of daily gas use (I have a accurate digital gas meter) vs. degree days posted by the local utility. I plotted Degree days on horizontal axis and gas use on vertical axis.

Result - A dead straight line that starts in the lower left corner with just the domestic hot water use at zero degree days and runs as a straight line up to the maximum gas use on the coldest winter day.

Some small scatter in the data due to sunshine, wind, cold fronts coming through etc - but a very accurate measure of the heating requirement of the building.

Then just pick your coldest design day and correct for the efficiency of the old and new boiler and pick the next largest size new boiler.

I would be very confident in this method.

Doug_7

p.s. I should have mentioned that in the graph, the Degree days are in degrees celsius and building is over 50,000 sq ft..

Boilerpro_3

I am well versed in your Method of Sizing

And it produces very accurate number, with some considerations. One, as you said is boiler efficiency on the design day you checked. Another is Solar gain.... how sunny was it and how much glass faces the sun. Another is internal gains... Electrical usage adds heat as does showers, cooking, and simply just being...ie. breathing.
In my Opinion, and I beleve this is the opinion of the industry in general, sizing a hot water boiler to the size of the radiation is just plain Wrong. A boiler should be sized to the heat loss of the home, and on some occasions be increased in size to handle things like thermostat setbacks and recovery, and domestic water heating loads. Knowing radiation capacity and heat load will give you an idea of what your maximum system water temperature will be and allow proper sizing the compression tank and help you decide if the extra cost of condensing or condesning/ recuperative boilers is going to be paid back in energy savings. In your case, if you are using gas for heat, I would say the answer would be yes.

As to outdoor reset, if you have a single zone system with oversized gravity radiation, I wouldn't bother with reset. The sheer mass of the system resets the water temps, as long as the boiler is sized properly. On warm days the rads run cool, and on cold days the rads run hot just by the nature of the system so what install additional controls to do the same thing? If you have a multple zone system, or have thermostatic readiator valves, I'd recommend reset.

No matter what, if you decide on a simple 80 to 84% AFue non condensing boiler, be sure that low temperature protection is used for the boiler. At minimum, I use a thermostatic mixing valve on the return to the boiler with two pumps, one to circulate the boiler water and a second to circulate the system water (Primary / Secondary piping).

I hope this helps

Boilerpro

scrook_2

hot water vs steam

For hot water the boiler is sized by the house loss. Steam the boiler must supply enough steam to satisfy the condensing capacity of the radiation, even if oversized for the house loss, so is sized to the radiation instead.

R. Kalia

thanks!

> In my Opinion, and I

> believe this is the opinion of the industry in

> general, sizing a hot water boiler to the size of

> the radiation is just plain Wrong.


Thanks for confirming; I will avoid using the contractors who did that. Thanks also for all the other info in your post.