boiler efficiency puzzle


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Old 12-04-09, 10:53 AM
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boiler efficiency puzzle

I recently had an energy audit done of my home. During the energy audit they tested my gas boiler (Burnham Holiday 172,000 input, 140,000 output, 106,000 IBR - cast iron approx. 1960 vintage) and found it operating at 81% combustion efficiency. This was a dissappointment to me because I hoped to replace it and reap much efficiency savings. If it's 80% efficient, an new one won't be much better.

The boiler was actually consuming only 1.5 CF of gas per minute or about 90,000 BTH/hr because I have turned the shut-off valve down some to reduce flow. The boiler is grossly oversized as, on the coldest day of winter the home requires only 12 CCF total to keep it warm, or about 50,000 BTU/hr.

Here's the riddle - while turning up the aquastat to test efficiencies at different water temperature, the boiler topped out at 160F (although the aquastat was set at 180F). This means that the hydro-air blower was pulling heat off as quickly as the boiler could produce it and therefore the boiler was in equilibrium and no longer heating up.

I looked up the rated BTU output of the air handler and found for a water temp of 160F, a water flow of 6gpm (generous considering it's a Taco 007 pumping through maybe 100' of 3/4" copper piping round trip from boiler in the basement to air handler in attic) the rated BTU output from the air handler was approx 50k but/hr. The boiler should have been putting out 72k btu at 80% efficiency (.8x90k btu). What happened to the extra 22k btu? Could it possibly be lost from the boiler jacket (which isn't noticibly warm) or in the 100' of mostly insulated copper pipe? It seems unlikely.

Is it possible that the combustion testing didn't give an accurate read?
 
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Old 12-04-09, 11:30 AM
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Hi ds and welcome to the forum,
My state doesn't allow me to play with the HVAC and we are only recently getting into NG so I see very little, but I do have some comments.

The combustion efficiency is just the burning process and everything goes downhill from there. With your heat exchange unit in an attic and 50 feet away, there is bound to be some loss in those insulated pipes. They operate under a much higher temperature difference and are probably insulated with one inch of foam rubber. The significant point is, are any of those pipes in an unconditioned space, ie the cold attic where the lost heat is truly lost.

Then there is the heat exchanger itself and any air ducts that may be associated with it. If any of those are in that attic, they should have huge amounts of insulation applied and be totally air sealed.

The bottom line is, that 81% efficiency is probably closer to 50%, when you consider the heat delivered to your home relative to the fuel burned. So, would a new high efficiency be worth the expense? When combined with an overall effort to air seal and insulate your home, a new MUCH smaller boiler and an improved delivery system, you could see a big difference in your heating costs. But, you have to look at it as a package. You can't replace just one component and expect all of those savings.

I'm also concerned about your comment of using the supply valve to attempt to reduce the combustion rate, but will let the HVAC guys handle that.

Bud
 
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Old 12-04-09, 02:46 PM
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Originally Posted by dstrutto View Post
Is it possible that the combustion testing didn't give an accurate read?
Yes. But if there are losses in the piping, they don't show up, either way, in the combustion efficiency test.

To check for piping losses, measure the temperature at the boiler supply and the the inlet to the air handler.

Does the 3/4" copper pipe go through the inside of the house, even if inside interior walls? Then, I'd expect heat losses would still wind up in the house.

I think there is plenty of uncertainty in your assumed 6 gpm flow rate. You'd need to compute the pressure head loss curve for your piping system and see where it intersects the performance curve for the Taco 007: Taco Pumps Performance Curves. Taco Hydronic Heating Circulators for Hydronic Heat Systems

Another way to estimate the flow rate is to measure the temp difference between the boiler supply and return. Knowing the Btu/hr output of the boiler - since you know the fuel consumption and can assume 80% efficiency - you can back into the flow rate.

80% efficiency for your boiler sounds reasonable to me.

Like Bud, I'm not keen on your idea of throttling the gas shutoff valve to reduce the burner ouput. If you're lucky, it wouldn't work anyway, since the fuel valve itself has a regulator to supply a reasonably constant gas pressure (e.g., 3.5" w.c.) within a range of line pressures. But, more of a concern is that your burner is calibrated to perform at its rated output. You're trying to achieve a modulating boiler without buying a modulating boiler. Open the shutoff valve all the way.
 

Last edited by Mike Speed 30; 12-04-09 at 04:06 PM.
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Old 12-04-09, 06:49 PM
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As the others said, the shutoff valve game is a no-no. I'd say even a potentially dangerous no-no. Even if it's been that way for years.

Hydro-air systems can benefit from higher efficiency equipment, but in a retrofit where you aren't replacing/resizing the hydro-air coil to run at lower water temperatures, the biggest benefit will likely come from appropriate sizing, as our friend rbeck so often states.

If it is new boiler time for this system, I'd suggest a properly-sized, ~50k output cast iron boiler if you keep the existing coil. Pending a real heat loss calculation, of course. There are several options (a Burnham Series 2 202/203 being an example of the most simple, but at least get the electronic ignition not the standing pilot version) that would probably realize significant savings over what you have now, even though the AFUE is fairly low by today's standards.
 
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Old 12-04-09, 07:44 PM
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Originally Posted by xiphias View Post
If it is new boiler time for this system, I'd suggest a properly-sized, ~50k output cast iron boiler if you keep the existing coil. Pending a real heat loss calculation, of course. There are several options (a Burnham Series 2 202/203 being an example of the most simple, but at least get the electronic ignition not the standing pilot version) that would probably realize significant savings over what you have now, even though the AFUE is fairly low by today's standards.
The Burnham ES2 has an AFUE of 85% and has an optional ODR add on card.
 
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Old 12-05-09, 01:45 AM
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AFUE is a small part of efficiency

First you should google AFUE and Brookhaven. You will learn that two boilers both within 2% of each other in AFUE can have 30 to 40% difference in actual fuel consumption.

The highest users are boilers that sit hot all the time, needed or not. That is why the European modulating systems such as Buderus and Viessman use less fuel than the old fashioned always hot American Boilers.

Another type of system are cold start boilers that sit at room temp when not needed.
Some boilers are a combination of modulating and cold start depending on the demand, or outdoor temperature.
 
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Old 12-05-09, 06:03 AM
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That is why the European modulating systems such as Buderus and Viessman use less fuel than the old fashioned always hot American Boilers.
First let's clear up the fact that old fashioned American boilers stay hot. The boilers have been designed to be cold started for many many years now. I am going to estimate 30 -40 years. In gas they were almost always cold start unless made to maintain in the field. In the last 30 years the only time boilers would maintain temp is if they were ordered that way, had a domestic hot water coil in the boiler or changed in the field. Even to day there are many boilers ordered to maintain temp or changed to in the field. I talked to a guy the other day that is maintaining the temp on a Burnham LE. This is a low water volume oil boiler. That probably adds 20%+ to the fuel bill on this application due to needless short cycling. For the last 20+ years everything is designed to operate cold start but that choice is not always the opinion in the field.
I agree the boiler of choice here would be a small cast iron boiler and also agree the 6 gpm may not be correct. It may be greater. If it was less the boiler would hit limit. If you increase flow you can get more btu's out of the fan coil or the fan may be moving too much air through the coil
I to agree with opening the manual gas valve open. You may be increasing the CO levels you are living with. Living with low level CO is dangerous and can cause long term irreversible physical damages.
 
 

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