Gas-Fired Condensing Boiler Question: Please Help!

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Old 12-02-07, 02:12 PM
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Thumbs up Gas-Fired Condensing Boiler Question: Please Help!

Hello all, and thanks in advance for your help!

We live in a 2,000 square foot, 80-year-old brick house in Pennsylvania. The house is 2 floors, plus a basement.

About 3 weeks ago, we replaced our boiler (hot water system, gas) with a gas-fired, Energy-Star, condensing boiler (Buderus #GB142).

Although the local gas company had recommended a larger Buderus model, the company we ended up purchasing from was firm about installing the smaller Buderus GB142, which they said they were certain was large enough for our home.

Now, however, we have some doubts, and any outside opinions would be greatly appreciated.

When the outside temperature is in the 30s or 40s, the Buderus raises the temperature of the house by about 2 degrees an hour. When it’s colder, it raises the temperature of the house by about 1 degree an hour.

We have a programmable thermostat—when we turn the temperature down at night, the boiler can’t seem to catch up in the morning.

Our questions are:

Were we given too small a unit for the house? Or is the new boiler heating at the right speed?

I’m told there’s a maximum set point the boiler will heat the water to—could mine be set too low? If it’s raised—could that help the boiler to heat the house faster?


Thanks in advance to everyone! And a warm, cozy winter to all of you.

Signed,
Chilly in PA
 
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Old 12-02-07, 02:22 PM
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I'm gonna make a quick guess and say that some of the settings on the BC-10 need to be 'fine tuned' for your house.

They did install the AM-10 and outdoor sensor, right ?

What type of heat emitters do you have ? fin-tube baseboard ? radiators ? radiant in-floor ?
 
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Old 12-02-07, 02:59 PM
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I`m with NJ Trooper on this,

sounds to me like the settings are off.

Hope he left the manual with you!
 
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Old 12-02-07, 03:00 PM
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PAchilly... I really doubt that your boiler is too small, but somewhere along the line someone whould have done a heatloss calculation to size the boiler. The software for doing this is available for free at SlantFin dot com and is called Heatloss Explorer.

Does you boiler have outdoor reset (ODR)? If it did then the recovery should be at about the same rate regardless of outdoor temps. What ODR does is base the boiler water's target temp on the outdoor temperature. Like NJ asked, does it have an outdoor sensor?

How many degrees of setback are you in the evening and how fast do you actually want the boiler to recover?

The faster a boiler can recover for setback, the more wasteful it is actually running... that's the paradox. ODR can help keep the recovery rate more constant, some t-stats can adopt when to start recovery as well.

Do you set back for sleeping comfort or economy?
 
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Old 12-11-07, 05:49 AM
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Thumbs up Gas-Fired Condensing Boiler Question--Continued

Hello All!

First off, thanks so much to everyone for your detailed replies to our question--your responses were so helpful!

It helps to know that you are all in agreement that our boiler is NOT too small.

In answer to your questions:

Yes, the folks who installed our Buderus boiler did put an outdoor sensor in, and it does have outdoor reset. But it sounds like what WHO is saying is that, if my boiler is heating more slowly on cold days, the ODR is either working improperly or is set too low. Do I understand that right?

Regarding the programmable thermostat, we are setting the temp back at night and while we are at work 5 degrees for economy only. We are no longer raising the temp in the mornings, only for evenings (after a week or so of programming the t-stat to raise the house temp for mornings, we realized that the boiler wasn't catching up in time and stopped doing that).

Our house has large cast-iron radiators.

Again, thanks to all of you for your help, and happy holidays!

Best,
PAChilly
 
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Old 12-11-07, 06:54 AM
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The ODR follows a curve (or sloped line) that raises the supply water temperature as the outdoor temperature gets colder. That curve can be tweaked a lot to fine tune how the house behaves. Best to spend some time reading up on how it all works.

http://buderus.net/OurProducts/GasFi...8/Default.aspx

That is a fine boiler, and cast iron radiators are a great match for it. The whole ODR philosophy takes a bit of learning about and getting used to, but it's great for comfort and efficiency.

The odds of the boiler being too small are about a billion to one. Getting your curve set up can take a couple weeks, especially as your thermostats learn as well.
 
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Old 12-11-07, 07:47 AM
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could I ask what is the whole model # on the 142/?

You may not have a large enough boiler for that old drafty brick home.

I'd think that the local gas utility company would not give you bad advice on boiler sizing if they did the estimate and energy workup.
 
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Old 12-11-07, 08:50 AM
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PAchilly, beware of the paradox of setback.

Sure enough... setting back saves fuel and the more you set back the more fuel you save but the recovery has to be carefully considered. Keep in mind however that with you already having an extremely efficient heating solution there really isn't much left of the pie to save.

In order for the house to recover its indoor temperature after a setback period, your boiler's water temperature for any particular outdoor temperature needs to be "quite" a bit higher than it otherwise would (at least for the recovery phase). If your return water temperature is in the 130° range then that could be enough to force you out of condensing and there goes several percent of your efficiency.

The other aspect is that different materials lose and recover heat at different rates. If your body is giving up heat to these higher mass colder materials/surfaces/areas then you'll end up feeling chilled resulting in a higher indoor temperature setting on the t-stat just to feel as comfortable as you would with a more constant temperature and there goes another bit of your efficiency.

I think a good way of looking at setback is thinking about a car driving in hilly terrain. When you set back, it's like going down the hill... a free ride, but you need to apply lots of energy going back up the other side. In fact you should use about as much energy reheating the house as you saved by letting the temperatures drop. Where the savings come from is if the temperature or the road flattens out in between, and this would be much more significant over a period of days instead of hours. At the lower temp, the heatloss is reduced, so that's where the savings are. During the setback the indoor temperature should plateau and a good sign would actually be the the boiler actually runs a bit while the temperature is set back. If it doesn't you are basically just doing a roller coaster.

The tighter a house is, the higher the mass of materials inside, the less setback you can use.

And with a condensing boiler, the less the setback the more efficient you can tune the outdoor reset curve.

Why not try putting the indoor temp on hold for a while? Your house will get very comfortable but before you get too spoiled start lowering the fixed indoor temperature to where it had a similar comfort level to what you have now.

Once you know what the magic indoor temperature number is to keep you comfortable while being efficient write it down. After that, if you want you can start playing with the setback, you'll have to keep in mind that you can't play with that magic indoor occupied temperature level. Adjusting the setback so that you don't have to compensate by higher indoor temperatures will mean that you will have to use less setback and probably initiate recovery sooner.

After that, then you could start tightening the outdoor reset curve if you know how, although the tighter the curve gets, the less recovery capacity you will have to play with for doing any setback settings.

It's a juggling act for sure...
 
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Old 12-11-07, 09:29 AM
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The smallest GB142 (the 24) has a flatout output of 72.5kBTU/hr. In a 2000sf house, it could theoretically (assuming the radiation is sufficient) satisfy a heat loss of 37.6 BTU/hr/sf. That's a pretty large number. Probably in the upper range of a rather leaky 80 year old brick home. But that level of heat loss should occur maybe a couple hours each year.

Do you have a heat loss calculation? I assume at least one or two (energy co and contractor) wre done to recommend a boiler. If not, where is the house and/or do you know what the local design outdoor temperature is? How is the insulation in the home, and how leaky are the windows?

This early in the heating season, my guess is that the reset curve needs a bit of tuning, and the thermostats (especially if they are the educable kind like Honeywell "adaptive recovery" models) need some time to figure out the new pattern of heating and cooling.

Example: my ODR system sets back about 2F overnight. It takes about 3 hours to recover those 2 degrees, so I had to start my recovery period a couple hours earlier than with my old non-ODR system in order to be at the desired temp by the time we are up and about. And we used to setback more than 2F with our old system, too. But over the course of a heating season, the ODR is saving anywhere from 12-23% per month compared to the old system. (And mine is not a super-efficient boiler like the GB.)

As Who so eloquently analogized, some of the real benefits of reset (especially with a condensing, high efficiency boiler) require a bit of rethinking. In many cases, it is probably more energy efficient to minimize or eliminate the setback. The EnergyStar and similar rules of thumb for setback are not based on high-efficiency modulating/condensing boilers.
 
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Old 12-11-07, 10:02 AM
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when he said 80 year old brick constuction.. balloon came to mind with no insulation in the walls and a no no to try to insulate. some of these old brick houses have very little insulation in the attics/ceilings either.

On top of that if you have (original?) single pane window framing with weighted sash pockets surrounding the windows the heat loss really tallies up even if you have storm windows.

Plus a full basement with maybe 80 year old piping that's another 1000sq ft
 
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