Ultraefficient furnace not so efficient
Last Post 03 Dec 2013 03:27 PM by sailawayrb. 15 Replies.
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AlexRUser is Offline
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02 Dec 2013 12:03 PM
I just had a Lochinvar Knight bolier put in our house, 97% efficiency model. After 1 month of use, the gas use dropped 30% from last year (not bad), but electricity use jumped 35%...not good! Far as I can tell the reason is the circulators are running much more than they were before (we have 3 zones so there are 4 circulators on the unit). First thing I did was drop all the circulators from high to medium. This will help a bit and probably better matches our system anyway. However the real problem is that this 'smart' boiler does not produce as much heat when the temperature is not too cold out, so as a result it just takes longer to heat up the house...and the circulators run longer. I found a way (I think) to turn up the power a bit (I increased he max setpoint from 160 to 180). Hopefully this will help some. Anyone else have any ideas?
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02 Dec 2013 01:17 PM
Ummmm....30% of the hydrocarbon consumption of a gas boiler represents quite a bit more energy than 35% of the electrical usage of same boiler. Are you saying that the outdoor reset causes the temperature to go lower and that makes the circulator run more?

Did you know that 100% efficiency isn't the theoretical maximum? Heat pumps can and do deliver 400% efficiency and higher.
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02 Dec 2013 01:37 PM
"Ummmm....30% of the hydrocarbon consumption of a gas boiler represents quite a bit more energy than 35% of the electrical usage of same boiler"
Unfortunately not true...the electricity bill increase almost entirely wiped out my gas bill savings. This is clearly not supposed to happen.

"Are you saying that the outdoor reset causes the temperature to go lower and that makes the circulator run more?"
Yes, exactly.
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02 Dec 2013 02:55 PM
Unfortunately not true...the electricity bill increase almost entirely wiped out my gas bill savings
Then, you've got a design problem. A small Knight can burn 50,000 BTU/hr, worth about 60 cents. To make them equivalent, you'd have be consuming, like 4kW an hour.
Dana1User is Online
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02 Dec 2013 03:40 PM
First, let's call a boiler a boiler- a furnace in most HVAC dialects refers to an air handler with an internal burner (or electric resistant elements) for hot-air heating, and a hydronic boiler is something that heats liquids, distributing the heat with pumped liquid (usually water, or mostly-water), or converting the liquid to vapor ("steam" heating.)

Overpumping is a common hydronic system design error. Most residential systems will deliver the heat just fine with ~1 gpm on the radiation, but it wouldn't be unusual in a system with a separate circulator per zone to find them cranking away at 6 gpm or more.

If your system is built around four identical 3-speeds, cut the radiation loops to "low", and start tracking the delta-T on the boiler under different conditions before dropping the primary loop flow. Most boilers are just fine running between 15-40F delta-Ts. If it's running 40F with it set to mid-speed you may need to crank it back up to "high", but if it's running a delta of 25F you'll probably be able to back off.


The boiler's pumping requirements are usually higher than that, but in a system plumbed primary/secondary you can set the flows accordingly. Some vendors will allow you to pump-direct if you do the math and keep the max delta-T/min-flow on the heat exchanger in the right range under all operating conditions, others will flat out void the warranty if not operated primary/secondary, which does add to the pumping power.

For a bunch of money a single programmable/adjustable ECM drive pump and zone-valves would cut the pumping power on the radiation loop(s) by quite a bit. If the primary loop pump is sized correctly there isn't a whole lot to be gained by changing it, but if it's being over pumped it would.

If you're going to try to cut power use by monkeying with the outdoor reset curve, raise LOW temp end, not the max temp end, since it's the long runs when it's not very cold out that have the least favorable power use per BTU-delivered ratio. But the higher you make the low end, the lower the average combustion efficiency. Above ~135F or so at the low end you may not get any condensing at all, depending on your delta-Ts at low output temp operation. To get any significant condensing the water entering the boiler has to be under 125F.

ICFHybrid: Discussing efficiency in those terms isn't really addressing the issue. While technically correct on a BTU basis, AlexR's interest is clearly a cost of operation issue. The per-BTU cost of electricity is typically 3x or more than of the source-fuel BTU cost of natural gas, but it varies quite a bit by local market. (In my neighborhood it takes a heat pump that averages 350% to have about the same operating cost as a well designed condensing gas hydronic system. Mini-splits can only come close to parity here but can beat hydronic gas on operating cost in some areas.)
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02 Dec 2013 03:55 PM
so as a result it just takes longer to heat up the house...and the circulators run longer.


That's the approximate goal of outdoor reset - to run the pumps continuously. This increases the boiler efficiency. Ideally the pumps would also slow down in mild weather.

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02 Dec 2013 04:12 PM
Mini-splits can only come close to parity here but can beat hydronic gas on operating cost in some areas.)
I'm aware of that, when electricity or gas is the source of the BTUs. In this case, it's the circulating alone which is apparently burning a significant portion of the kWs. My entire 12 zone system is using 25 to 50 watts to circulate. That's essentially negligible compared to the heat generation needs.
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02 Dec 2013 04:40 PM
Posted By ICFHybrid on 02 Dec 2013 04:12 PM
Mini-splits can only come close to parity here but can beat hydronic gas on operating cost in some areas.)
I'm aware of that, when electricity or gas is the source of the BTUs. In this case, it's the circulating alone which is apparently burning a significant portion of the kWs. My entire 12 zone system is using 25 to 50 watts to circulate. That's essentially negligible compared to the heat generation needs.

That's not as negligible from a pumping dollars per BTUs-delivered point of view, but still not bad. I'm going to guess it's probably not 13 Taco-010s, right? :-) 

Keeping a 12 zone system from short-cycling usually requires high-mass radiation- I assume it's all concrete slab?

At about 100 watts per pump for "typical" hydronic heating systems it can turn into measurable power use. Homes using condensing boilers but distributing the heat with a kilowatt air handler with a hydronic coil has an even less favorable kwh/btu ratio than pumped-radiation-only systems.  A good hydronic system designer will factor in the pumping costs to avoid the "gee, saving a lot on gas, but my power use has a big uptick" situation.

But most hydronic systems  that started out with an oversized cast iron beast and a presumption of 180F output water aren't optimally pumped for a mod-con, and  this may be Alex's situation. You can't blame the boiler for the deficiencies of how the system is pumped, but with 3-speeds you can still make some adjustments rather than going straight for the pump-swap/re-design.
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02 Dec 2013 06:08 PM
does not produce as much heat when the temperature is not too cold out

That is the plan for outdoor reset. Lower temps longer runs tend to make for more efficiencies.

What is controlling your set up? Some controllers on OR need to be assigned a number representing the type of emitter it will be using. If the controller is set for high mass and your set up is low mass, it will not be sending out the correct water temps.


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02 Dec 2013 07:19 PM
Thanks for this great reply. I will set everything to low and see what happens. For now the delta t is under 10 degrees, but we'll see what happens when it gets really cold out. As for this: "If you're going to try to cut power use by monkeying with the outdoor reset curve, raise LOW temp end, not the max temp end, since it's the long runs when it's not very cold out that have the least favorable power use per BTU-delivered ratio. But the higher you make the low end, the lower the average combustion efficiency. Above ~135F or so at the low end you may not get any condensing at all, depending on your delta-Ts at low output temp operation. To get any significant condensing the water entering the boiler has to be under 125F. " Any idea how to do this? For the Knight, the user has only 3 settings (plus 1 how water setting) they can change , and I can't exactly figure out what they correspond to.
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02 Dec 2013 09:36 PM
I'm going to guess it's probably not 13 Taco-010s, right? :-)
They are a couple of Grundfos Alphas. What ridiculously fine pieces of equipment they are. If they were brunettes I'd probably be in love with them.

I assume it's all concrete slab?
All finest kind concrete. 'Ceptin the drypack in the second floor bathrooms.

ETA:  Might as well do the current performance on the Daikin Altherma unit.  That's all that is running for heat right now.
Here is what the system is doing currently with temps in mid 20's at night to mid 30's in the day.

Utility consumption on the meter, 14 day averaged: 52 kWh/day
LESS
340 watts of lighting, left on 24/7 (8 kWh)
Hot Tub (10kWh)
Circulators (1 kWh)
Control Boxes, Security, etc (3 kWh)
8 + 10 + 1 + 3 = 22 kWh/day for other things
TOTAL: 52 kWh - 22 kWh = 30 kWh going to heat

30 kWh X 3,412 BTU = 102,360 BTU/24hrs = 4,265 BTU/hr utilized by the Altherma indoor and outdoor boxes.

Hydronic System run parameters

7 gpm X 8.34 lbs/gal X 60 min X 5.5F (dT) = 19,265 BTU/hr outgoing to radiant slab
joe.amiUser is Offline
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03 Dec 2013 08:23 AM
Alex,
Did you change anything else around the same time? What do you do for domestic hot water?
Joe Hardin www.amicontracting.com We Dig Comfort! www.doityourselfgeothermal.com Dig Your Own Comfort!
Dana1User is Online
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03 Dec 2013 11:10 AM
Posted By AlexR on 02 Dec 2013 07:19 PM
Thanks for this great reply. I will set everything to low and see what happens. For now the delta t is under 10 degrees, but we'll see what happens when it gets really cold out. As for this: "If you're going to try to cut power use by monkeying with the outdoor reset curve, raise LOW temp end, not the max temp end, since it's the long runs when it's not very cold out that have the least favorable power use per BTU-delivered ratio. But the higher you make the low end, the lower the average combustion efficiency. Above ~135F or so at the low end you may not get any condensing at all, depending on your delta-Ts at low output temp operation. To get any significant condensing the water entering the boiler has to be under 125F. " Any idea how to do this? For the Knight, the user has only 3 settings (plus 1 how water setting) they can change , and I can't exactly figure out what they correspond to.
A sub-10F delta at the boiler implies a high pumping rate on the primary loop. Dial the primary pump back, keep an eye on it, but it's probably still not going to grow to anything boiler-stressing under any operating conditions.

Assuming you have the outdoor temperature sensor installed, start reading how to set up the outdoor reset curve starting on P18 of the manual.  The adjustment for the min-output temp is on P19:

Minimum SH set point

When the outdoor temperature rises to or above its
maximum setting (F2) the water temperature will be at this
point (FIG. 1). This parameter can be changed by the
installer by accessing parameter
F4. The temperature range of this parameter is 0°F to 190°F.
The default value is 70°F.


So, if you bump the temp at the warm-temp end of the  outdoor reset ramp to say, 110F it'll still get pretty good condensing efficiency, but shorter pumping times. As long as the burn times are longer than three minutes and fewer than 5 burns per hour you won't be trashing the boiler by over-cycling/short-cycling it.  Then, start dialing down the max temp number at the cold-outdoor temp of the curve so that it can condense as much as possible as the outdoor temps run cooler.  As long as the output temp is still high enough to heat the house at the 99% outside design temp with the radiation you have, the lower you can set that temperature, the better the combustion efficiency.

BTW: What type, and how much radiation do you have on each zone?
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03 Dec 2013 12:06 PM
Is there an optimum delta-T to aim for for condensing boilers in general?
Dana1User is Online
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03 Dec 2013 01:18 PM
Posted By seiyafan on 03 Dec 2013 12:06 PM
Is there an optimum delta-T to aim for for condensing boilers in general?

The delta-T will vary with output temp, radiation type, and flow.  With pumps operating at a constant speed, and radiation that changes output with temperature, when you vary the boiler output temp (as with outdoor reset control) the delta-T will vary, since the radiation puts out more heat (= bigger temperature drop) at a high temperature than when it is at a low temperature.  If you design the flows for 20-25F deltas at the mid-point of your outdoor reset range, most boilers will operate effectively across the whole range without blowing over it's specified max delta-T, even well below the design temp, and still have some delta left during mild weather operation. 

It's not all set in stone, but a sub-10F delta-T should only happen at very low boiler output temps, temps at which the radiation can't shed much heat.  The amount and type of radiation make a difference- this is a system design problem with many variables to factor in- calculations that are best done you you even specify the boiler, let alone install it.
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03 Dec 2013 03:27 PM
Interesting, this is actually about boilers and hydronic radiant heating systems. I initially lost interest when I read the title and given where it was posted… Yeah, “efficiency” should never be used in any discussion involving heat pumps...which move or convey heat and do NOT convert energy. Efficiency is used for energy conversion processes and has a very specific thermodynamic definition (i.e., the ratio of energy output per energy input) and maximum efficiency is always 100%. When discussing energy conveyance processes such as heat pumps, one should only use the term “coefficient of performance (COP)” which more appropriately describes the ratio of heat conveyance per work input which should always well exceed 300% for heat pumps. A 100% COP would mean you are expending the same amount of energy to move the energy that you want...an energy gain and benefit of nothing...  Air-source heat pumps typically have COPs in the 300-400% range. Ground-source heat pumps typically have COPs in 300-500% range. The real question you have to answer for each situation is whether a 90% efficient heating system is a better choice than a 450% COP heating system... Otherwise, Dana has this one well covered like always.
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