Homeowner Heat Loss and ODR Curve Learning

Dan_G

I'm am a homeowner trying to find, figure out and learn how to properly dial in my Munchkin 80M R1's ODR curve. I'm quite aware of the proper way or I should say, type of heat loss calculations that should be done (Manual J), but I don't know exactly how to do that.



I understand how to program my boiler, but am unsure how or if I can tweak it without doing a comprehensive heat loss calculation.



My Outdoor sensor is mounted on the North side of my house and is unobstructed.



It is currently set for the default curve:



WWSD at 68*F

Design Supply water temp at minimum design outside temp: 201*F

Differential of 30*F

Min Outside Design Temp of 5*F

Max Outside Design Temp of 68*F

Design supply water temp at Maximum outside temp 95*F



My heating design temp for my area is 13*F with Medium swings of 16-25*F.



We don't usually even turn the heat on until the outside temp hits 60*F or below. We currently have the t-stat set to 66*F.



The house is furnished with a mix of old and new SlantFin tube/fin HWBB. I will be changing out one of the units (~12,000 BTU output at 180*F/~9400 BTU output @ 160*F) in the living room to a flat panel radiator. Total house square footage is 2400 sq. ft.



Any insight would be appreciated!

Jean-David Beyer

What I have and how I got there.

My Cape Cod house is in Monmouth County, New Jersey where the design temperature is 14F. I did a heat loss calculation of my house three different ways.

1.) I looked at the existing oil-fired boiler that was going to be replaced and it had a 1/2 gallon per hour nozzle in it, so that comes to about 70,000 BTU/hour. Since that boiler always provided enough heat, that was the upper limit of what size I needed. The installing heating company recommended a 105,000 BTU/hour unit "to be on the safe side." This is the worst way to size a system, I think.

2.) The manufacturer of the mod-con gas fired boiler has some worksheets to calculate heat loss, but they are not very accurate. I got something like 40,000 BTU/hour if I remember correctly.

3.) I used the Slant/Fin program that used to be available and got something like 32,000 BTU/hour. This was probably the most accurate calculation.



Calculations 2 and 3 are all somewhat too high because they assume design temperature of 0F and around here it is 14F. I did not bother fixing that because the smallest boiler I could get in the product line was 80,000 BTU/hour input (about 72,000 BTU/hour output).



I figured out what temperatures at the high end and low end I would need from the size of the baseboard units in the upstairs. When this system was installed, I increased the baseboard in each of the two rooms from 3 feet to 14 feet units so I could run lower water temperatures and get more condensing.



For downstairs, where the slab at grade contains copper tubing (1/2 inch) I had to guess the length and spacing of the tubing.



In any case, when I used the results of the calculations, it did not work all that well. I did not make any calculation errors, but estimating the transmission of my walls was problematical because I could not see inside them. They are pretty well insulated, and I had just installed Marvin gas-filled double windows, so they should be pretty good. But there are some air leaks, and I do not know how much.



I imagine with the heat loss calculations more accurate than mine (mine failed because that data I put in was not accurate enough) you could compute the water temperatures at each end of the temperature range you need to match the supply water temperatures to the heat loss of the building using simple high school algebra. But it sure did not work for me. (By the way, my boiler supports a separate reset curve for each of the two heating zones, and a fixed (adjustable) temperature to drive the indirect hot water heater.)



 In any case, the water temperatures were by and large too high for optimum condensing, and the thermostat in the baseboard area shut down pretty quickly. So I lowered the temperatures at both ends of the upstairs reset curve, The big problem is that I could not (of course) adjust the outdoor temperatures. So on a pretty warm day, I set the end of the reset curve down until it worked OK, but as it got colder outside, that was off, so I adjusted that, etc. It took about two years of tweaking to get it to match the heat loss of the house so that the supply water temperatures were as low as I could get and the thermostat would call for heat for many hours at a time. Ideally, if I could get it right, I would not need a thermostat at all, but that assumes the wind outside blew at constant speed because outdoor reset cannot cope with variable air infiltration.



Downstairs in the radiant zone was even worse because it takes about 24 hours after adjusting a temperature until the system recovers from the change. And by then the outdoor temperature has changed again, adding to the confusion.



When all is said and done, I have, in the same format as yours, given first:



Your system

WWSD at 68*F

Design Supply water temp at minimum design outside temp: 201*F

Differential of 30*F

Min Outside Design Temp of 5*F

Max Outside Design Temp of 68*F

Design supply water temp at Maximum outside temp 95*F



My system

WWSD at 70F (manufacturer's default: I never changed it.) Both zones.



Baseboard zone:

Design supply water temp at minimum outside temp 136F

Differential +7F, -8F (total 15F)

Minimum outside temp of 6F

Max outside temp of 50F (if it gets hotter outside, it holds the setting until WWSD is reached)

Design supply water temp at maximum outside temp of 110F. The reason I have it this high is to reduce short-cycling. Also why I have such large differential.



Radiant Slab zone

Design water temp at minimum outside temp 120F

Differential +5F, -5F (total 10F)

Minimum outside temp of 6F

Max outside temp of 50F (if it gets hotter outside, it holds the setting until WWSD is reached)

Design supply water temp at maximum outside temp of 76F.



If you lived next door to me, I would suggest starting with these settings (same builder, same basic house design) and tweak them from there. It would take you perhaps two days if the outside temperature would be very warm on one of the days, and extremely cold on the other day. Unfortunately, those two days might not even be in the same month, but over a year or two you can get close enough.

Zman

Heat loss

You can run a heat loss and get a pretty good idea of you temps.

You can also just use trial and error since you live in the house.

The max I would set the boiler  to is 180. You can probably go to 150-160.

Baseboard heaters don't seem to do much below about 110.

The goal is to get nice long boiler cycles at the lowest possible return temps.

Carl

Dan_G

tweaked

Jean-David,



Thanks for the insightful information, especially since you're one county over from me. How did you know you had good results and did you notice any difference in your utility bill?



Carl, I went ahead and make some tweaks to my system last night, but am unsure on how to measure the results. Here are the changes:



WWSD at 66*F

Design Supply water temp at minimum design outside temp: 160*F (was 201)



Differential of 30*F (no change)



Min Outside Design Temp of 13*F (was 5) - I set it to the Design Degree low as per the Manual J specs for my area. Not sure if it should actually be lower, thereby making the curve more shallow.



Max Outside Design Temp of 60*F (was 68)



Design supply water temp at Maximum outside temp 95*F (no change)



I also have a default setting of 68*F for the Lowest Temp for Central Heating (adjustable range is 32*F - 194*F). I'm not sure what this parameter represents.



I'm wondering if I should go through each room, measuring the tube/fin units to determine their estimated BTU output (I think they're all SlantFin units). I have 3 zones. One for two rooms upstairs, one for all of downstairs, and then one for a utility room that I have shut off (valves to the circuit are closed and T-stat is off).

Jean-David Beyer

did you notice any difference in your utility bill?

Sure I noticed a difference in my utility bill, but that means very little. Very little because I took out a 60 year-old oil burning boiler when heating oil was almost $4/gallon and installed a brand new W-M Ultra-3 modulating condensing boiler burning natural gas at about $1.30/Therm  and for all I know the number of degree-days was different from one year to another. Also, I replaced an electric hot water heater (with a very slow leak) with an indirect running off the new boiler.



"How did you know you had good results"



That depends on what you and I mean by good results. For one thing, since I have a radiant slab at grade heating the downstairs, I want to run that slab at a constant temperature. Otherwise the temperature overshoots by about 5F  after the thermostat us satisfied, and undershoots several degrees when the thermostat calls for heat. That is what it did with the old boiler with no outdoor reset. With the outdoor reset, I tried to supply the water to the slab at just high enough temperature to make up for the heat loss, and it took a while to determine those temperatures and adjust the reset curve accordingly. A side benefit of doing this is that the supply temperature is often about 76F (until it goes below 50F outside) so I get lots of condensing.



One way to judge is to notice when the thermostat is calling for heat and if it is set up "right" it will call for heat for a long time. For my slab, it sometimes calls for heat for 18 straight hours.



For my baseboard zone, I cannot do it as tight because the boiler is too oversized and will not modulate down far enough. To get around the rapid cycling, I put a minimum of 110F into the baseboards and let it get up to 136F on really cold days (it never got that cold around here since I got the boiler about 5 years ago). Otherwise I might let it get down to 90F or something on warm days. But the idea is to get the supply temperatures there as low as possible, consistent with getting enough heat. Upstairs, it may run 4 hours or so sometimes, but I have never gotten as much as 12 hours. If that zone had more heat load, perhaps I could get it to run longer, but I do not choose to leave the windows open there in winter just to make the boiler run more efficiently. I guess if I felt like burning that much gas, I would put snow melting under my short driveway and my 180 feet of sidewalk. But I am too cheap for that.



"Differential of 30*F (no change)"



Mine is 15F, and the default from the manufacturer is 10F. The greater the differential, the greater the expansion and contraction noise from the baseboards and associated piping. I would lower my differential to 10F, but then the boiler cycles too much when heating only that zone. My boiler will modulate down to only 16,000 BTU/hour and that zone can take only about 6.250 BTU/hour on the coldest days, so I had to do something.

Dan_G

Differential?

How do you determine whether or not the differential should be adjusted?



Mine is default at 30*F.

Dan_G

Buzzing pipes

I'm occasionally getting a low frequency buzzing in my pipes since changing the heating curve. Any ideas how to dial this out?

Jean-David Beyer

How do you determine whether or not the differential should be adjusted?

Several different ways.



1.) Default was +|- 5F from the manufacturer. I kept the default values for everything unless I had a good reason to change them.



2.) The greater the differential, the more expansion and contraction noise in the baseboard zone.



3.) I did increase it some in my baseboard zone because that zone is way too small for the modulation of the boiler. I can make it go down to 20% firing rate, but no lower. And since that zone consumes only 6250 BTU/hour when it is 0F outside, and since design temperature is 14F, it cycles too much. The lowest the boiler will go is to 16,000 BTU/hour. I did several things to help that, but now I have it run +7|-8F as part of what is needed to reduce the cycling rate to an acceptable amount. It cycles a little too much on very warm days outside, but that does not happen much.

Dan_G

differential change

Jean-David, thanks for the information. I'm still trying to learn the rational behind the adjustments you've made and how they might apply to my system.



ok, I think I might reduce my differential from the 30* default, to 25*, and/or bring up the Design supply water temp at Maximum outside temp from 95*F to 110*F (SlantFin's lowest temp rating/output on their ratings chart: - since I've heard that anything below this or 105*F and the output isn't much effective anyway.



http://www.slantfin.com/images/stories/Technical-Literature/ratings_fineline30_r.pdf



Any thoughts on this tactic?