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Outdoor reset vs setback on single-stage boiler

user-5059898 | Posted in Mechanicals on

I’m trying to figure out what would be the more efficient thing to do in my situation. I’ve recently heavily insulated the house and installed a radiant hydronic system (making sure to do a detailed load analysis and system design by the Sigenthauler hyrdonic book). While the radiant system and boiler loop are new, the boiler is a 20 year old cast-iron boiler that I want to nurse along for a few more years (mostly because of current budget).

With my new 4-way mixing valve, I have the option to run the radiant with outdoor reset varying water temperature. WIth my Ecobee smart thermostat, I have the option to do setback during work and sleeping hours. Last winter, before insulating and while using the hot-water baseboard setup, I was able to track the temperatures and found setback saved me approx 9% (average temperature was 62 – thermostat set at 50 at night and while gone, 68 while at home). My guess is that the newly insulated house would have a savings of maybe 4% with setback due to much less heat loss from insulation

The catch is that for the outdoor reset to work, the thermostat would be calling for heat most of the day. While the radiant system can run at low temps, the non-condensing boiler needs hot water and the boiler loop would be circulating and constantly giving of heat with 140-180F water. My gut says that the added heat loss from the boiler would offset any savings from the outdoor reset. The heat from the boiler is going into a spray foamed rubble foundation basement – so some might make it into the house but much will go down through the old uninsulated slab.

Obviously this will change when my budget can afford a new mod-con boiler.

Any thoughts?

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  1. user-2890856 | | #1

    Eric ,

    What 4 way valve are you using ? Where are you located ? What water heater are you using in the home ? Setback is not a good strategy for radiant , you will actually use more fuel doing this , depending on the install method used in a particular house .

    Your mixing valve should enable you to send low temps to the floors while also protecting the boiler . A cast iron boiler has a bit of mass within the water and cast iron . The boiler should be able to heat the home for quite awhile without firing considering the low temps required for the floors . You will suffer stack losses however .

    Set up properly you should only be using one circulator and the 4 way . i do however know that a few manufacturers still only show the 4 way valve being used with Primary / secondary piping . Unfortunately , if your installer / boiler tech did not know this he hasn't a good enough grasp on Hydronics to call himself either . The boiler loop need not have a circulator , the circ on the system side of the pump pulls hot from boiler and colder return water at the proper ratios . As long as the boiler is set up to fire when the average temp in the unit is >135* it will not condense and all will be fine . So , in summation , use the ODR and forget the setback if you have a floor with mass .

  2. user-2890856 | | #2

    I forgot to say . There are water heaters that are modulating / condensing that are made for exactly your type of situation . No need to spend 10K on a boiler and all kinds of stuff that the average HVAC man thinks you need because some salesman told him it is necessary . Siggy knows how to do it simply , I will tell you also that he has adopted newer better ways to do things than are within the pages of MHH 3rd Edition .

  3. Dana1 | | #3

    With the outdoor reset (or not), on the mixer (and not the boiler) you'll do better with ODR than with setback. But there's rarely enough thermal mass in the system for cast iron boilers to deliver long efficient burns, and setback approach isn't going to improve that aspect.

    It's common to need the thermal mass of a buffer tank to lengthen the minimum burn cycles and lower number of burns/hour (especially if the house is cut up into zones.)

    To maximize the burn times and get the most out of it's thermal mass you'll need to maximize the temperature differential between high & low limits on the boiler, no matter what what temp your radiation needs. If it's not getting at LEAST 5 minutes per burn (10 is somewhat better) it won't be hitting very near it's AFUE tested numbers, and if it's doing something like 8-10 burns or more an hour or more (fewer than 5 is better, 3 is great) it'll be slipping over an efficiency cliff.

  4. charlie_sullivan | | #4

    You are right that the setback savings will be less with the new insulation. As your insulation gets better, the % savings with setback get lower and lower.

    Is your floor heat under wood floor or tubing embedded in concrete? Setback is more feasible with the former than the latter.

  5. user-5059898 | | #5

    I removed the check valve and disconnected from power what was the primary boiler loop pump. After several hours, the radiant is clearly still functioning based on the supply temp to the manifold. Looking at the instruction for the Taco i-series 4-way valve with ODR, it shows a primary pump - but that seems like a clear waste of energy in this case.

    Richard - I have been working off the 3rd edition of MHH. Is their a good online source of Siggy's newer ideas. That would be interesting to see. I was assuming that in a few years I could have a much better sized mod-con boiler (current is 120K at perhaps 75% efficiency, detailed house load at 99% worst case looks more like 35K). Would something like the Polaris Water Heater (96% efficient and could replace the aging domestic water heater) make more sense?

    Unfortunately, the aquastat has a fixed differential of 15 degrees.

    I am located in Vermont with a design temp of about -5F. The radiant is a mix. The downstairs is staple-up under 70% wood floors and 30% thin-set tile with R19 above a heated warmish basement, the upstairs is plates above the subfloor with R13 insulation between floors.

    The boiler fires for 5-6 min. at a time and approx 5 times per hour.

  6. user-5059898 | | #6

    I meant to say - Is their a good online source of Siggy's newer ideas?

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