Baxi optimization

Big Ed

Hydro air will need to run on a higher curve then say a radiator. The sweet spot would be setting the curve to run the hydo air on the coldest day of the year with no recovery from a set back...Then cut the output of the baseboard by closing the air dampers on enclosers to match there cycles.... Or install a boiler bypass on baseboard to temper it water temperature ...Either way you need to set curve for its largest load which is hydro air.....

Steve Goldstein

Baxi optimization

OK, my new system is installed and running. I replaced a 23-yr-old Utica boiler (125k BTU) and a crappy forced hot air furnace with a Baxi Luna HT modulating/condensing boiler and an air handler to handle the FHA zone. I also kept my MegaStor DHW tank in service. So I have 5 copper fin baseboard zones, the air handler hot air zone, and the MegaStor zone all connected to the Baxi.

There is an outdoor sensor connected to the system.

The Baxi has a dial on the front that allows me to control the water temp of the system, so I can control that. It's also possible to put the system into a configuration mode that allows control of the curve, but I haven't dared mess with that yet.

I can already tell that the air handler isn't as responsive to temperature changes as the FHA furnace was. This isn't necessarily a bad thing--it's possible that the FHA furnace was grossly oversized, and that the air handler is correctly sized. I'm simply commenting that the FHA furnace responded to temperature changes much more rapidly than the air handler. I've got the air handler's fan speed at maximum right now (which is most similar to the fan speed of the old FHA furnace), but it still responds more slowly.

What's the best way to optimize the system for efficiency? I'm aware that the copper fin baseboards and air handler will force me to set the system's water temperature higher than for optimal efficiency, but I'd like to find the "sweet spot" between efficiency and comfort. Now that the system isn't as oversized as it was with the Utica Boiler and FHA furnace, I can't get away with the deep thermostat set-backs I used to do.

So I'm trying to figure out how to systematically tweek the configuration of the system. I look forward to your advice.

Thanks,

Steve

Mike T., Swampeast MO

As long as you have reasonable insulation and infiltration control, I'd try to keep daily setback (most particularly in the hydro-air zone) to an absolute minimum.

I realize that "deep, daily setback saves" has been drilled in people's heads--particularly by makers of thermostats. The claims made [seem] to be based on poorly insulated, drafty old homes with hideously oversized forced air furnaces...

Deep daily room temperature setback runs contrary to the efficiency goals of condensing/modulating boilers where "slow and steady" is the desire.

With less setback the hydro-air will run more frequently, but very likely the required supply temperature will be lower. In that vein, the highest blower speed may not be ideal. While it will transfer the most heat, the discharge temp will likely be lower than if the fan were operating at a slower speed. As long as a lower speed gives sufficent heat and throw from the registers, such might be more efficient and comfortable if less daily setback (and a correspondingly lower supply temperature) are used.

Steve Goldstein

Not yet

Thanks for the info.

I don't have reasonable insulation and infiltration yet. That's something I plan to work on over the next several months, so that it's much improved by next fall. But currently, I have a drafty old home which used to have a hideously oversized heating system (consisting of both a forced air furnace and boiler).

So now I have the condensing/modulating unit w/ hydro-air serving one of the zones. I will reduce (or even eliminate?) the temperature setback on the hydro-air zone. I will try bringing the air handler's fan speed from Hi back to Medium.

Then I will reduce the boiler's water temp. How far can I reduce the boiler's water temp? Should I be expected to adjust the water temp for different seasons/outdoor temps, or do I base my decision on design temp (zero-degree outside temp), then leave the water temp there during the shoulder seasons?

Can I continue to do some setback on the copper fin zones? Those zones behave more like they did with the old boiler, and therefore seem more capable of recovering from a setback than the hydro-air zone is.

Brad White_9

I think the simplest way to conceive all of this, Steve

is to set your highest water temperature to suit your hydro-air zone. That seems to be the most "emotionally needy", right? That will set the boiler, you can always mix-down for reset elsewhere.

For now I would treat it all as one system using the hydro-air zone as the sentinel. See how far you can drop the HWS temperature and what the net result in the space is. I would submit to you that you will not have nearly the reset capability relative to comfort.

The other zones especially copper/fin zones will behave similarly. They can take setback more than most of us have believed. The hydro-air zone is less forgiving (blowing cool air takes away some of the good graces that a radiant system would not think of imposing upon you.

I agree with Mike T. re: not taking too deep a setback with ModCon boilers, but it all really gets down to you trying it, taking good notes and seeing how long it takes you to recover. (Repeat all this after you tighten up the house.)

Random thoughts, randomly offered, apparently

Mike T., Swampeast MO

Am not familiar with the Baxi, but does it not have a reset curve? Presuming it does, most outdoor reset curves are a simple combination of ratio (how much the target moves with changing temperature) and shift (how much the target moves at all temperatures).

Ratio is usually expressed by a simple number like 0.5, 1.0 or 1.5. 0.5 means the target moves ½° up with each 1° that outside temp falls; 1.0 means that the target moves 1° up with each 1° fall; 1.5 means the target moves 1.5° up with each 1° fall.

As someone else mentioned your hydro air will define your curve as it's almost guaranteed to need the highest supply temp.

I've never worked with hydro air, so keep that in mind. Would appreciate any corrections/suggestions...

To adjust your curve, I'd suggest starting in moderate weather (say highs in the 40sF). Find the lowest supply temp that still produces acceptably warm air when the hydro-air unit is running (this is where lowering the fan speed may help).

Then make a guess at the supply temp needed in the coldest weather, then compute ratio and shift.

For example: say you find that 130°F supply produces acceptably warm air in 45° weather. Then say you need 170°F supply at 0° outside. 45 - 0 = 45 degrees of change outdoors. 170° - 130° = 40 degrees of change in supply temperature. 45 / 40 = 1.125 reset ratio (round to 1.1).

For the shift in this circumstance: The reset curve usually originates at the desired room temperature, say 68°F but this may be different for different boilers--check your manual! Again, say that it's 45°F outside and 68°F is the curve origin. 68 - 45 = 23 degrees worth of ratio or 23 * 1.1 = 25.3° worth of ratio increase. 68° (the origin) + 25.3° = 93.3° target. BUT 93.3° isn't high enough--at 45° outside you want 130° supply. 130 - 93.3 = 36.7 so use 37°F of shift.

In moderate weather the hydro-air will probably cycle considerably. Why? Because the curve is most likely too high in such weather to keep the unit from blowing "cold air". (It would still be heated, but the velocity could make things drafty.)

As the weather gets colder, the hydro-air unit cycles should get longer and longer--ideally it will be running almost constantly in the coldest weather. If the unit still cycles frequently in colder weather decrease the reset ratio; if it "runs out of heat" increase the ratio.

Regarding setback: I'm not saying "never use setback", just tailor your setback to both your house and lifestyle. Zones in unoccupied space can bet set back as deeply as you want (without any freezing problems of course).

If your hydro-air serves sleeping rooms you may well want some night-time setback for comfort as you sleep--just keep such to a minimum--say 3-5°. I would also use a programmable thermostat and have the hydro zone recover EARLIER than any other zone in the house--that way it can receive the full output of the boiler (if needed).

Once you get your insulation/weatherization in reasonable condition, I truly believe you'll find that you'll need less setback--both for efficience and comfort. Some setback thermostats have a run timer (often used for filter replacement reminder). You can use this to determine the effectiveness of your setback amount. With "average cold" weather, reset the timer right before the setback period. Then check the timer BEFORE recovery begins. If there is zero or nearly zero run time then use less setback! Reduce setback 1-2° and time again. Keep reducing setback until you get some reasonable amount of run time (say 20-30 minutes) during the setback period.

Also, as previously mentioned, a mixing valve for your baseboard will likely increase both comfort and efficiency. Depending on your system and your definition of cost, cost for this will range from "modest" to "high". Depending on fuel cost, weather, your particular system and how you use that system, monetary payback will be highly variable--I'd guess from years to decades. Comfort payback would be immediate with proper adjustment...