Prestige Solo 175 boiler not making temp with all zones on.

[skihawx]

Hello,
Found this website and looking for help with my Prestige 175 boiler. I recently moved into that house that had an older forced air oil burner and several propane wall units. The house is about 4200 sqft. I met a TT representative at a grand opening and he did a quick heat load and recommended the 175. I've learned this is not the best way to approach this but it is what it is. The house is very unusual with odd shaped rooms with tall ceilings, part of the living room is two stories with an overlook from the bedroom. I'm not sure this would fit the standard heat loss programs. The first year I installed the boiler, an indirect hot water heater, a radiant loop in the kitchen floor and one air handler to replace all the propane wall units. It worked fine all last season. This season I replaced the oil burner with another air handler. It seemed everything was working reasonably well and then something happened. Now when all three heating loops are on the boiler will not maintain the 140 temperature. It hovers around the 130's. When I force the boiler into the full fire mode it does come up sometimes but very slowly. I really think that it was maintaining temp until the beginning of December. My delta T's were are always around 15 to 20. I know the radiant loop is too small and may have to address this someday but I though I'd deal with the temp problem first. When I force it into the full fire mode the temp comes up very slowly but still no where near where I think the temperature should be. If I wanted a higher temperature it would never get there. In the year and a half it has never thrown a single error code. First I thought that it wasn't getting enough propane. But I can put five burners on the stove at full fire and one very low and do not see the flame height change when I turn on and off the gas to the boiler. The gas comes in on a 3/4 line with less than 1 foot from the regulator to a tee and then less than 4 or 5 feet from the tee to both the stove and boiler. Could something have happened to the controller?? It is definitely not a hard failure and does work but it does not seem to modulate. Flue temp is usually close but a little higher than the return water. The flue temp never gets more than 4 or 5 degrees above the boiler supply temp. It is set up P/S with only a Taco 007 boiler loop circulator. At the time this seemed like it would give me the 8 gpm required. There are two First Co FWA50-HW air hanlders with 1-1/4" piping and two Taco 008 circulators. I'm an electrical engineer and usually do everything around the house my self. But this hydronic heating is becoming a challenge. If the boiler was going over temp I'd think the main loop didn't have enough flow. But this is not the case. I guess a manometer would be nice but I don't have one... Any ideas on things to check???

[Dana]

If I understand correctly, last year you had both a hot air oil furnace and the boiler, with the boiler running an airhandler & some some radiant, and now you've added another air handler where the oil-burner had been?

At 140F entering water temp, each of the air handlers pulls 53KBTU/hr out of the boiler for a total of 106K plus whatever the radiant is pulling, whereas last year with only one air handler you had 53K + radiant zone. The rated output of the 175 is 134K, so last year you were probably only pulling half the rated output, and now you're knocking on the limits pretty hard when all 3 zones calling for heat at once. A 53K step in load is a pretty big chunk, especially if it's taking you close to the full-fire rated output. The system behavior is probably about what you should expect now that the heat loads are higher, and not a flow problem. You could test the combustion efficiency with an analyzer, but it's probably working just fine.

During periods of lower heat load the zone calls probably weren't overlapping as often. Starting up the radiant zone cold it would be pulling far more heat for the first several minutes as it heats up the thermal mass of the water in the loops, and you're just plain pulling more than the boiler can deliver if you start them all up cold all at once. If the system is still able to satisfy the thermostats, I wouldn't sweat it if it's output is running in the 130s rather than dead-nuts-on 140F with all zones calling for heat at the same time. If they're pulling 53K per @ 140F, they're still pulling more than 45K per @ 135F.

Your true heat load is probably under 100K at design condition even with the oddball tall rooms, etc., but without a heat loss calc or a season's worth of fuel-use data to apply against weather data it's tough to say for sure what it is.

3/4" line on the propane is fine- it'll deliver well over the 175K input on such a short run.

[skihawx]

At 140F entering water temp, each of the air handlers pulls 53KBTU/hr out of the boiler for a total of 106K plus whatever the radiant is pulling, whereas last year with only one air handler you had 53K + radiant zone. The rated output of the 175 is 134K, so last year you were probably only pulling half the rated output, and now you're knocking on the limits pretty hard when all 3 zones calling for heat at once. A 53K step in load is a pretty big chunk, especially if it's taking you close to the full-fire rated output. The system behavior is probably about what you should expect now that the heat loads are higher, and not a flow problem. You could test the combustion efficiency with an analyzer, but it's probably working just fine.

Thanks for your reply. I never thought about the output being only 134K. I was thinking 170BTU in 154BTU out but that is at a much higher water temperature. I ran the boiler this morning and with the two air handlers on the output temp was 130 the return temp was 116 and the flue temp was 126. The set point is fixed at 140. It ran steady this way for several minutes. The blower sounded quiet. I entered the boiler into the full fire mode. The blower speed picked up dramatically and this time the boiler went up to 146 with a return temp of 124 and then reached the upper set point at 148 and shut down. During those few minutes it was condensing like crazy. Hopefully in the heat exchanger.

If it can make a higher temp why isn't it??