Update 100 year old high mass gravity system with ModCon Boiler

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Fitter30

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Boost mode if the boiler isn't keeping up with load it will start raising setpoint by 10* till it catches up. Boiler satisfies when boiler call again starts at reset temp. Anti cycle will keep the boiler from short cycling this is a set time
 

PC7060

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@Dana / @fitter30 - Thanks for the information. I've found the Outdoor sensor tends to read significantly higher than outside ambient when temps are in mid to low 20's (e.g Sensor =40F, ambient = 25F). I believe the error is due to heat loss from the house coming through the foundation. I'm trying slightly different locations / orientation to see if I can get more accurate reading. In the mean time, I've enable Boost with a 5F step after 40 minutes to address the situation.

I've attached a couple picture of the completed system, seems to work well and is much quieter than the old beast. The system is piped per @Dana original recommendations.

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PC7060

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I've moved the Outdoor sensor to a temporary location where it appears to be reading ambient more accurately. I'll need to come up with a permanent solution that can be tucked in a discrete location since the sensor is on the front of the house near front door. I'm planning to mount the sensor on small piece of aluminum angle and use thermal paste to improve sensor accuracy.

I also disabled Boost Mode (more on that later) last night and let the system operate per design. The temperature last light was in the mid 20's and the boiler held temperature very well running for very long periods circulating 109F water at at 5-7GPM.

I currently have max water temperature set to 130F with lowest outside temperature of 5F (default). This gives me approximately 120F water temperature for a design day temperature of 15F with a bit of insurance if temps drop further.

Note: Boost mode drives the water temp up too quickly causing the unit to short cycle. Based on my analysis and tests, there appears to be a bug in the firmware that controls boost mode temperature steps. I've found that the boiler always uses the default values for 10:bd Boost Degree (10F ) and 11:bl Boost Interval (20 MIN ) regardless of what is programed in (5F / 40min).
 

Fitter30

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I don't know if this has any effect on the boost mode but a setting to slow ramp speed of burner. When ever making setting adjustments make baby steps see how everythings acts.
 
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PC7060

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@fitter30 - enabled the step modulation for both Heat and DHW. All operating smoothly. Have about 37 heating hour with 50 starts or about 45 minutes average.

DHW is working well too with tank temperature set to 130F and mixer output at 115F. I reduced the DHW heating temp to 160F. At the default 180F the boiler was running full out and was getting a lot of noise on intake / exhaust line. I understand this will impact DHW performance but it is still meeting normal demands. Not sure how it will handle marathon showers, Kids will just have get out after 40 minutes. ;)
 

Dana

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@fitter30 Not sure how it will handle marathon showers, Kids will just have get out after 40 minutes. ;)

Installing an occupancy/vacancy sensor switch for the bathroom lights set to time out in 10-12 minutes keeps the marathons to about that length. :cool:
 

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You must have better trained kids than I @Dana!
I switched to Leviton moisture sensing fan controllers after giving up on getting the kids to just push a button to turn on fan and light. Even when they did remember, the timer would shut off after 15 minutes and I’d end up with Amazonian rain forest levels of humidity.

The sensors work very well; I use in all our bathrooms now.

Leviton IPHS5-1LW Decora In-Wall Humidity Sensor & Fan C... https://www.amazon.com/dp/B00H3QQD64/ref=cm_sw_r_sms_api_glc_fabc_ndT2FbA4VGB01?_encoding=UTF8&psc=1
 

Dana

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You must have better trained kids than I @Dana!

Not really. This morning my son's girlfriend took a 50+ minute shower- she figured out that when showering during daylight hours it doesn't matter if the lights stay on.
 

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After two weeks still had ongoing problem with ODR sensor sensor not getting true outdoor temperature reading. Adjusting slope didn’t address problem since error was not linear (got worse as temp dropped).
Determined I needed to move ODR sensor away from house 2’ and location of the outside AC compressor made it a good option. Mounted the sensor to underside of wire connection box and connected back to boiler using spare wires from low voltage control lines for compressor. Now reads very accurate and my 120F max at design temp slope is good match. Boiler has been running almost continuously and was running at 25-35% capacity during recent 25F nights.
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Dana

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Boiler has been running almost continuously and was running at 25-35% capacity during recent 25F nights.

So now what would you estimate the design load or peak heating requirements to be? (75,000 BTU/hr, right? :rolleyes: )
 

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So now what would you estimate the design load or peak heating requirements to be? (75,000 BTU/hr, right? :rolleyes: )

Ha! As you originally mentioned, the original Manual J / Heat loss projections by SpacePak were very conservative on high side at 75K. Also agree the increase footprint was more than offset by the better air sealing and insulation. You were right on with your recommendation for a FireTube ModCon with 10:1 turndown. The direct pumping optimized performance and greatly simplified piping. Also glad I listened to knowledgeable advice a and ignored the calls for bigger boiler to handle the DHW requirement. That UFT80 does great job. No hot water run outs even with four of us (including 2 shower zombies).

I’ll look at the gas utilization number in Jan / Fed which are typically our cold times but I’m guesstimating I’ll be at 45-50KBTU at 99.6% design temp of 11F. This also where I’ve got my 120F heating point dialed in.
 
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Dana

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I’m guesstimating I’ll be at 45-50KBTU at 99.6% design temp of 11F.

If going by ASHRAE's recommended 1.4x oversize factor (more appropriate for 1 or 2 stage burners than fully modulating systems, but OK) 45-50K @ design condition would call for 63-70K output on the boiler.

At higher non-condensing temps the UFT-080 puts out ~70K, so I'd say you're damned near perfectly sized any way you care to slice it!
 

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If going by ASHRAE's recommended 1.4x oversize factor (more appropriate for 1 or 2 stage burners than fully modulating systems, but OK) 45-50K @ design condition would call for 63-70K output on the boiler.

At higher non-condensing temps the UFT-080 puts out ~70K, so I'd say you're damned near perfectly sized any way you care to slice it!

Agree! Right now I’m just keeping an eye on the performance bottom end of the slope to ensure btu input/loss match up at that end. Softly softly catchy monkey! :)

Only remaining issue I’ve note is tendency for thermostat to short cycle boiler (15-20 minute runs then start again in 5 minutes) in evening as temperature of day starts to taper off.

I set the anticycle time (setting 16) up to 20 minutes since this is masonry exterior house and temps change pretty slowly.
However, UFT80 doesn’t seem to follow the time out and will fire up up again when TT is triggered.
Have a email into HTP tech support but no response. Not a huge issue but I’m watching it.
 
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Dana

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Agree! Right now I’m just keeping an eye on the performance bottom end of the slope to ensure btu input/loss match up at that end. Softly softly catchy monkey! :)

Only remaining issue I’ve note is tendency for thermostat to short cycle boiler (15-20 minute runs then start again in 5 minutes) in evening as temperature of day starts to taper off.

A 15-20 minute burns at a rate of 3-4 burns per hour isn't a short cycle, and nothing to be concerned about.

Outdoor temperature reset is an imperfect load-matching model, and truly continuous burns for days or weeks are rare. Things like solar gain, wind-washing adding to heat losses affect the model, so some amount of cycling is perfectly normal, even when the ODR curve is tweaked pretty finely.

Efficiency & boiler destructive short cycling would be something like running 1.5- 2 minute burns with half-minute off times 2o-25x per hour. That's the same duty cycle for the burner as the cycling that was described, but with an order of magnitude more ignition cycles & flue purges.
 

PC7060

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Thanks Dana, it is a very helpful to have objective definition of short cycling. My average cycle time was around 45 minutes the the last time I ran the numbers. Typically runs long cycles at night with shorter ones during sunny or warmer days.

Just got my bill for first full month of boilers operation with DHW. Gas utilization is 32% less that same period two years ago with a 800SFT larger envelope. Nice! 50% less that last year same time, but we had framing going with opening into basement so gas usage was atypical.

Reduction is due to combination of higher efficiency boiler running low water temps (120F SWT @ 11F outside) and the benefits of the well sealed and insulated addition.

And of course with the long boiler runtime, the house has much more even heating and the cat always has a warm radiators to snooze on. Indirect DHW is also big hit with the family (not so much for the cat).

I appreciate all of your help on my project. Met or exceeded all of my objectives and I love the simplicity of a single pump. The Grundfoss Alpha2 is probably overkill since I just run it at constant speed but well worth the extra $100 for low wattage ECM motor and the GPM display. Constant speed Level one moves 4 GPM at cold startup (70F return water temp) and progresses up to 6 GPM at 100F SWT / return temperatures of 80F. Flow rate tops out at 8GPM few times it got cold enough for SWT >110F. Good numbers overall for a 80KBTU boiler.
 
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