Triangle Prestige internal CH pump speed and ODR hi limit effect?

[Machinist]

Just hooked up outdoor reset on our Triangle Tube Prestige 110 boiler. I'd appreciate recommendations regarding the Prestige's internal circulator speed setting. Specifically I'd like to know whether running it on low speed would provide adequate flow through the boiler. I realize there are factors to take into consideration. All near boiler piping (1") is according to the TT manual with Primary Secondary. Three radiant zones each use their own 3 speed Grundfos circulator and manifolds to monitor flow. Loop lengths are within spec using predominantly subfloor pex in plates. The 110 boiler is oversized for our heat loss of 70K BTU, but it supplies a Smart 40 gal. indirect tank also. I ask about reducing the internal pump speed because the boiler cycles slightly more than I prefer in the fringe season (currently 40's) if not all zones are calling for heat. I thought that a low internal pump speed would permit the secondary zone pumps to pull the heat from the boiler to reduce short cycling. I'd rather not increase the secondary pump speeds in order to preserve electricity costs. My biggest concern is harming the boiler or reducing it's longevity.
One more factor. Previously, without ODR enabled, the boiler's hi temp setpoint was 140* for condensing. It's kept our home warm for two winters. I only increased the hi temp limit to 150* after hooking up ODR. Factory default is 186*. Would changing to the higher setting have any impact on boiler cycling? Thanks for any help.

[jadnashua]

I have a different brand, but the manufacturer specified the primary loop pump speed based on the max output level...you do not want to increase it beyond that. High flow can cause cavitation and erosion of the pipes and boiler heat exchanger. You'd know if it was too slow, it would likely start to get some flashing to steam, and would be very noisy.

You want the temp to be as low as possible, while still heating the house adequately. If it cycles too much, or short cycles, you may need to look into a way to add some mass. The delta between high/low may have changed. If that's too small, it will cycle more.

There's generally no big reason to oversize a boiler to run an indirect. The only reason I can think of is if you use hot water continuously, like say in a spa or commercial laundry. Otherwise, it generally runs as a primary zone, and gets all it needs, and in your case, that's over twice a typical stand-alone gas WH burner would have...so, it heats much faster, and the system can return to heating the house before you'd notice.

[Machinist]

Thanks for the reply.

you may need to look into a way to add some mass. The delta between high/low may have changed. If that's too small, it will cycle more.

I did pipe a large cast iron radiator into the basement zone prior to last winter. Also, I wired the basement zone pump to run whenever the internal pump runs. The basement needed more BTU's anyway and the boiler delta T increased, which improved the short cycling condition.

There's generally no big reason to oversize a boiler to run an indirect.

For a few reasons, the Triangle Tube was my boiler of choice in 2010. The 110 (86K output) is oversized but the next size smaller was 60K input. I believe the newer Trimax model will modulate lower than the Prestige. A few contractors quoted about the same size boiler (with indirect) based on heat loss. They would have been oversizing also. A far too common occurrence it seems.

[Dana]

It's the low-temp end of the curve that has the biggest effect on cycling, not the high end. When the water temps are low the radiation puts out less heat, and when the water temp is low enough that the radiation can't put out the min-fire output of the boiler the thing cycles. If you can program the hysteresis around the setpoint, increasing that will lengthen the burns by "exercising" the thermal mass of the system a bit more.

The minimum output of the thing in condensing mode is about 28,000BTU/hr, which is a significant amount of heat- could be more than half your actual heat load at design temp if that 70K heat load is a Manual-J type heat loss calc rather than calculated from fuel use. (The 99% heating design temp for Ithaca is 0F, 28K is more than 80% of the heat load of my house at 0F.) The minimum burn time depends on the thermal mass of your smallest zone (most of which is the water in the tubing), and the programmed temperature hysteresis. If you have say 1000' of half inch PEX in the smallest zone, that's about 85 lbs of water. Now 28,000BTU/hr is roughly 470BTU/minute, so at low temp when the floor isn't emitting much heat into the rooms, at min-fire that water heats up at (470/85=) 5.5F per minute. With the hysteresis is set to 10F you're looking at 2 minute burns, but if it can be bumped to 25F you'd hit 5 minutes. Whatever the boiler's controls allow for hysteresis, max it out.

Do the lipstick-on-napkin-math of your actual system, assuming you know the tubing layout. In very rough terms you're looking at 8.5lbs of water-equivalent thermal mass for every 100' of half-inch PEX, 8.5lbs for every 50' of 3/4" PEX. The boiler itself has about 20 lbs of water-mass. Add up each zone separately, since they can all call for heat separately. The key is find the water temp and hysteresis to get the min-burn on your smallest zone up to the 5 minute range.

If there's a programmable hard low-limit to the boiler to block the reset curve from taking it any lower, start by setting it to 120F, then see what happens- time the burns when it's just maintaining (not a cold-startup). If you get 5+ minute burns out of it serving only your smallest zone, drop the temp 5F at a time until you drop below the 5 minute threshold and go no further.

The actual whole-house heat load could be calculated from mid-winter fuel usage measured against degree-day data for the billing period, and from there the water temp requirements estimated from the square footage of radiant floor, the type of heat transfer plates, subflooring, and finish flooring. But since you know it keeps up with 140F, starting with 140F @ 0F is a good place to start on the high-temp end of the reset curve, and you can work down from there. The output is roughly linear with temperature above 70F, and from the fuel-use numbers we can calculate the heat load at any arbitrary outdoor temp. But if it needs 140F at 0F outdoor temps it will only need 85-90F water at +35F, and the hysteresis of the thermal mass of the smallest zone becomes critical. If you run the experiments and time the burns you'll know where to set the hard-bottom of the curve.

[Machinist]

Thanks for taking the time to reply.
I'll experiment with the curve's low end temp and minimum boiler temperature this weekend. Are you saying that a five minute burn time is a good target for the smallest zone (given it's not in a cold startup condition)?
Our home's heat loss figures came from an average of three manual J calculations between two contractors and my own. Our exterior walls should be better insulated and the home is often subjected to brutally cold winds off of one of New York's finger lakes (except last winter) Nonetheless the boiler is still oversized at design temps.
FYI...I did attempt to drop the boiler's internal pump (Grundfos 15-58) to low speed to permit the secondary pumps to pull more heat from the boiler. The boiler began to make some faint hissing sounds which I assume was steam flashing. Back to medium speed with the internal pump and the boiler is smooth.

[Dana]

The way it works is this: At a given water temp every flue purge and ignition cycle throws away a fixed amount of heat in unburnt fuel and heat extracted from the boiler and blown out the exhaust vent during a purge. In a low-mass boiler like the Solo 110 running at minimum fire if the burn is 5 minutes that's about a half a percent hit in efficiency, maybe a bit less. If the minimum burn time is only a minute that's a few percent of the source fuel energy for the entire burn blown out the flue. In the extreme short cycling where the burns are a few 10s of seconds you can easily be taking a double-digit hit in efficeincy.

Aside from the efficiency hit with a short-cycling boiler you're putting a lot of wear and tear on the boiler just in raw numbers of ignition cycles. Ideally you'd be modulating with nearly continuous burns when it's below 40F outside, but with 28K of min-fire output that might not happen until it's colder, depending on just how well-tweaked the reset curve is. But even during the shoulder seasons to be nice to the boiler you want to keep it do a handful of burns per hour.

Most of the time Manual-J methods overshoot reality by 15-25%, sometimes more. You can use the boiler's fuel use against heating degree-days (HDD) to MEASURE the heat load, or at least put an upper bound on it.

If you have a mid-winter gas bill from last winter with the meter reading dates and a zip code (for the closest weather station data).

First divide the fuel use by the number in days in the period for source-fuel BTUs per HDD. (It's 100,000BTU/therm, or 102,000BTU/ccf)

Then divide by 24 converting that to degree-hours.

Then multiply BTU/degree-hour times the difference between the balance point base (assume 65F), and your 99% outside design temperature (0F according to the ACCA datasets), for source-fuel BTU/hr at the design temp.

Multiply by the efficiency of the boiler (assume 92%, if you were running 140F water), and you have the approximate heat load.

Since you also use gas for heating hot water and cooking that will be an upper bound on heat load.

You can download a spreadsheets of daily HDD from degreedays.net from a weather station near you for the period covering the billing period, clip the relevant dates and sum them to come up the HDD. Alternatively, if your gas provider gives you the average temperature during the billing period and the number of billing days, multiply the difference between 65F and the average temperature times the number of days, use that as the HDD- it'll be close enough.

If slowing the pump down got you some sizzle &/or bang it was definitely too slow, or the pressure was too low. Most 1-2 story houses can do just fine at 13-15psi (static, when the pump isn't running), but some low-mass boilers want to run it a bit higher than that, especially if it's plumbed where the pump is pumping away from the boiler (and expansion tank), making it the low-pressure point of the system when the system is running. Pumping away is a classic configuration for low-head high mass boilers, but sometimes with higher-head low mass boilers it's better to pump toward the boiler. (Read the installation manual for the manufacturer's recommendations.) In a short loop primary/secondary setup it may not matter.

If you can't get the burn times up at low temps by programming more hysteresis it may be time to consider adding mass. The raw combustion efficiency is primarily a function of the water temp entering the boiler- lower==higher efficiency. Ideally you'd be able to run it with return water averaging well under 120F during low heat loads to hit the mid-90s for average efficiency. With 120F return water (probably where you've been running with 140F output) you're over 90%, but not by a whole lot. With aluminum extrusion heat transfer plates on the subfloor and a hardwood finish floor you should be able to hit the mid-90s during the shoulder season, as long as you aren't short-cycling the boiler to death. With 20F of hysteresis centered around a setpoint of 110F and 5+ minutes of minimum burn you'd be hitting about 95%- if you can make it work at lower temps without short cycles, go lower. Then finding the max-efficiency point becomes a matter of tweaking the curves ever lower to determine where it stops keeping up with the load, then pulling back up a bit so that it just keeps up.

[BadgerBoilerMN]

Never up-size a boiler, condensing or otherwise, to "compensate" for an indirect water heater without professional, experienced advice. One size does not fit all, which is why we install several manufacturers of condensing boilers including Triangle Tube.

If a proper heat load were perform, for each zone, it may have been determined that the TT boilers low output was not low enough and short-cycling predicted with confidence. It is all fine and good to parse the particulars of why the boiler is short-cycling but I hope others will get the real problem; to spite the nice looking mechanical work and astute perusal of the installation manual, condensing boiler with a low-fire output too high for the load is a mistake.

If the heat load were in fact 70mbtuh, I would have chosen on of several 80mbtuh condensing boilers on the market and tied zones together to make a functional load for the boiler chosen.

The Triangle Tube Solo 110 will tolerate a 40°delta T, and also very high flow rates without erosion. There are some boilers that may suffer from flow rates over spec. but the boilers with internal circulators will not.

[Machinist]

Thank you for your experienced input. The kicker is that three local heating contractors who quoted the boiler install would have gone with almost the same btu boiler. One did an actual detailed heat loss on our home. They deal in Buderus or Weil McClain and felt the indirect made them lean toward the larger boiler. I now know they are wrong and it's a very common mistake. After seeing their huge labor markup I decided to install the entire system (with two sons running pex...ugh) as I could afford (over a year), while utilizing our 32 year old Cast Iron Dunkirk supplying baseboard fin tube in the meantime. Our terrible economy and jobs going oversees is creating the worst conditions I've seen in 35 years as a U.S. factory machinist... money is tight. As a homeowner, there are only a few manufacturers that I could buy from. Research showed how well built the Triangle Tube Solo is made and I have a heating contractor on call if needed and to perform some annual maintenance and safety inspections. So far the TT has run flawlessly with only reset needed at power outages.
Perceptions on what constitutes short cycling varies dramatically. I had the impression that perhaps a 10 minute burn time before reaching hi limit was too little when the smallest zone was calling. By incorporating the basement zone to run along with either upper zone, the boiler runs nicely. As mentioned before, the basement always needed more heat, so it worked out nicely. On the downside, at least two pumps are always running in addition to the internal pump. At least the secondary pumps run on low speed. I realize a professional installation would have avoided this correction and fuel bills would be reduced more than they have been.
Per Dana's recommendations, I've been seeing improvement in run times as I adjust the outdoor rest curve. I'll keep watching the cycling behavior as the temps drop also.
Thank you again... and to any homeowners reading this, don't always assume your local heating contractor is sizing your boiler correctly.

[BadgerBoilerMN]

Times are tight and local service is a benchmark of ours. A ten minute cycle on condensing boilers is not a short cycle. Naturally, the longer the better.

[Dana]

A ten minute minimum burn cycle would be decent even for a high-mass boiler, but getting it to where you get pretty-much continuous modulating burns out of a mod-con when there's a real heat load is a holy-grail worth tweaking the system toward if you can. But the greater the degree of oversizing, the colder it has to be before that happens.