Hi All, I have some questions with my Laars Mascot II Combi system...some background first. 1800 sqft house, baseboards with two zones one for each floor of the 2 story house. The system was installed 3 years ago right when we bought the house and the problem is with short cycling. Last year I had a local company (on Laars approved contractor list) take a look at it and he said ASC (anti-short cycling) was turned off, cleaned the combustion chamber and said it was running very cleanly. During this time we also realized the inlet and outlet piping was incorrectly sized, using 3/4" on both instead of the recommended 1 1/4". So over the summer my dad and I replaced both inlet and outlet, made a new manifold, redid everything from top-bottom with the piping leading to and from the boiler hoping this would also help (and it looks better than the previous PEX install :)). The tech said between the ASC being off and the incorrect piping it was the reason why it was short cycling. Well it's still short cycling. I have a hunch something is wrong with the math...delta T is somewhere in the 8-15 degree range which I know is low. Any ideas as to possible solutions would be greatly appreciated. I am very much a DIY guy and my dad is an old GC so we have an idea of what we're doing but my expertise lies more with kitchen cabinets, decks, and floors.

not knowing much about your system but a bit about the boilers,
assuming you now have a 1 1/4 primary loop, and are pumping each zone (check the sizing on your pumps vs head required,).
Here are a couple solutions/ideas
slow pump speed to 1 (Slowest speed)
set short cycle delay to max , I think it might go 3 miniuts read the manual or call lLAARS and have them talk you through it,
Spread delta T to about 15 degrees
Lower delivered water temp to a point where the house stays comfortable but out bound water temp is at a minimum. Start at 135 work your way up,
Dan

Hi Dan,

thank you for your quick reply...Yep got a 1 1/4 primary loop with 1 taco 007 pump for each.

How do I check/do the math for the pump sizing?

I'll adjust the short cycle to a longer setting, hopefully it's not already 3 min haha. Install brochure online says the option to change it is in the "user" menu but I have never seen it there, I'll call LAARs when I get home.
The output temp is 150, I'll change it to 135. One day I'll get around to setting up the outdoor temp sensor.

Check the pump speed on the pump built in to the boiler,
slow it down.

Newbie question Dan, how does one change the pump speed on the internal pump?

open the front of the boiler panel,
see the red pump, there should be a small black dial with settings 1 11 111
set it to 1
easy

You're the best, I'll update when I'm home later...thanks so much! Paul

Dan has you well covered. However, if you still have short cycling, it is likely just the result of the heat source capacity exceeding the heat capacity of the zone(s) that are calling for heat. If this is the case, the only solution is to reduce the heat capacity of the heat source or increase the heat capacity of the zone(s) by using a buffer tank.

Borst Buffer Tank Design Software

Okay here's my update with suggestions thanks to Dan...

1.) Lowered temp to 135
2.) The internal boiler pump was set to "High/III", I switched that to "Low/I", ran one zone and boiler ran for at least 4-5 minutes without cycling once, but once DeltaT got in the 8 degree zone it started shutting off
3.) Went into the set up menu and ASC was turned on to .3, which I assume is 30 seconds, I set it to 3 minutes but it does go higher up to 4 (didn't try more than that)

Would short cycling lead to higher gas usage, I would assume so right? I hope this cuts down on it

Yes, short cycling reduces boiler efficiency and hence results in higher fuel usage. However, the real issue with short cycling is significantly reduced boiler life because of the mechanical components in gas and oil fired boilers. Electric boilers on the other hand are much more tolerant of short cycling.

Okay so update#2, with ASC turned on and the internal pump set to min...the boiler will fire and run for about 4-5 minutes unless both zones are on in which case it's closer to 10 and then shut off when delta T is around 5-7 degrees. Again this is an improvement where it would cycle every 1-2 minutes.

Is this acceptable? @sailawayrb you mentioned the zones might not be big enough for the boiler. I did a heat loss calc courtesy of the SlantFin HeatLoss express and my total BTU is 46,524.

Yes, if you know the zone(s) and boiler BTUs, you can use the software link that I provided to determine if a buffer tank is needed and, if so, determine the minimum buffer tank size for the heat source minimum cycle time that you enter into the software. Heat pump manufacturers typically recommend a minimum of 5 to 10 minutes. Boiler manufacturers typically recommend a minimum of 10 minutes. Longer cycle times will typically result in higher heat pump Coefficient of Performance (COP) and boiler efficiency. The instructions explain all this and how to use the software.

So here's where I am

Option 1: Buffer tank: I'm assuming adding a buffer tank is an expensive solution and labor wise lots of re-plumbing too
Option 2: Lowering heat output would mean changing boilers I'm assuming? Expensive!
Option 3: Increase btu's...add a few extra radiators off of the first floor zone for the basement? Our basement is COLD and mostly above grade since we have a walk out so this might make it less raw down there.
Option 4: Do nothing, tick up the ASC to 5 minutes and set up the out door temp sensor.

Ugh...I hate this! But thank you for your guidance...I've asked plumbing "friends", HVAC buddies, other forums and everyone has been dumbfounded and certainly no one came up the easiest solution of them to slow down the boiler pump! Many thanks...

Yes, those would be the post installation phase options. Of course, the best option is to address this during the design phase (i.e., before the installation phase) and size the heat source with this in mind or design for a buffer tank if required.

Agreed...design would have been nice. What's my best cost-benefit option?

If you need additional heat gain in zone(s) that will be calling for heat the majority of the time, installing additional radiators in these zone(s) may increase the boiler ON time sufficiently. Otherwise, adding a buffer tank would be the solution for short cycling. While doing nothing may reduce boiler life and will reduce the operational efficiency, this may well be the best ROI option. It is difficult to quantify how significant the reduced boiler life would actually be.  So maybe do nothing, learn from the experience, and select a more appropriate boiler when a replacement is needed...which hopefully will not be for a long time.  It is always best to simply avoid creating this dilemma.

Dumb question, if it's a modulating boiler it adjusts the output correct? The tech data for the boiler says it can run at 20% of full modulation...so that would be about 24kbtu. I'm assuming since I have about 20-22kbtu/zone that's why it's exceeding the need. So for your tank calculator what btu's would I use?

Paul

Without running at least the napkin design numbers, it's hard to say for sure what's going to be the solution with the best cost/benefit.

Run the heat loss numbers PER ZONE, and compare the radiation of each zone to it's load. If it's under-radiated you may never get the advertised condensing efficiency without short-cycling it into an early grave.

To balance 24,000 BTU/hr of boiler output with fin-tube baseboard at 120F AWT (necessary to hit the mid-90s for efficiency) you'd be looking at about 100 feet of baseboard per zone(!). I'd be surprised if you had anywhere near that much, but you might have half that. Measure up the total baseboard length per zone, and confess the design-sins of the installer- we'll be able to tell if there is absolution in his/her future, and gain some condensing efficiency without resorting to major hackery with buffer tanks.

Run times of 4-5 minutes aren't an efficiency disaster for a low-mass mod-con (1-2 minutes IS a disaster), but if it's getting on to 10 burns per hour or more it'll still be a wear & tear issue. Ideally you would have fewer than 5 burns/hour under all load conditions, and a minimum burn time 5+ minutes. But if it's 4-minute burns and 7 burns/hour don't sweat it too much. The return on investment of retrofitting a buffer tank won't be there (unless it's a propane fired boiler in a high propane price market.)

Dana’s advice is great and consistent with mine. Yes, as the instructions for the software state, you should use the minimum BTUs if you have a mod boiler. So 24 KBTU would be appropriate.  Of course, this raises something else you had best verify...  Is your boiler actually modulating and reducing the heat output like it should?

Thanks guys...the boiler does modulate well and you can tell when the "load"/delta T is higher it adjusts nicely. Also, came home and turned both zones on and they ran together for 31 minutes before going into a cycle, from that point on it was once every 10-12 minutes for a cycle, this is with the 3 minute ASC turned on. Delta T was between 12-30


Below are my heat/loss calcs from the slantfin program with their fineline 30 baseboard.
1st floor
Heat Loss BTUs 21,935
Baseboards required 59
Baseboards present 54

2nd floor
Heat Loss BTUs 27,481
Baseboards required 62
Baseboards present 69