Rinnai M090C short cycling

Montana

I'm having some trouble with a new install of a M090C Combi Boiler and could use some help. The boiler is short-cycling (see below).

System:
I've attached a sketch of the system as designed and built. This is installed in the basement of a new addition which has only three, 265' radiant staple-up loops thus far. (We will be adding 5-6 more loops in the second phase of the remodel, outfitting the original structure with radiant this spring.) The home is located in Montana with the interior temp of the addition at 15-20 dF. (No insulation so the heat load opportunity is very high. Theoretically, we should be able to easily dump 20-30k BTUs (or more?) into the space from the system.)

As you see in the sketch, there is a separate primary loop pump (GF Alpha2 in addition to the M090 built in circ pump) to ensure adequate flow and suction head in the primary loop. There is another GF Alpha2 pump in the secondary loop connected to the primary loop with a closely spaced Tee with built in 3-way valve (Webstone). A Caleffi mixing valve is included in the secondary loop to provide mixing in the incoming secondary manifold supply. The radiant system is distributed with a Bluefin 6 port manifold.

Problem:
The boiler is short-cycling.

I've tried various system configuration settings but it persists. Here is a sample of the settings:
System set to "Auto"
Reset monitor reading 18dF (in one of the rooms)
Set point therefore comes up at 142 dF
Jumped system Tsat terminals for continuous "heat call"
Boiler inlet pressure 26.7 psi
Closely spaced tee valve manually closed (forcing full flow through the radiant system)
Mixing valve set to "Min" (closed bypass we assume)
Both pumps are set at max constant pressure (III) and reading 5 gpm (!?)
Manifold flow indicators ~0.7 gpm ea (2.1 gpm total)
Flame indicator on the panel shows 1 out of 6 bars (modulated to the lowest right?) (TDR of 3.8:1 right?)

Results:
First run this am, closed the t stat contacts. The system ran up to 90 dF (indicated on gauge) and shut off (went into CH pump overrun mode). Took a few minutes then started again. Second run up to 105 df then shut down. After several minutes it started again up to 110 dF, then finally up to 120 dF. After a couple more cycles it shut off at lower temps as well. I've tried running it with the "Auto" function shut off and a set point of 140 dF - same results. These cycles are approximately 5-10 min apart. Delta T across the manifolds is 20 dF with a flow of 2.1 pm (using the indicators on the manifold). That should work out to 2.1*(20 dF)*500 = 21,000 BTU's (right?)

Please comment - I'm open to workarounds to get to a basic system operation without short-cycle. On one hand, when we get all the loops running, we should have much greater heat load in the cold but I am also concerned about spring or fall with moderate temps.

Added note: DHW is not connected but will be added later.

Thanks

Steve

GGross

Alright lets start it off, not piped right. Your boiler should be piped into the heating sytem via close space tees, you wouldn't need pump 5 on your drawing, in fact that pump will cause flow issues with the boiler as it is flowing right through it and not separated at all. On you heating loop you wouldn't need close spaced tees, or a mixing valve as long as your system is all low temp. On your mixing valve you need to have the pump on the mix outlet side of the mix valve. Your expansion tank is located between 2 pumps which is also not good design.

Onto short cycling, if the unit is not cycling due to pressure changes from the pumps/flow keep in mind that your boiler only turns down to 23,500 BTU/hr on natural gas, and 73.500 BTU/hr on LP, so more than likely you are cycling because there is not enough load.


from the rinnai manual to give an idea about proper piping

Jamie Hall

On top of all of which, I'm just a wee tiny bit concerned about your reported pressure -- which is at least 10 psi too high.

GGross

Ah yeah thats a real problem if the boiler pressure is at 26 PSI, that should be the first issue to get addressed

Montana

Thanks all.

The boiler pressure is specified by Rinnai - they require >19psi inlet pressure on this boiler.

GGross - I am not understanding. Please comment on this article and design.
https://www.pmmag.com/articles/85102-primary-proportions-john-siegenthaler

dko

Pass

Montana

Here is what it ways in the manual. Please comment.

"The boiler is in normal operation between 14 PSI and 43.5 PSI.
− Below 10.1 PSI, the boiler will have an error code (Fault 108 on the display) and be
blocked from operation. Increasing the heating system water pressure will be necessary
for operation.
− Between 10 PSI and 14 PSI, operation will be limited to 80% (Alert Warning 1P4 will
appear on the display).
− Above 43.5 PSI, the boiler will have an error code (Fault 109 on the display) and be
blocked from operation as the pressure is too high.
− The maximum pressure permitted inside the heat exchanger is 45 PSI.
− The pressure relief valve supplied with the boiler is rated to 30 PSI. "

GGross

Montana said:

Here is what it ways in the manual. Please comment.

"The boiler is in normal operation between 14 PSI and 43.5 PSI.
− Below 10.1 PSI, the boiler will have an error code (Fault 108 on the display) and be
blocked from operation. Increasing the heating system water pressure will be necessary
for operation.
− Between 10 PSI and 14 PSI, operation will be limited to 80% (Alert Warning 1P4 will
appear on the display).
− Above 43.5 PSI, the boiler will have an error code (Fault 109 on the display) and be
blocked from operation as the pressure is too high.
− The maximum pressure permitted inside the heat exchanger is 45 PSI.
− The pressure relief valve supplied with the boiler is rated to 30 PSI. "

"The pressure relief valve supplied with the boiler is rated to 30 PSI." this is what matters the most. if you set your pressure to 26 PSI you risk blowing off the relief valve when it hits 30 PSI. You set the pressure to the system, not the minimums and maximums that manufacturer will allow. So a standard home, residential, you will generally see a cold fill pressure between 12-18 PSI, with a 30 PSI relief valve installed on the unit. 26 PSI is way too high for a cold static fill pressure on a boiler with a 30PSI relief, it's too close to the blowoff pressure. It is wild to me that Rinnai would spec out a 19PSI minimum, it is just asking for trouble in a hydronic system