Pri-Sec pump mismatch or gravity back flow - homeowner needs help [long]

Gcracker

First post, thank you for the incredible resource you provide here. What a place for answers!

Equipment list:

• Bosch Combi 151 ZWB42-3A using LPG
  
• Combi internal circulator (0-8GPM@~20'-0')
  
• Grundfos 15-58 FRC circulator (0-17GPM@19.5'-0') with three speeds
  
• Braeburn programmable thermostat
  
• Taco single zone controller sending demand to combi and controlling Grundfos pump
  
• Outdoor reset is not installed
  
• Supply pipe temp sensor is not installed
  
• Rough estimate of H[ft] is 9.06 (~151' of pipe X 1.5 X .04)
  

This is new construction single level 1100sq-ft ranch modular with the boiler install completed in the hottest summer months. Heat is sent into a single zone to baseboards with what seems to me to be a standard primary-secondary setup. The spacing on the tees is not the recommended less-than-or-equal-to 4", but 6" to 8". The installer did not use the Bosch controls (his words for not using the Bosch CRC200 and outdoor reset, and why he used the Taco instead were that a small system like this doesn't need "all that."). I am struggling to determine the cause of my problems in gaining heat in the home now that the cooler months are here in the northeast. My loop has been twice-bled (hose from return drain into a 5g bucket), so I think air blockages are out. All baseboards get warm, even hot, sometimes, but not enough heat transfer is occurring. We have a kick-space heater in the kitchen, and if I turn that on, there is a very noticeable improvement. Trouble is, it's optional, and it's somewhat noisy. We are hoping it isn't required.

1. Nighttime set point of 64F
   
2. Morning set point of 67
   
3. Afternoon set point of 64
   
4. Evening set point of 69
   

When I wake up, roughly 45 minutes after the morning set point engages, the temperature in the home is still 64 degrees with 28F outdoor temperatures. A look at the boiler LED shows the temperature swing from 140F to +/-190F. When the boiler turns off, the temperature drops about a degree every two seconds, slowing down over time. When fired back up, the temp climbs strangely fast, making me wonder if the system is not pulling water through the boiler, and its hot water sits going between the supply and return through the unrestricted tee.

This past weekend, with the first dip into the twenties, we tested without the kick-space blower on and found that from 64 to 69 took five hours.

Further examination led me to use tape on several points of copper and PEX and use my IR thermometer to gauge actual system temps. During firing, in order of exit from the boiler:

• A - Copper Supply line as it exits boiler - 170
  
• B - Copper Primary line just downstream from 1st tee - 169.5
  
• C - Copper Primary line on other side of expansion tank and air vent - 169
  
• D - Copper Primary line 10" above Primary circ pump - 169
  
• E - PEX Primary line 20' away - 147
  
• F - PEX Halfway point of loop - 145
  
• G - PEX Primary line 20' away on return - 135
  
• H - Copper Primary return ~4' away from return line to boiler - 172
  
• I - Copper return line 3" from entrance to boiler - 172
  
• K - Copper common pipe between supply/return - 172
  

It's not easy to take all these measurements and record them, all while the boiler is running, so there may be some room for error there. The takeaway for me isn't that the return is sometimes hotter than the supply, although that seems pretty obvious too, but that 20' upstream, it is 40F cooler.

I now realize (having watched extensive YouTube videos and read several blog posts and other documents on old-school expectations of new condensing boilers) that I may need to change the way I think of a heating system regarding setback at night and during work hours, but I still feel that something is off here.

My main questions:

• Can this boiler work without the primary secondary setup?
  
• If not, is the tee spacing critically important? - it is not what the manufacturer recommends
  
• Is gravity circulation a possibility?
  
• How fast is too fast for a boiler temp to climb running at a claimed
• How fast is too fast to cool off?
  
• Is there a rule of thumb for pump speeds, like integral low/primary high?
  
• Is the CRC200 a better fit for this system than a simple thermostat switch?
  
• What is my next step?
  

If you made it this far, I thank you very much. I can't believe I wrote this wall of text. I look forward to hearing from some of you with your ideas and tips.

Pics and some extra info below

hot_rod

Quite a few of your questions can be answered by reviewing the piping drawings in the manual, including dimensions for the closely spaced tees, etc.

At first look I'd say the P/S tees are too widely spaced.

https://s3.amazonaws.com/s3.supplyhouse.com/product_files/ZWB-42-3-install.pdf

Rich_49

Could you include a wider picture also ? We can probably provide much more information seeing all the near boiler piping . In particular , where do the pipes from the farthest right and left tees go if this is a one zone system , should those tees not be 90*s or is it me ?

EBEBRATT-Ed

The spacing of the tees isn't that critical. I believe your pumps are incorrectly sized.

You have a pump sized for 17 gpm on the house side and an 8gpm pump on the boiler side.

Your only moving enough water through the boiler to get a 40 degree td. But you are using baseboard for heat. This means that your average water temperature is running about 150 which may not be enough when the weather gets cold and it will make the system slow to respond.

I think the designer/installer didn't pay attention to what he was doing.

Start from scratch. Do a heat loss of your home. The boiler is plenty big enough but we need to determine how much baseboard you have. Count how many feet of finned element baseboard you have. That will determine how much water needs to be pumped. Post a sketch of the piping if you can. that will help

Gcracker

Rich said:

Could you include a wider picture also ? We can probably provide much more information seeing all the near boiler piping . In particular , where do the pipes from the farthest right and left tees go if this is a one zone system , should those tees not be 90*s or is it me ?

Ironman

Mod/con boilers work more efficiently when they are allowed to modulate as low as possible for as long as possible. This requires the outdoor reset function to be properly setup.

Your probably loosing at least 20% efficiency, while actually sacrificing comfort, by not using the ODR function. Using setback with your thermostat actually compounds the issue; that's for non-modulating equipment.

Connect the OD sensor, get the curve set right, and don't use setback and you'll be much more comfortable and efficient. What you're doing now is defeating the design of the boiler.

The onboard pump in that boiler is only good for about 45k btus when connected to high temp emitters.

Have you done a scientific heat loss calc?

Gcracker

I have eliminated the setback for now with "hold" on my thermostat, and also as an experiment, set the zone pump back to its "2" setting and the boiler pump to the equivalent of 50% of the heat zone flow, for a theoretical increase of flow through the boiler loop of 2x the flow through the p/s bridge.

Al that is probably pointless without the scientific h/l calc you mention, and I am getting to that this evening (busy week). I have some basic figures from last week, but was using calculation placeholders instead of my actual figures. Will post back here this evening.

Thank you all for your comments and suggestions.

How much can the spacing of tees affect this setup?

Ironman said:

Mod/con boilers work more efficiently when they are allowed to modulate as low as possible for as long as possible. This requires the outdoor reset function to be properly setup.

Your probably loosing at least 20% efficiency, while actually sacrificing comfort, by not using the ODR function. Using setback with your thermostat actually compounds the issue; that's for non-modulating equipment.

Connect the OD sensor, get the curve set right, and don't use setback and you'll be much more comfortable and efficient. What you're doing now is defeating the design of the boiler.

The onboard pump in that boiler is only good for about 45k btus when connected to high temp emitters.

Have you done a scientific heat loss calc?

Noel

In addition to the loop length, what is the diameter (you mentioned pex) and how much baseboard is in this loop. The temperatures indicate that you might not be moving enough water on the system side.

Ironman

As long as the Tees aren't more than 12" apart, no effect. Yours may not be perfect, but I don't see an issue.

Ironman

I'd go ahead and connect the OD sensor and set the curve to the factory setting for high temp emitters. Then you can tweek it as you go. You'll still be a LOT more efficient on the factory default curve than not using it at all.

Gcracker

Noel said:

In addition to the loop length, what is the diameter (you mentioned pex) and how much baseboard is in this loop. The temperatures indicate that you might not be moving enough water on the system side.

Here is a very crude drawing with baseboard relative locations and measurements. Sorry if it's tiny font...

Gcracker

Ironman said:

I'd go ahead and connect the OD sensor and set the curve to the factory setting for high temp emitters. Then you can tweek it as you go. You'll still be a LOT more efficient on the factory default curve than not using it at all.

In addition to setting the boiler temp to MAX, where would you recommend I try to land the pumps? Leave them at the high settings they were at when we bought the house, or is the primary pump likely too many GPM? Fluid dynamics and heat transfer laws are quite confusing to me.

If I am out of line asking, I understand - I've just been researching for weeks and not absorbing some of this knowledge...

PS, I don't have the ODR sensor, but I may ask the HVAC installer if he brought some of the supplies home from my setup. I thought they all came with the CRC200, if not the ODR.

Leon82

Its my understanding that anything under 400k btu ships with and odr sensor. They probably have a whole box filled

Brewbeer

Only 30 feet of baseboard plus the kick space heater? In the whole house? I would have put in more baseboard, it would really help with set back recovery and keeping supply temps lower.
Not sure of the output of your combi, but you are going to use a tiny fraction of its output with only 30 feet of baseboard.
The room by room heat loss cal would also be good info to post.
This zone might perform better if it were piped in a split loop configuration.

Noel

The 151,000 BTUH boiler isn't the problem. 30' of baseboard plus a kick space heater with a 35° loop temperature difference is the problem. You aren't moving enough water through the loop to get full output from the emitters.

Ironman

The toe kick has 1/2" tubing. It should not have been piped in series with the loop. It's restricting flow.

Set the system pump on high speed.

Gcracker

Ironman said:

The toe kick has 1/2" tubing. It should not have been piped in series with the loop. It's restricting flow.

Set the system pump on high speed.

Sorry for the poor diagram. The toe kick is not in series, but as a secondary loop off the 3/4".

Is there a calculation I should be using to set the pump speeds, or do I need more measurements/readings from rooms? I feel out of my element, but I need to find if the "tune" or setup is just wrong or if physical changes need to be made before my 1-yr protection expires and getting things sorted will be my own responsibility.

Before I even understood that condensing boilers are supposed to run cooler water, and that pri/sec piping was a thing, I just thought the plumbing was wrong, and not pulling through the boiler as it should. Now, given how much the boiler temp reading swings, I still think it would work better without the pri/sec, but that is not for my level of understanding to decide.

Basically, I need to be able to be in a position to head of any "just wait and see" requests before we get into the really cold weather, because as it stands right now, unless I have the toe heater blower on full, the temp of the house changes so slowly or not at all, that I don't think a -15F day (or week!) is going to be pleasant.

According to the boiler, it runs at 30% when it's on, and I haven't seen it do more than that. The temp of the boiler water comes up from 80F to 190F in less than a minute, so I thought, "Not enough actual flow through the boiler," but then I read more about pri/sec and now I'm not so sure.

I have eliminated setback, and there seems to be no problem maintaining temperature now, but we are too warm sleeping set at 67-69F.

Thanks all for your help. I will keep trying to learn so I can get the most of the eventual return visit from my installer.

Gcracker

Leon82 said:

Its my understanding that anything under 400k btu ships with and odr sensor. They probably have a whole box filled

What hardware other than the CRC200 and the ODR should I be asking to be returned to me?
I should be within my rights to ask for that hardware, right? Even if I wasn't the owner of the home yet when the Combi was installed?

neilc

so you're loosing 22*, 20 ft from the boiler supply,
sounds like a flow restriction in the house loop,
you only loose another 10 degrees before
getting back to the boiler return,(145 - 135)
is that pex kinked somewhere?
rads at 145? or 180?