munchkin boiler set point question for you guys

[TT_Vert]

i use a munchkin boiler to heat a 2K sqft garage and a 600sqft bathroom floor. Both also have dedicated forced air HVAC systems which i rarely use. In winter I have the bathroom floor (referencing a slab sensor) to 90 deg. at the sensor. This gives a nice warm floor for me. In the past I "believe" I've always been able to achieve this in very cold Illinois winters. This year I'm noticing the slab temp won't go above 85. I checked set point and it was at 135 for some reason. Now I don't know if it's been this way or I somehow changed it by power cycling the unit at point. Now on this model the default set point is 180 so i cannot see that being the case. My major question is, is there a reason the HVAC tech would have originally set this to 135 deg. if this was in fact how it's been for years? I've checked the inlet and outlet temp at both a gauge and at the unit temp sensors and everything seems to be following the programming as set. I'm noticing a lot of cycling going on as the delta temp is 20 degrees so it hit the 135 outlet water temp, lower to ~115 and repeat the process quite often. I did some testing by increasing the set point to 150F and the slab has already gone up 1 deg. I don't want to go back to the default of 180 until I pick your brains as to any reason a tech may do this. The tubing is standard PEX as used in these systems w/ the same manifold systems I've seen in others.


My initial thought was my zone value may be acting weird and not opening just allowing the water to deadhead and not really heating up the water on the other side but given I'm at 85deg at the slab and the other side of the valve is hot I know that's not the case. Pump pressure is 12PSI, never noted what it was previously but 10-25 I believe is spec pressure for this system.


My though to prevent this constant cycling is:
1. Reduce slab temp to a maintainable temp given the 135F set point
2. Increase set point to default of 180

Thanks for any input you may have.

Dave

 

[Jim greengo]

When was the last time the boiler was serviced/flushed out?

 

[TT_Vert]

If by flushed out you mean serviced, just today i tore it down, cleaned the boiler, coils, condensate line and flame rectification probe. Nothing looked out of the ordinary and very clean as I service it annually. Flame pattern looks good and no other mechanical issues.

Dave

 

[Jim greengo]

Yep,that's what I was referring to.
Is the thermostat reading slab temp or air temp of the rooms?

 

[nadogail]

Curious about "munchkin boilers" were they built for heating the set of The Wizard of Oz?

 

[Shiftless]

I’m even more curious about a 600 sq. ft. bathroom.

 

[TT_Vert]

Yep,that's what I was referring to.
Is the thermostat reading slab temp or air temp of the rooms?

Slab sensor as far as I know. I've not traced it but it's under the door somewhere.
Dave

 

[bmwpowere36m3]

Is the temp, floor or air, being satisfied? How is it short cycling, thermostat or sensor temp satisfied or is it meeting the set point temp?

If it’s the latter and it wasn’t doing it before... I’d guess a flow issue, either thru boiler or in system. Would need for info on boiler and system loops and how it’s plumbed.

 

[TT_Vert]

Is the temp, floor or air, being satisfied? How is it short cycling, thermostat or sensor temp satisfied or is it meeting the set point temp?

If it’s the latter and it wasn’t doing it before... I’d guess a flow issue, either thru boiler or in system. Would need for info on boiler and system loops and how it’s plumbed.

With the set temp at 135 it only satisfied water outlet temp out of boiler at this outside temperature (~25F). I set the set point to the default of 180 as a test and it is satisfying the slab sensor which is good and not continually cycling to try to reach that 90F slab temp call. I guess my question would be WHY was the set temp so low? What are the economics and physics behind the two? I started to think about it and I don't know if it's better to continually cycle w/ the lower set temp (Outlet water temp) or to increase the set temp to factory default 180 and let it eat. It will certainly not cycle as much as it is satisfying the heating call but it'll run longer to satisfy that 180 deg outlet temp. The way this unit works is it will run until it satisfies the defined set temp then shut off until the delta temp (In this case 20 deg) is met. So at 159 it'll turn on again and repeat until the heating call has been satisfied. So needless to say when the set temp was 135 it was constantly cycling the water temp from 114F to 135F and never satisfying the thermostat which utilizes the slab sensor which does make sense due to heat losses.


I'm fairly sure I do not have a flow issue as I am monitoring input and outlet water temp at the boiler and the inlet temp comes up to meet the outlet fairly quickly which tells me that warm water is flowing through the system, plus the floors are warm. There is one feed from the zone valve to this bathroom where the feed goes into the crawl space of the bathroom then goes into a wirsbo tee manifold. Then those two feeds zig zag through the underside of the floor and go back into another identical wirsbo tee manifold and return to the burner. All 4 tubes are hot so we have good circulation.

 

[bmwpowere36m3]

The boiler temp (outlet) needs to be as high as to satisfy the zone temp (whether air or floor temp). Part of the boiler set point is based on BTUs required and flow (# of loops, length and pump size).

If you want to run a constant circulation, low and slow... then you set the boiler temp as low as possible. The only downside, it won't be able to heat a space quickly. Though, if your constantly heating, then it isn't an issue.

Now, I wouldn't set the boiler at 180*... if you truly are getting a delta of 20* (boiler outlet vs. boiler inlet), then your flow is good. So you might have to raise the boiler temp as 135* is not enough to maintain the floor at 90*. Maybe set it 145* for a day and see how it goes.

 

[TT_Vert]

Thank you. I never need to heat a space quickly, it's always on. I have forced heat in each of these spaces if it is ever needed but the radiant heat flooring always keeps both at a stable temp. I just don't know if either economically or mechanically it's better to run it low and slow or hot and fast? I did raise the temp up a bit and lowered my floor temp to 85 and it's still maintaining a good temp in bathroom (72 at night) in 30 deg weather. I just seem to recall the floor being warmer last winter. Maybe i'm wrong there.

As far as inlet/outlet what i mean is this. The boiler monitors inlet/outlet temp. The boiler stays on until the predefined outlet temp is reached (In this case right now I think I have it on 155). It will then stay off until the predefined delta T is reached. I have delta at 20 so the outlet temp has to be at 134 or less for the boiler to re-ignite and start heating again to prevent constant short cycling.

Dave

 

[finn]

Don’t know what your system looks like, but mine has three Taco circulation pumps to cover three zones in my 48x75 foot shop.

One of the circulators failed, unknown to me, but the room temp was still ok.

Ends up that the way it was plumbed, there was enough parasitic flow from one of the other zones to keep the floor somewhat warm. It (the floor) was only something like seven degrees less than what I normally set it to.

I uncovered the problem when I couldn’t get a crisp outline of the tubes with the thermal camera while locating the tubes for the installation of the two post lift.

The tubes in the other two zones showed up relatively crisply, by comparison.

 

[TT_Vert]

Don’t know what your system looks like, but mine has three Taco circulation pumps to cover three zones in my 48x75 foot shop.

One of the circulators failed, unknown to me, but the room temp was still ok.

Ends up that the way it was plumbed, there was enough parasitic flow from one of the other zones to keep the floor somewhat warm. It (the floor) was only something like seven degrees less than what I normally set it to.

I uncovered the problem when I couldn’t get a crisp outline of the tubes with the thermal camera while locating the tubes for the installation of the two post lift.

The tubes in the other two zones showed up relatively crisply, by comparison.

I definitely don't have a pump issue. I am seeing 15PSI of line pressure and I see a rise in water temp on the inlet side shortly after the boiler ignites indicating that we have good circulation.

Dave

 

[finn]

I definitely don't have a pump issue. I am seeing 15PSI of line pressure and I see a rise in water temp on the inlet side shortly after the boiler ignites indicating that we have good circulation.

Dave

But do you have multiple pumps and multiple zones? In my case, three zones and three pumps. Any one pump will build pressure, but if one pump is out, that zone only got parasitic flow, and that low flow resulted in a low slab temp.

The others were fine.

There were no check calves in the system at the time.

 

[TT_Vert]

But do you have multiple pumps and multiple zones? In my case, three zones and three pumps. Any one pump will build pressure, but if one pump is out, that zone only got parasitic flow, and that low flow resulted in a low slab temp.

The others were fine.

There were no check calves in the system at the time.

Two zones, two zone valves, one pump. When a call for heat from one zone is called it'll open that valve and fire the pump/boiler. Pressure rises and water flows. This is confirmed on both zones.

Dave