Help me understand mod/con boilers and load

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Metal Man Greg

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Hello to the forum.

I am not a heating or plumbing pro.

Recently my system wasn't working and I was researching all the info I could, I ended up reading several threads about boiler sizing, I have developed a desire to learn more and possibly tweak my system to improve it's performance.

I have a Monitor Mz 25c commissioned in 1999. Through the years I have done only a little maintenance and repair and it has performed fairly well. The boiler supplies 5 zones in approximately 3200 sq ft.

I believe my load is in the 65K range. The boiler is rated at 94k input with max efficiency of 95%.

The current plumbing is output from the boiler to a manifold with 5 zones and 2 stubs for future zones. In floor 1/2" pex, 1-1/2" gypcrete. Some carpet, some tile, some wood.

The boiler has an internal circulator and there is a second circulator in the system. Individual zones do not have their own pumps.

So my first question is:
- My condensing boiler has an analog temp control and I was wondering how an outdoor reset could be added to the mix and would I benefit? My output water temp is currently set at 115-120F. Based on the (my) systems ability to relase heat into the living space in the 2 larger zones, a slightly higher water temp seems desirable during cold snaps.

Next
- I suspect that in 3 of my 5 zones demands are too small for the boiler's output or the ability to transfer heat within the zones isn't great enough. I suppose a simple answer could be to combine the zones so as to increase the demand, but that sort of defeats the original intent as some zones are preferred to be less warm. Is the addition of a storage tank something that might be relevant to this kind of issue? How does primary and secondary piping relate to the described system?

- When my boiler was down I researched some mod/cons. Hypothetically, if one's load was 50K, then that would be all the output needed. If a mod/con with a 100K max out and a 10:1 turn down were installed, would that boiler be as efficient in supporting the 50K load (aside from initial cost) as a 50K max out with a 5:1 turn down?

Some of these questions are from my thoughts about short cycling. Is short cycling in the system I described more of a shoulder season phenomenon? I don't know if it's a problem, but from time to time it seems the burner cycles somewhat frequently.
 

Dana

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If your output temp is set to 115-120F, your return temp is something like 100-110F, and there's little left to be gained efficiency wise by going lower using outdoor reset. Bumping it up 5F isn't going to reduce efficiency by much, and bumping it up to 10F it'll still be running in the condensing zone, north of 90% efficiency. If it's still in good shape there's no "payback" on replacing it with a mod-con boiler, but given that it's already a teenager it's not insane to be scoping out the possible replacement equipment.

With gypcrete radiation the radiation itself has substantial thermal mass. With a low-mass condensing boiler you'll be fine with burns as short as 3 minutes, but not with burns of 30 seconds. Turn down the T-stats on all the zone except one of the smaller zones, then time how long or short the burns are when the boiler is serving just that zone. If it's under 3 minutes you can think about tweaking the high/low limit differential to make it bigger, or adding a buffer tank, but let's not get ahead of ourselves here.

What makes you think the load is as high as 65K?

A 2x oversized boiler 10:1 turn down with the same min-fire output as right-sized boiler a 5:1 turn down can indeed be set up to run at the about the same seasonal efficiency. But they all need to be dialed-in to get that efficiency.
 

Metal Man Greg

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Hi Dana,

Thanks for the quick response. Your posts in some other threads have been helpful as well.

The 65K load was from a quick calc done on USA boiler website IIRC. It also seems to be in line with what I remember from a J form done before construction, but not sure.

When I was looking at the online form, the biggest variable seemed like it as air exchanges per hour. I know I haven't sealed everything up as well as it should be. It appears that's going to be the number one energy saver and comfort improver for me, so I will put it on the short list.

I had some mud wasps build in my concentric intake. That caused the exhaust high limit to trip and shut down the boiler. It took me a while to locate the problem and the switch, but in the effort I cleaned the exchanger and condensate trap.

We had a cold weekend with 2 overnight lows of under 30F. Well Sunday was when I got the boiler back online. It warmed things up fine by late afternoon. Monday was significantly milder and when I returned home at the end of the day much of the house was around 72. The stats were typically set under 68. I suppose the thermal mass from the prior days heating needs had a latent effect, but that prompted my thoughts about outdoor reset, for both higher and lower swings.
 

Dana

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Most boiler manufacturers' load tools are fairly crude I=B=R method variants, that will overshoot reality by 15-35%, sometimes more. You have a pretty good instrument for MEASURING the heat load- the boiler itself. If you have some mid to late winter gas bills (or propane fill-ups) with the exact meter reading dates (not the billing dates) and quantities, you can look up the heating degree-day information for those periods to correlate fuel use to heat load. It's fairly straight forward to then make a linear approximation of the heat load at any arbitrary outdoor temp based on the BTU per degree-hour constant, which can be derived with 5th grade arithmetic.

With high-mass radiation it sometimes takes a PID algorithm thermostat to limit the under & overshoots (Tekmar has several variations of that theme). Outdoor reset may help, but it's not a perfect solution.
 
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