Insanely BAD efficiency and short cycling from a Buderus G115 boiler

jinbtown

Hey everyone
Friend of mine just bought a house, approx 2000 square feet ranch in climate zone 6 with OD design temp of -3F. 1990ish build, standard R13 construction. Buderus G115/4 boiler installed by previous owner a couple years ago. Standard baseboard. He has insanely short cycle times, like 45 seconds at the shortest, and 1.5 minutes at the longest from initial fire to high temp cutoff, and I'm saying the majority of the times I've timed it, it's been around 45-60 seconds, then maybe 3-5 minutes off while the pump circulates before the boiler fires again. He is microzoned into 4 zones with a single circulator pump and 4 stats. I think P/S piping is closely spaced tees but I'm not positive about that. There is an outdoor reset control on the Buderus that is currently inactive.

There is very little separation in the zones, not even doors between most of them. I'm guessing that a big part of this issue is zoning and I'm guessing that zoning is probably costing him fuel $$ per year vs jumping out the zoning panel and controlling the whole house off a central stat. The unfinished basement is also an entire zone. I don't have a lot of experience with oil, my background is EE in refrigeration so all sparks and pixies, not so much dinos and fire. Are these short cycle times going to reduce efficiency to the point where it's killing overall efficiency? Last winter we increased water delta T on the controls and killed the outdoor reset in a bid to get longer run times and I think, essentially saw little change in run times. It was late season so no change in efficiency that was calculable.

The other part of the equation is that there's an 80 gallon Buderus indirect tank sitting under the boiler unused, since he switched to a HPWH. Would piping this in as a buffer tank (since it's sitting there free) increase efficiency to the point where it was worth it to do? I probably have 90% of the copper sitting in my basement so only investment would be time.

What say you? Delete zoning only? Pipe in the 80 gallons as a reverse buffer tank? Both? We don't have super cold temps here yet but we are anticipating some major frostiness in 8-10 days so I want to have a game plan in place before that hits.

thanks in advance for your help!

Jamie Hall

I'd bet that that boiler is oversized for the house, for starters -- that's almost 50 BTUh per square foot. I wonder what the heat loss really is?

Second, I wonder if there enough baseboard in the house to absorb the output of that boiler. How many total feet of baseboard is there?

Never mind the microzoning, which isn't going to help at all.

Yes, using that big tank as a buffer will at least extend the run times to something more reasonable. Reducing the number of zones -- down to one, if the house is really open floor plan or two, keeping the basement separate will also help.

I don't see anything about outdoor reset for the boiler. Does the outdoor reset control a mixing valve?

jinbtown

Jamie Hall said:

I'd bet that that boiler is oversized for the house,

Sure is, based on fuel oil usage last winter - full year 664 gal 664*.80*138700
73-80mbtu (80% - 87% eff)
29,500 BTU at 65 balance.
So yeah, grossly oversized, but it's only a few years old and swapping it out probably isn't in the budget.

I can get the baseboard info more exact, but definitely not enough, just spitballing, maybe 150 ft. Guess what one zone is? Yep, a 12x12 bedroom with maybe 10 ft of baseboard in it...

Outdoor reset I haven't messed with since last year, it reduced water temps based on outdoor temps. Perhaps more accurately called an economizer? I'll check the exact model number next time I stop over there.

bburd

664 gallons for a full winter seems pretty reasonable oil use for a house that size in climate zone six. Especially for a single story house with a big roof.

Steamhead

Outdoor reset (ODR) would be my first step, since if the control is there (standard aquastat or HydroStat?) it's fairly easy to hook up.

jinbtown

bburd said:

664 gallons for a full winter seems pretty reasonable oil use for a house that size in climate zone six. Especially for a single story house with a big roof.

It doesn't seem crazy. The thought right now is that the boiler hasn't broken because it's basically new but will if it keeps running for 60 seconds at a time. Then 664 gallons and $3000 will seem like a small expense compared to $12k for a new boiler

Steamhead said:

Outdoor reset (ODR) would be my first step, since if the control is there (standard aquastat or HydroStat?) it's fairly easy to hook up.

Ah thanks for that name drop, it's a Hydrostat. I think 3200 or 3250. The economizer function is turned off to have control over water Delta T. when ODR is on, Delta T's are automatic. Honestly, I don't think he saw much difference in fuel usage or run times with ODR on vs off.

Hot_water_fan

You probably know the levers at this point, but run times increase as the mass increases and/or as the delta T increases. So you can pull either of those - if it’s easy enough, 80 gallons of buffer tank would go a long way. Lower temperatures are better too. While wildly oversized, it’s about as small as oil boilers get unfortunately. Hopefully you can see some results! Standardized by heating degree days preferably . 

Steamhead

What's needed here is a way to tell the HydroStat that the indirect zone is calling. Then you can use the Economizer feature.

The easiest way I know of is to take a small RIB relay such as this one:

https://www.functionaldevices.com/product/ribu1c/

and wire it up so the coil activates when the indirect circulator is powered up. You may need to install a junction box in the indirect circulator cable if there is no room in the zone relay panel.

Poke the NO wires from the relay (yellow and orange in this case) out of the panel or box thru a bushing or Romex connector, to prevent abrasion. Run a thermostat cable from these to ZC/ZR on the HydroStat.

This will disable the economizer feature whenever the indirect calls, allowing the boiler to reach full temperature to heat up the tank.

The HydroStat manual is here:

https://hydrolevel.com/wp-content/uploads/2019/11/HydroStat-3250-Plus-Instructions-web-111919.pdf

jinbtown

Hot_water_fan said:

You probably know the levers at this point, but run times increase as the mass increases and/or as the delta T increases. So you can pull either of those - if it’s easy enough, 80 gallons of buffer tank would go a long way. Lower temperatures are better too. While wildly oversized, it’s about as small as oil boilers get unfortunately. Hopefully you can see some results! Standardized by heating degree days preferably . 

Do you think standby losses are negligible with that 80 gallon buffer? I guess it's all going into the envelope, but this part of the basement isn't finished or living space. no insulation in floor joists but probably losing some heat through rim joist (another project) and bare concrete walls.

Steamhead said:

What's needed here is a way to tell the HydroStat that the indirect zone is calling. Then you can use the Economizer feature.

The indirect is not plumbed in, he has a heat pump water heater. It's just sitting there empty. I'd obviously like to add ODR back in but not at the expense of short run times. the hydrostat prevents you from setting water delta T if the economizer ODR is turned on. So we turned it off last year to try and get longer run times with higher water temps.

Steamhead

Try this, then- it's not expensive:

https://www.supplyhouse.com/Hydrolevel-48-140-OS-100-Outdoor-Sensor-Kit-for-HydroStat-3200-PLUS-and-3250-PLUS?gclid=Cj0KCQjwy5maBhDdARIsAMxrkw39vfXmF129-lSPoA0VycdToaD6ECZ8ZmjrNDMA1Be9ByiKUm9XITUaAnAkEALw_wcB

Hot_water_fan

Do you think standby losses are negligible with that 80 gallon buffer? I guess it's all going into the envelope, but this part of the basement isn't finished or living space. no insulation in floor joists but probably losing some heat through rim joist (another project) and bare concrete walls.

They’ll be pretty low, and the lower the water temperature is, the lower the losses will be. See if you can figure out the heat loss / linear baseboard length and what water temperature that corresponds to. 

EBEBRATT-Ed

@jinbtown

If it was my house I would find a way to make that boiler work...no guessing.

Do a heat loss of the house
see if there is enough baseboard to support the heat loss...and at what SW Temp?

only then can you decide what the right procedure is, buffer tank....outdoor reset....more baseboard....different control or pumping/piping?

Why guess? it's not Majic and you want it to work.

Start from scratch and find the issue

as @hotrod always says the heat emitters drive the system

If you guess you will go around in circles

jinbtown

Steamhead said:

Try this, then- it's not expensive:

https://www.supplyhouse.com/Hydrolevel-48-140-OS-100-Outdoor-Sensor-Kit-for-HydroStat-3200-PLUS-and-3250-PLUS?gclid=Cj0KCQjwy5maBhDdARIsAMxrkw39vfXmF129-lSPoA0VycdToaD6ECZ8ZmjrNDMA1Be9ByiKUm9XITUaAnAkEALw_wcB

he has an outdoor temp sensor and the hydrostat is set up for ODR, just has to be turned back on if we want that option.

Hot_water_fan said:

Do you think standby losses are negligible with that 80 gallon buffer? I guess it's all going into the envelope, but this part of the basement isn't finished or living space. no insulation in floor joists but probably losing some heat through rim joist (another project) and bare concrete walls.

They’ll be pretty low, and the lower the water temperature is, the lower the losses will be. See if you can figure out the heat loss / linear baseboard length and what water temperature that corresponds to. 

ah super good thinking. I might propose that we pipe that in as a reverse buffer. I'll do some calcs on the linear baseboard and get back to you.

hot_rod

Does Buderus have a minimum return temperature? If so the ODR would help but only to a point
The buffer idea is a good option since you own it. As I recall that tank has a 1” coil so it could be used as a reverse indirect 

Id pipe it as a 2 pipe buffer for direct to load capability 

That 5500 btu zone room is what is the motivation for a buffer

Zman

Outdoor reset is going to make the short cycling worse. The lower water temps will reduce emitter output making it even harder for the boiler to shed heat.

A wider on/off differential or a buffer tank are the only easy fixes

jinbtown

hot_rod said:

Does Buderus have a minimum return temperature? If so the ODR would help but only to a point
The buffer idea is a good option since you own it. As I recall that tank has a 1” coil so it could be used as a reverse indirect 

Id pipe it as a 2 pipe buffer for direct to load capability 

That 5500 btu zone room is what is the motivation for a buffer

Return water temp is specced at 150. Makes a lot of sense to use the buffer to me too, was hoping to avoid the work since he's not much of a DIY'er. I'm going to get all the pertinent info on how much emitters he has and then get back to y'all.

Jersey2

I can't imagine a 2000sf ranch needing 6 zones. Does he rent out rooms? What I think happened is that 5 zones were satisfied, and a tiny zone that holds hardly any water was calling. Maybe he can change it to have two or three zones, and in essence the other zone areas as the buffer. He can have his bedroom and 3 other rooms on one zone for example, and the living room and another area, like the basement, on another zone.

HVACNUT

jinbtown said:
 He is microzoned into 4 zones with a single circulator pump and 4 stats. I think P/S piping is closely spaced tees but I'm not positive about that. 

P/S piping and only one circulator? Need pics to see how its actually piped.

DJD775

Probably the easiest thing to try first is to reduce the amount of zones. Figure out what zones make sense to combine and wire the existing pumps or zone valves that you want to combine to a single thermostat.

hot_rod

Combining zones would help some, I think your boiler is twice the size it needs to be fit that size home? 

Your homework assignment was a load calc and measure the heat emitters, how’s that coming along?🤓