Efficient gas boiler with real-time data

Efficient gas boiler with real-time data(emoncms.org)

47 points by pandora-health 2 years ago | 43 comments

I would like to share with you the real time performance of my condensing gas boiler, managed by SAT and OpenTherm.

Many gas boilers are oversized which means it's more difficult to run them efficiently. Also, many gas boilers have capability to run more efficiently but are not configured to.

This particular boiler does not seem to respect the MM=0 command (despite OpenTherm's such specification requirement) which is not uncommon. SAT sends the MM=0 command for the boiler to work at its minimum capacity. CS= minimum_setpoint and then PWM controls the ON/OFF times of the boiler. However, I have a workaround which ensures that most of the time, the boiler is indeed working at its minimum modulation, unless the target flow (heating curve + PID) demand is high [higher than current flow temperature] during warm up.

The boiler supplies heat to the central heating and domestic hot water tank, but not at the same time. The hot water tank capacity is 255 litres. When DHW call is made, boiler ignores OpenTherm instructions and runs at 80 C degrees. There are three zone valves in the house: Hot Water, Upstairs and Downstairs (not talking about TRVs here). When DHW call is made, Hot Water valve opens, and Upstairs & Downstairs close; then if CH call is made, Hot Water valve closes and the other two open.

The hot water system is also fitted with "Waste Water Heat Recovery System". In essence, the heat energy extracted from showers and baths is used to preheat cold water that is then sent to the hot water tank. The extraction recovers about 60% of heat normally lost. You can watch examples of how it works here - https://www.youtube.com/watch?v=G-jKlCoRvEM

SAT project - the brain behind the efficiency https://github.com/Alexwijn/SAT

OpenEnergyMonitor - for making possible to share real time boiler data https://community.openenergymonitor.org/t/my-gas-boiler-data...

Any questions, let me know :)

blitzar 2 years ago |

> The hot water system is also fitted with "Waste Water Heat Recovery System"

I really want one of these - the >40C water going down my drain being replaced with <20C water into my hot water tank keeps me up at night (figuratively). Such an easy win to exchange some of he heat out. It is on my shopping list for the next house / next renovation.

You should add dhw water temperature sensors to the mix too.

alliao 2 years ago | |

I've stopped worrying about it, I now use the heat with a packet of bio-enzyme to help dissolve any random bits of lard blocking up the waste drains, this is one of those unintended consequences scenario. I recall reading about California sewage got clogged up when enough people bought low flush toilets but the bigger street sewage pipes were never designed with so little flow.

NoMoreNicksLeft 2 years ago | |

Can substantial heat be recaptured? The sink drain's often pretty far away from the hot water heater in most homes. There might even be multiple faucets (not to mention shower/tub drains) in the mix. Seems less than feasible, but my intuition's often shit at figuring this stuff.

blitzar 2 years ago | | |

You can preheat the cold flow into the shower so it is just local within the bathroom. Your hot water tap flows at the customary ~55C and the cold feed to your shower gets a boost to 25C or something.

Or you could send all the waste heat to a big sump and extract it with a heat pump (dont think that actually exists though - or would be some form of ground source heat) - at extreme temperatures would be worth it.

rainbowzootsuit 2 years ago | | |

If you recover heat to the cold supply then you need less hot supply to get to the same outlet temperature. If you have an updated fixture with a thermostatic mixing valve then I think this would track pretty much seamlessly, but I've not ever had the opportunity to use a shower equipped this way.

btbuildem 2 years ago | | |

Something as common as automotive coolant/"antifreeze" could efficiently travel via thin lines alongside regular plumbing.

I think a big part of the challenge would be in having multiple heat exchangers. Perhaps (if plumbing codes allowed) a plain p-trap could be swapped out for an exchanger unit that also serves as a sewer gas stop.

I don't know anything about thermodynamics; how long would it take to transfer a reasonable amount of energy from waste water into the coolant?

Another issue again, running all these coolant lines back to a central water heater w/another exchanger. It does all sound pretty complicated and expensive.

pandora-health 2 years ago | | |

There are three showers and two baths in the house. All upstairs, which allows for the pipes to recover some of the heat on the way down. This excludes sinks where you most likely to use cooler, if not cold water. I take the maximum efficiency of heat recovered with a pinch of salt, but the hot water tank seems to last for longer heat-wise, compared to the one I had in the previous house, without such system.

choffee 2 years ago |

It feels like that is the least that is required at the moment. There should be more effort getting our current boiler stock up to a lot closer to it's theoretical COP of 1. At least until they can be replaced with something more efficient. Some focus on tuning boiler settings for performance, maybe some aftermarket controllers to do some of this and display the results.

That recoup thing looks really good it always seems a shame to let all that heated water go to waste.

pandora-health 2 years ago | |

I agree, most boilers, left without proper tuning are running below 90% efficiency, most likely about 80%. A 10% increase at the country level would be significant.

throwway120385 2 years ago | | |

I've always wondered if my boiler is set up correctly. I have a closed loop hydronic heater and an indirect-fire tank and when I shut off water to the house ahead of a winter storm I noticed the boiler controller was throwing warnings about short cycling. I'm guessing there's a procedure to tune it correctly but it was probably never followed by the HVAC company since it's very unusual to have a hydronic system with boiler in the US.

blitzar 2 years ago | | |

As someone with their heat set to a comfortable 18.5C you could also knock a few degrees off the 23C room temperature and save a few of the ice caps.

hn_throwaway_99 2 years ago |

When anyone is talking about a "100% efficient" anything, I really wish they would clearly define what the numerator and denominator are. I just looked at your site and it said "103.4% efficiency in window". Which is obviously bullshit for how most people define "efficiency".

I think it's fine if you define efficiency by some other metric where it can rise above 100%, but if so you should clearly elucidate how.

pbowyer 2 years ago |

Nice work! I'm in the UK and just about to get a new 4-pipe gas boiler fitted. Can you share how much adding SAT and OpenTherm cost? I've got HomeAssistant up and running.

Edit: Also, is this a new build or did you retrofit your waste water heat recovery system? I'd like one but in my house the bathrooms are on opposite sides of the house upstairs, so would need to fit one in each and the costs don't stack up. If I built a house with the bathrooms backing on to each other it would be a no-brainer.

pandora-health 2 years ago |

Here are two other boilers I found for comparison:

https://emoncms.org/app/view?name=GlowWormBoiler&readkey=88a...

https://emoncms.org/app/view?name=TomsBoiler&readkey=484aa31...

boringg 2 years ago |

I have a question for OP. On a total energy used perspective how much are you saving per annum on this setup?

I have no doubts about the interest level of it (kudos!) -- and I am curious what the total gains are versus LOE. I have a renovation coming up and I have interests in putting together a lot of custom work but its seems like it would be a labor of love (though it might be end up a project of frustration :) ).

pandora-health 2 years ago | |

Thanks for your question! It’s definitely been more about passion than payback. The energy savings are there for sure, but for me, the real value has been in the learning and doing as well as the result, house climate control with constant comfort.

I will have to wait for a year to compare the data of total gas cost with the previous year.

If I was renovating and starting from scratch, I would probably invest in a ground source heat pump, which can offer climate control in summer and winter.

boringg 2 years ago | | |

I figured something along those lines - great project. I always find it amazing how much more efficient we can make home energy systems but there is no economic incentive or level of talent in the market to make it work.

I mean at the end of the day home energy is more about comfort then efficiency and is relatively small money economics.

calvinmorrison 2 years ago |

Did OEMs give away efficiency for reliability (eg, cycling things more frequently?)

pandora-health 2 years ago | |

No, I haven't seen that data for my boiler but it would be useful. Even though cycles might be shorter, note that the temperature that the whole system is exposed to is much lower on average. A typical boiler will run at 70 degrees, compared to mine at about 40-45.

topspin 2 years ago | |

Almost certainly yes. It's entirely possible to make frequent cycling reliable, but it costs more. Higher cost is bad when the decision process is essentially "sort by price and pick the lowest."