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Efficient gas boiler with real-time data (emoncms.org)
47 points by pandora-health 66 days ago | hide | past | favorite | 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 :)

> 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.

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.

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.

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.

No, the hot water gets heated in the boiler which is usually not in your bathroom.

People who use tankless electric hot water heaters in their bathrooms to avoid waiting for hot water spent quite a bit for the privilege.

Recovering significantly energy without causing plumbing issues with clogged hair etc seems difficult however.

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.

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.

In vertical stacks surface tension makes flowing water cling to the perimeter of the pipe. The supply can then be coaxial like I think you are saying.

This is one product (linked below) like this although I think plastic would be fine if a little less thermal conductivity and less expensive than this much copper.


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.

Technology Connections has a pretty good rundown of how electric hot water heaters work.

You’re using hot water at X gallons per minute while the heater can heat Y<X gallons a minute but the water is extracted from the top of the tank so it stays fairly hot until you’ve used more than half of the tank, then drops like a rock at the end when the supply is exhausted.

Anything that warms the cold water going to your shower slightly reduces the rate of hot water use, which extends the amount of time before you run out, which is more time to heat more water.

Yes, that's what I noticed in the previous house, where the water tank was connected to a solar thermal panel. Solar thermal panel heats the oil inside it and the heat is exchanged between the water tank and the panel during the day when it's sunny. That system was about 60% efficient throughout the year but I did notice that the temperatures would go down really fast and that didn't feel efficient.

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.

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.

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.

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.

I use something called Summer Simmer Index, which means that my target temperature takes into account indoor humidity. I feed Home Assistant RAW indoor temperature, and SAT (the integration), converts it to SSI.


21.3 – 25 °C Somewhat cool. Most people feel comfortable. 25 – 28.3 °C Optimal. Almost everyone feels comfortable. 28.3 – 32.8 °C Somewhat hot. Most people feel comfortable.

I find it hard to believe that most people feel comfortable with 25 – 28.3 °C, how to come up with this numbers? Based on the link above? It seems rather hot. Also it would depend what the outdoor temperature is. But different people have different preferences https://comfort.cbe.berkeley.edu/ (comfort calculator)

I agree, 25 SSI adjusted and above begins to feel too warm but that's why I set it at slightly cool instead.

ahh 19/20/21 based off the chart - that is more reasonable. I probably would not have bothered with the conversion as humidity indoors is fairly consistent and it appears the index is more designed for summer / higher temperatures rather than winter clothing indoors etc.

Yes, it's between 40-60% on average. Here's a snapshot https://snapshots.raintank.io/dashboard/snapshot/OvZOk9eb6gL...

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.

Regarding gas condensing boilers and their efficiency, the notion of them achieving over 100% efficiency, as measured by the Lower Heating Value (LHV) of the fuel, really showcases the value of recapturing latent heat. When we burn natural gas, besides generating heat, water vapor is produced as a byproduct. Traditional boilers expel this vapor and its associated latent heat through the flue, wasting a significant portion of energy. Condensing boilers, however, are designed to cool the exhaust gases to a point where the water vapor condenses back into liquid, reclaiming that latent heat. This process effectively allows the system to extract more heat from the combustion than the calorific value of the gas would suggest possible, if you only consider the LHV, which excludes the heat contained in the steam. Consequently, when this extra captured heat is added to the overall heat output, it can push the efficiency of the boiler beyond 100% using LHV calculations.

> Regarding gas condensing boilers and their efficiency, the notion of them achieving over 100% efficiency, as measured by the Lower Heating Value (LHV) of the fuel, really showcases the value of recapturing latent heat.

At least in the US (not sure where you are), combustion boiler/furnace efficiency is required to be calculated using the HHV, which includes the heat of vaporization, not the LHV. The result is that even the highest efficiency condensing combustion heating systems all have rated efficiencies below 100%, which makes sense thermodynamically.

It seems that the standard in Europe is to instead calculate efficiency using LHV, resulting in the efficiency rating over 100% for condensing boiler. > 100% doesn't make sense thermodynamically, but is good enough for comparing when the same calculation is used.

Either way, it should be made more clear in the UI that it is using LHV.

To calculate the cost using a UK gas price we would want to have assumed an efficiency in HHV. This is because UK gas prices are given on an HHV basis. Either convention can be used as long as all of our fuel consumptions, efficiencies and energy prices are given on the same basis. Consistency is crucial.

> This is because UK gas prices are given on an HHV basis. Either convention can be used as long as all of our fuel consumptions, efficiencies and energy prices are given on the same basis. Consistency is crucial.

If your audience is global, perhaps consider adding a toggle to switch the efficiency calculation to HHV. Think of it like the °C/°F toggle on some sites and apps.

Could you integrate a heat pump or solar tank pre-heater to this system and calculate how much it saves on gas? especially if you have on-demand pricing for electricity. it would be interesting to see it dynamically optimize depending on the price per BTU or some other metric.

Yes, there are hybrid heat pumps that can work with gas boilers. Such system can help to reduce gas usage by up to 70%. I am considering it :)

The OP was too kind to you. Most furnace efficiency ratings come from calculating the input energy and the output energy. It is pretty obvious from the site that the calculation is a simple, energy in vs energy out calculation.

Maybe you should educate yourself some before writing posts you know nothing about.

I see where you coming from and even if the simple calculation is not what it is in real life, it's definitely running more efficiently than it would otherwise. Also, there are other benefits. Mainly, climate control, which means that the room temperature is very precisely at the target level at all times, within 0.3 C degree on average. And when you have constant temperature at all times, it feels very comfortable, and you can't tell whether heating is on or off.

> Maybe you should educate yourself some before writing posts you know nothing about.

Except another commenter, obviously more versed in the field than I, specifically highlighted how this efficiency is calculated differently based on what is specified as the denominator, and the rules are apparently different for the US vs. Europe: https://news.ycombinator.com/item?id=40055464. So perhaps it is you that should educate yourself.

My mistake, I thought it was more common knowledge how HHV and LHV are calculate and equivalents when comparing to heat pumps. I tend to respond too harshly when low level words are thrown. Sorry!

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.

Me too. Following the links, it seems it's an open source design [0] that's available as a fully populated PCB from the Netherlands for €30 [1].

The only other hardware requirement afaict is room & outdoor temperature sensors (for the Home Assistant component). But you probably have to be willing and able to wire it up yourself, or else ask around and maybe with a bit of luck find a willing plumber - I don't expect most would touch it.

[0] - https://otgw.tclcode.com/schematic.html

[1] - https://www.nodo-shop.nl/nl/48-opentherm-

Thanks! SAT (Smart Autotune Thermostat) is a free integration you can add to Home Assistant, which can work with your existing thermostat but the intention is that it replaces it altogether (SAT becomes your virtual thermostat). There are many more features but it's best to head to https://github.com/Alexwijn/SAT for details.

OpenTherm is a standard communications protocol which SAT uses, however, you need this hardware:

https://www.nodo-shop.nl/en/featured/211-opentherm-gateway.h... - board

https://www.nodo-shop.nl/en/our-products/213-wemos-d1-mini.h... - WEMOS D1 MINI

https://www.nodo-shop.nl/en/solder-service/202-soldering-ser... - soldering service

It costs €29.50 + €6.25 + €7.50 soldering service + delivery , so that it comes as a ready kit for you.

You also need an indoor temperature sensor. You can get a zigbee device, such as - https://www.zigbee2mqtt.io/devices/WSDCGQ12LM.html

For outdoor temperature, Home Assistant will cover that out of box.

The installation is usually very simple, you connect two cables to the boiler and its OpenTherm module in the boiler and then it uses MQTT to connect to Home Assistant via WiFi. If you are not comfortable touching the boiler, ask your plumber to help you.

By the way, that hardware is much cheaper than Hive which does not support OpenTherm at all. Google Nest does but is so extremely limited, and from my experience, it was cycling too often and did not autotune which is the main reason why you want SAT.

If you check the manufacturer of your boiler and its model, you should be able to find out if your boiler supports OpenTherm.

This is new build~ (2017) and the heat recovery system was installed when it was built but we are not the first owners of the house and personally, I would probably not try to retrofit such system.

That's way more reasonable than I expected, I'd looked at OpenEnergyMonitor hardware before and while nice it's not cheap.

My new boiler will be a Viessmann 100-W which looks to support OpenTherm, but some online posts say once you link it to an OpenTherm thingy you lose use of the controls on the boiler. I'll try and check that out.

Edit: Argh, nodo-shop.nl don't ship to the UK. **** Brexit.

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 :) ).

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.

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.

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

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.

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."


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