How to set up my 'new' gas boiler

[Radian]

So now I've finally got some half-decent control over my heat-only gas boiler thanks to ebusd:

 

...now I just need to learn how to drive it 😂

 

While the electronics I've added hasn't transformed the 15 year-old boiler into a state-of-the-art heat source, it has enabled me to adjust the flow setpoint and monitor all its vital signs to assist with optimising the efficiency of the heating system. Now I have to confess that I find the electronics much easier to get my head around than the hydronics. I simply came to this with the rudimentary understanding that the efficiency of condensing boilers is inversely proportional to the return water temperature - which in turn is a function of the flow temperature and the ability of the heat emitters to dissipate the heat energy pumped through them.

 

Before I got digital control over the boiler, the lowest I could set the 'one and only' flow temperature was to several degrees higher than the temperature required for the Domestic Hot Water - otherwise the boiler would never reach the DHW set point. By just looking at the temperature of the DHW and how long the boiler took to achieve it, I had already determined that I needed a flow temperature at least 10^oC higher. Interestingly, setting the DHW thermostat to 55^oC and boiler flow to 65^oC was resulting in a average return temperature of 55^oC although the peak was as high as 60^oC with a probable efficiency hit according to the typical efficiency curve such as this one:

 

 

This seems to be a fundamental limit to the efficiency that can be achieved with a condensing boiler heating DHW? If you need water hotter than 55^oC  coming out of your taps you have to accept sub-optimal fuel use. After all, SWMBO's not going to be particularly happy with 30^oC bathing water in order to get a 10% energy saving.

 

However, now that I can partition the DHW and space heating processes with their own flow temperatures, I can start to see how low I can go with the radiators. But what how do I asses this? Presumably it's going to be a trade-off between how long the boiler fires for and the gas consumption over that period. The product must still be such as to achieve the heating demand. I can picture there being a ripple with varying frequency. And a response time that can be compensated for with timing adjustments. Oh, and weather compensation!  TBH I really haven't got a clue what to do next 😔

[SteamyTea]

Newton Law of Cooling can be modified to become a heating curve.

 

Play about with this to get a feel what happens when heating something.

 

 

Heating Power Curves To Radian.xlsx

[JohnMo]

9 hours ago, Radian said:

If you need water hotter than 55^oC  coming out of your taps

I thought you shouldn't have water coming out the taps hotter than 43 deg?

 

For efficiency treat the boiler in same way as you would a heat pump. Store at a lower temp. 

 

Or fire the boiler at 80 for a short duration. On your graph there is only 1% difference between a 53 and an 80 return temp. You may find the return temp doesn't alter that much, from the previous lower flow temps as you coil is more efficient at higher flow temps.

[S2D2]

Something I've been meaning to look at for a while, but never got round to, is where exactly the optimum lies from a cost perspective. From that graph alone you'd assume a boiler running 24 hours a day at the lowest temp to meet heating demand, but actually my boiler uses 100W extra electricity when firing/pumping, so that would be 2.4kWh a day, or around 7kWh in gas money. Clearly there is likely to be a point of diminishing returns but I haven't done the sums yet.

[S2D2]

Using some extremely questionable extrapolation from smart meter data I looked at a random heating cycle on the boiler to give a reference point:

• 45 degrees flow temperature
• 4kWh gas consumed
• Firing/pumping for 0.65 hours at 100W, so 0.065kWh in electic, or ~0.2kWh in gas money
• ~5% of consumed energy (in cash terms) is the electricicty consumption at this flow temp

I'll have a go at tweaking the flow temperature to get a flow temp -> efficiency function which could then be combined with the above graph for an overall optimum. As a side note, there are a massive number of assumptons here, my boiler is not in the heated envelope so I'm assuming that electrical input energy is lost. Maybe the pump instead loses energy to the flow water? I have no way of telling.

[Radian]

11 hours ago, SteamyTea said:

Play about with this to get a feel what happens when heating something.

 

I think I understand that except for what criteria make for the 'ideal'?

[Radian]

2 hours ago, JohnMo said:

I thought you shouldn't have water coming out the taps hotter than 43 deg?

 

 

Well it's rather subjective except that for vented HW cylinders (like mine) the water is supposed to be stored at 60^oC to prevent Legionella bacteria colonisation. In practice this would be very energy inefficient and can be addressed with occasional cycles at that higher temperature. Putting that to one side, 50^OC (not 55^OC which is actually the absolute lowest return

temperature I can get away with to heat the water to 50^OC at the cylinder sensor) is the setpoint I've been using but I will be lowering it little by little to see what minimum can be comfortably used.

 

2 hours ago, JohnMo said:

Or fire the boiler at 80 for a short duration. On your graph there is only 1% difference between a 53 and an 80 return temp. You may find the return temp doesn't alter that much, from the previous lower flow temps as you coil is more efficient at higher flow temps.

 

This is a good point - although one other thing I'm trying to achieve (I forget that I hadn't mentioned it) is to range rate the boiler down to ASHP equivalent power levels. It was originally at it's maximum 30kW but I've taken it down to 20kW and I'm aiming for 15kW. 80^OC is therefore off the table but I'm allowing 65^OC for DHW for now.

 

1 hour ago, S2D2 said:

my boiler uses 100W extra electricity when firing/pumping

 

Really? My boiler draws a maximum of 36W when at 66% modulation (its current range rated maximum) and falls to roughly half that at minimum modulation. The circulating pump draws 16W and the three valves are less than 1W each. But I agree, it shouldn't be forgotten.

 

[SteamyTea]

32 minutes ago, Radian said:

I think I understand that except for what criteria make for the 'ideal'?

That is where the 'testing for data collection" comes in.

 

 

[S2D2]

1 hour ago, Radian said:

Really? My boiler draws a maximum of 36W when at 66% modulation (its current range rated maximum) and falls to roughly half that at minimum modulation. The circulating pump draws 16W and the three valves are less than 1W each. But I agree, it shouldn't be forgotten.

Sadly yes, it uses around 5W idle and 105W when firing. Using smart meter data so accurate reading too. No idea where that's going. I've always thought it was a lot but the specs have it rated at 115W so it's performing "as expected", just inefficiently. It is 9 years old now though. You can probably just ignore it then, only 1-2% max in your case which may only become relevant at the extremes of flow temp.

[SimonD]

10 hours ago, Radian said:

This is a good point - although one other thing I'm trying to achieve (I forget that I hadn't mentioned it) is to range rate the boiler down to ASHP equivalent power levels. It was originally at it's maximum 30kW but I've taken it down to 20kW and I'm aiming for 15kW. 80^OC is therefore off the table but I'm allowing 65^OC for DHW for now.

 

Not necessarily, a 15 kW output boiler will still deliver a flow temp of 80 for your dhw and it'll be better for recharge time and probably efficiency. Just like you, as I've experience with my own installation, the problem is when there's a small differential temp between flow and store and the boiler just runs forever. Increasing this differential has made a big difference for me especially as I have a thermal store.

 

Then as per probably the main point of your OP, focus on the efficiencies of the heating system as that's where you gain the most . Personally, I'd start with a brief calculation to estimate the output of your radiators at various Delta Ts compared to your calculated heat losses at a variety of exterior temperatures, then do some calcs to look at the mass flow rates required at various temperature points to assess feasability compared to your current pipe sizing and circulating pump (is your pump single speed or does it have some modulation tied to flow/pressure and/or boiler modulation?). A bit onerous but possible to derive some clunky initial theoretical values to develop a curve. But as you've indicated, ideally you'd want some measure of temperature rise over time so that you can then begin to throttle back boiler output as the room approaches setpoint, with feedback to learn how quickly the measured space responds to a range of heat inputs. Rather you than me...and a process you've probably already thought through 😁

 

I'd also have a good look at balancing the system to minimise return temps, a process so often overlooked and somewhere you can experiement with different Delta Ts too.

[Radian]

16 minutes ago, SimonD said:

Not necessarily, a 15 kW output boiler will still deliver a flow temp of 80 for your dhw and it'll be better for recharge time and probably efficiency.

 

Ah, sorry, what I meant was that I'm trying to simulate switching over to an ASHP. This is jointly an exercise in squeezing as much as I can out of the gas and seeing if my house could feasibly be heated by an ASHP when the boiler inevitably pops its clogs. Flow temperatures in the 80's are therefore not a realistic proposition - unless heat pump technology has made huge advances since I last looked!

 

26 minutes ago, SimonD said:

I'd also have a good look at balancing the system to minimise return temps, a process so often overlooked and somewhere you can experiement with different Delta Ts too

 

I'm busy right now making a measuring device with a couple of digital temperature probes for this exact purpose.😃

[SteamyTea]

5 minutes ago, Radian said:

unless heat pump technology has made huge advances since I last looked

And the Ideal Gas Laws are rewritten in an Alternative Fact manner, I think we will be hard pushed to find a refrigerant gas batter than CO₂ for the foreseeable future.

One problem with heat pumps is that they cannot contain too much flammable fluid if they are inside the house.

 

 

From https://en.wikipedia.org/wiki/Ideal_gas_law

 

where:

•  is the absolute pressure of the gas,
•  is the volume of the gas,
•  is the amount of substance of gas (also known as number of moles),
•  is the ideal, or universal, gas constant, equal to the product of the Boltzmann constant and the Avogadro constant,
• B is the Boltzmann constant,
•  is the Avogadro constant,
•  is the absolute temperature of the gas,
•  is the number of particles (usually atoms or molecules) of the gas.

[Radian]

6 minutes ago, SteamyTea said:

I think we will be hard pushed to find a refrigerant gas batter than CO₂ for the foreseeable future.

Some tricky issues though, with a triple point at 4.2bar CO₂ can change from a gas directly to a solid which isn't an easy thing to circulate 🤔 Going to take some skilled installers to roll this one out.

[SteamyTea]

3 minutes ago, Radian said:

Some tricky issues though, with a triple point at 4.2bar CO₂ can change from a gas directly to a solid which isn't an easy thing to circulate 🤔 Going to take some skilled installers to roll this one out.

The ones that have been in service for a few years, initially had problems, but I think they dropped the pressure, which rather defeated the object, to make them more reliable.  Probably why they have not been taken up in the domestic market, don't want people fiddling with the controls to get an extra 5°C out of them.

 

The easy way to boost performance is to make them larger, or fit two separate units.

[markocosic]

On 03/02/2023 at 22:36, Radian said:

This seems to be a fundamental limit to the efficiency that can be achieved with a condensing boiler heating DHW? If you need water hotter than 55^oC  coming out of your taps you have to accept sub-optimal fuel use

 

Two ways to hear water cylinders:

 

Fully stirred vessel (heat the whole lot up gradually)

 

Plug flow (heat a small part directly from cold to finish temperature)

 

Conventional cylinders are designed as fully stirred vessels.

 

Combi boilers do plug flow heating so can benefit from list returns.

 

Some big commercial water heaters are heated using an external plate/pump in plug flow regime for efficiency, for power density, and for cleanability of the plate hx.

 

The YouTube "urban plumber" chap has an example of a cylinder heated by a combi in this manner.

 

It's all moot though. You'll be binning the cylinder and replacing with one that has a larger coil and it heated as a fully stirred vessel when fitting a heat pump.

 

Unless you decide to convert the existing one into a fully stirred vessel heated via high flow rate external plate hx and pump. Probably not worth the hassle though!

[Radian]

2 hours ago, markocosic said:

It's all moot though. You'll be binning the cylinder and replacing with one that has a larger coil and it heated as a fully stirred vessel when fitting a heat pump.

 

Thanks. You've helped me realise I can't productively model the DHW side of things with my current setup. I shall focus on the space heating and try and save a few pennies on the HW with whatever it takes for now. The sun is shining as I type this so the PV divert is already doing some of the work and it's only the first week in February!