My Viessman boiler journey

[marshian]

Well I have a new HW tank to fit soon but I've been trying to optimise the HW a little more - Average consumption is 4kWh per day to do the water heating for two people occupancy.

 

Flow temp is typically 66 Deg C return temp peaks at 61 Deg C, 115 Litre tank and 30 Min Cycle with the tank ending up at 52 Deg C (I've tried the lower and slower approach and to be honest it actually uses more gas but I think that's because of overshooting the target temp when I did it)

 

However because of the X plan set up it does mean even in the summer I put a slug of 68 deg water into the rads straight after the HW cycle is finished. In winter no issues - it helps make up for the HW cycle time out of CH - summer it contribute to heat gain in the house......

 

I decided to do an experiment..........

 

If I run the HW for 30 mins and ten kill the boiler and continue to circulate the HW system with no boiler activity I can get an increase in the tank temp of 2 deg C and the water hitting the rads is down to 50 deg C

 

I'm now thinking about putting a delay timer on the HW valve closing 10 mins after HW cycle is completed to extract the last bit of energy out of boiler and circuit

 

Mad idea?

[marshian]

New Tank Fitted over the weekend

Grant Tank - 180L 2.7m2 coil surface area 

 

 

couple of errors in pipework configuration that are resulting in increased heat loss but I'll fix them over the next couple of weeks.

 

meantime a quick experiment on boiler efficiency (compared to old tank data) revealed

 

Boiler efficiency

 

Old tank when heating HW - efficiency was 88.97% (condensate volume 157 grams - condensate stopped after 10 mins of a 30 min cycle due to return temp being above 54 deg)

 

New tank when heating HW - efficiency is 93.29% (Condensate volume 332 grams was generating condensate all the way thro the 45 min cycle)

 

Can show workings if required....... 

 

Heat Loss

 

Old Hot water tank fully lagged pipework - london loops everywhere (U loops on any hot outlets from tank) best I could get in term of temp decay in the tank was 0.4 deg per hour (most of the time it was 0.5 to 0.6 deg per hour) so heat a tank to 50 deg and 20 hours later it would be down to 40 deg (thats a just bearable shower according to SWMBO)

 

New Tank - no lagging on pipework yet and minimal london loops and the tank losses are down to 0.19 deg per hour - so heat a tank to 50 deg and 24 hours later it's still over 45 deg - I think I need a better hobby!!!!

 

PS Yes I need to fit a tank stat

 

PPS The tank is set up as a vented tank (so no G3 certification is required) 

[marshian]

Was having a conversation with a mate on whatsapp and he decided to put part of the comments above into AI

 

Yes — those numbers suggest the new cylinder is substantially better, not just marginally better. The nice thing is that his temperature-decay measurements let us put sensible numbers on it.
For a hot-water cylinder, a first approximation is
\[
Q = m c_p \Delta T
\]with water \(c_p \approx 4.186\text{ kJ/kg·K}\). So, for every 100 litres of water, a cooling rate of 1 °C/hour corresponds to about:
\[
\frac{100\times4.186}{3.6}=116.3\text{ W}
\]Therefore, at his measured rates:
Cooling rate    Heat loss per 100 L    200 L cylinder equivalent
0.60 °C/h    69.8 W    140 W
0.50 °C/h    58.1 W    116 W
0.40 °C/h    46.5 W    93 W
0.19 °C/h    22.1 W    44 W

So if, for example, these were roughly 200 L cylinders, he's effectively gone from somewhere around 90–140 W of standing heat loss to around 44 W at that part of the temperature range.
That's roughly a 60–65% reduction in heat loss, based simply on the measured cooling rate:
\[
1-\frac{0.19}{0.50}\approx62\%
\]And it may be nearer 68% if the old cylinder was normally doing 0.6 °C/h.
There is another interesting way of looking at his actual example. If the old tank went from 50 → 40 °C in 20 h, a 200 L tank lost approximately:
\[
200\times4.186\times10 = 8372\text{ kJ}
\]or:
\[
\approx2.33\text{ kWh}
\]So that's about 2.3 kWh of stored hot-water energy disappearing every 20 hours.
With the new tank at 0.19 °C/h, after 24 h:
\[
50-(24\times0.19)=45.44^\circ C
\]A 200 L tank has then lost:
\[
200\times4.186\times4.56
\]≈ 1.06 kWh.
So you're comparing roughly:
old: ~2.3 kWh lost in 20 h
versus
new: ~1.1 kWh lost in 24 h
That's a pretty striking difference.
There's one subtlety, though. Heat loss isn't actually constant in °C/hour. It's approximately proportional to the temperature difference between the cylinder and the room:
\[
\dot Q = UA(T_{tank}-T_{room})
\]so the temperature ought to decay exponentially rather than in a perfectly straight line. At 50 °C in a 20 °C room, there's a 30 °C temperature difference; at 40 °C there's only 20 °C. Consequently the old cylinder losing 0.5–0.6 °C/h near 50 °C is actually worse thermally than that simple 20-hour average makes it look.
The London loops are relevant too. They're intended to reduce thermosyphon circulation along hot-water pipes — hot water rising out of the cylinder pipework, cooling, and creating a slow natural circulation loop. But if the new cylinder manages 0.19 °C/h even before the associated pipework has been fully insulated, that's particularly impressive. It suggests most of the improvement is inherent to the cylinder itself: better factory insulation, fewer thermal bridges, improved penetrations/fittings, etc., rather than just clever pipework.
I'd be quite interested to see what happens after he lags all the new pipework. If he got down to, say, 0.15 °C/h, a 200 L cylinder would be losing only about 35 W at that operating point.
So I'd reply to him something along the lines of: your new hobby may be slightly obsessive, but unfortunately your measurements say it's working — you've apparently cut the standing heat loss by something like two-thirds. 😄
If you know the actual cylinder volume, I can calculate the losses in watts, kWh/day and £/year quite accurately from his figures.

 

the bit in bold made us both laugh

[marshian]

Grant documentation says 1.61 kWh over 24hrs and energy lost at a rate of 67 W

[marshian]

Next job is to understand how to wire this into the system

 

 

As a temporary upper limit safety measure I'm using the original tank stat as a pipestat on the coil return

 

 

it's not resting there it's got a spring wire underneath to hold it on 🙂