My Viessman boiler journey

[JohnMo]

5 minutes ago, marshian said:

see no benefit to a higher DT at the expense of increase cycling but I maybe missing something

Me neither, every start equals wasted heat out the flue and stabilising metal work temperature, instead of real work heating the house. Long runs equal efficient use of gas.

[Gus Potter]

5 hours ago, marshian said:

The house didn't overheat but I can't see the point of increasing the level of cycling for a higher DT

Did anyone open doors in the house, go out and get mucky and need a shower, was it a windy day or not, have you moved the furniture about or got a new sofa. 

 

Nice idea but at my end it's just like a theoretical, like structural design model, it's there to inform, and only that. 

 

 

[SimonD]

14 hours ago, marshian said:

 

I'd probably agree based on his content

 

However today I've tried to get a wider DT (I manged to get it to just over 10 by reducing pump speed and lowering the flow rates thro the rads) and whilst the boiler managed the situation it did it by increasing the cycling and shortening the burn times (Flow temp 36 - return temp 26)

 

The house didn't overheat but I can't see the point of increasing the level of cycling for a higher DT

 

I suppose the fundamental question is whether the DT in a heating system actually matters if the system is running efficiently, producing and distributing the required heat and not causing excess wear and tear, or noise in the system.

 

I'm actually trying to find the engineering origins of delta T 20 in the first place, or even Delta T 11 and I don't seem to be able to find a specific engineering reason why this has to be. It seems more like a hereditary thing that has been passed down from boilers of old. In the imperial view the maths was simplified to using calcs based on 10,000 btu factor - divide the btu by 10,000 give you the flow rate in gallons per minute at delta T of 20 degrees F which is 11C. There are some boilers, like the Ideal Logic that specifically ask for DT 11 in commissioning.

 

Cycling in an of itself is not necessarily a bad thing - short cycling yes. I read a paper a while ago that showed that 6 cycles per hour was not detrimental to the efficiency of the test system, but obviously systems vary greatly. There was a minimum burn time given where I recall something like 7 minutes but I could be wrong here as just like you I think it;s sensible to just aim for a long burn time as you can achieve.

 

And as you'll already know the overall efficiency of the system isn't just down to the cycles, but also pressure drop, flow rates etc. so it's all just a balance of these. Like on my system, I generally have a DT of about 2-3C, even though emitters are all balanced to about 5. My system is currently showing a SCOP of 6.6 for the last 2 months including DHW - seems implausible and surprising so I need to dig deeper to confirm.

 

I'm interested in whether you did a gas rate to compare gas usage between the two set up?

[marshian]

7 minutes ago, SimonD said:

I'm interested in whether you did a gas rate to compare gas usage between the two set up?

 

At a DT of 6 Deg at the boiler for the period before the change usage was 2.0 kWh (OAT was 10 Deg) Boiler burning once per hour - Boiler at start fires at 58% and then over the the 90 secs it slowly modulates down before settling on the min output of 10.8% for the rest of the cycle.

 

At a DT of 11 Deg at the boiler for the period after the change usage was the 2.2 kWh (OAT was still 10 Deg) But the boiler was cycling 3 times in an hour (I think the additional gas used was as a result of the more frequent initial fires when for the first 90 sec it fires at 58% and then over the the 90 secs it slowly modulates down with the higher target temp the modulation stayed at a slightly higher rate for longer before settling on the min output of 10.8% for the rest of the cycle.

 

I actually learnt something from the exercise (or rather remembered previous boiler behaviour) sometimes it helps to write down my thoughts.

 

So my heating circuit is pretty close to 130 Litres with all rads open 

 

with a flow rate of 9 litres per min it takes 15 mins for the return water to start to rise and the boiler flow temp starts to increase until the over temp point is reached 

 

At WC Flow temps the boiler allows an overshoot of the target temp of 4 Deg C

 

Target flow temp is 27 Deg C but the boiler runs until the flow temp hits 31 deg C (Basically extending the burn length)

 

When the burn is finished the circuit cools down to around 4 deg below the target so 23 Deg C (this takes around 20 mins ish)

 

With a higher flow temp and the same return (so a wider DT) and a slower flow rate 3 lires/min the return water stays fairly low and as the boiler is targeting a higher flow temp it runs a little harder for longer. 

 

The time it takes for the return temp to drop to the restart point is much quicker - I think possibly because the warmer water never gets back to the boiler during the burn period (Typically the restart temp point was reached in 10 mins)

 

I wouldn't call it short cycling and at 3 cycles per hour it's still much lower than 6 cycles per hour limit

[marshian]

15 hours ago, JohnMo said:

Me neither, every start equals wasted heat out the flue and stabilising metal work temperature, instead of real work heating the house. Long runs equal efficient use of gas.

 

My thoughts exactly - with an open system every rad getting the correct flow rate to meet the heat loss of the rooms I wasn't sure why a higher DT was going to drive any more efficiency.

[SimonD]

1 hour ago, marshian said:

 

At a DT of 6 Deg at the boiler for the period before the change usage was 2.0 kWh (OAT was 10 Deg) Boiler burning once per hour - Boiler at start fires at 58% and then over the the 90 secs it slowly modulates down before settling on the min output of 10.8% for the rest of the cycle.

 

At a DT of 11 Deg at the boiler for the period after the change usage was the 2.2 kWh (OAT was still 10 Deg) But the boiler was cycling 3 times in an hour (I think the additional gas used was as a result of the more frequent initial fires when for the first 90 sec it fires at 58% and then over the the 90 secs it slowly modulates down with the higher target temp the modulation stayed at a slightly higher rate for longer before settling on the min output of 10.8% for the rest of the cycle.

 

I actually learnt something from the exercise (or rather remembered previous boiler behaviour) sometimes it helps to write down my thoughts.

 

So my heating circuit is pretty close to 130 Litres with all rads open 

 

with a flow rate of 9 litres per min it takes 15 mins for the return water to start to rise and the boiler flow temp starts to increase until the over temp point is reached 

 

At WC Flow temps the boiler allows an overshoot of the target temp of 4 Deg C

 

Target flow temp is 27 Deg C but the boiler runs until the flow temp hits 31 deg C (Basically extending the burn length)

 

When the burn is finished the circuit cools down to around 4 deg below the target so 23 Deg C (this takes around 20 mins ish)

 

With a higher flow temp and the same return (so a wider DT) and a slower flow rate 3 lires/min the return water stays fairly low and as the boiler is targeting a higher flow temp it runs a little harder for longer. 

 

The time it takes for the return temp to drop to the restart point is much quicker - I think possibly because the warmer water never gets back to the boiler during the burn period (Typically the restart temp point was reached in 10 mins)

 

I wouldn't call it short cycling and at 3 cycles per hour it's still much lower than 6 cycles per hour limit

 

It would be interesting to see any difference in colder temperatures with greater load on the system, but extrapolationg that 10% over a heating system would be significant.

 

What was the rationale for suggesting a higher DT to begin with?

[marshian]

9 minutes ago, SimonD said:

It would be interesting to see any difference in colder temperatures with greater load on the system, but extrapolationg that 10% over a heating system would be significant.

 

I think at colder temps it would have been less of an issue but with current set up this isn't an issue either.

 

Fundamentally the boiler "burn" time just increases as it gets colder outside and the "coast" period gets shorter - it still pretty much cycles once per hour - length of the burn and coast periods just change.

 

9 minutes ago, SimonD said:

What was the rationale for suggesting a higher DT to begin with?

 

He'd read some research by Viessmann indicating that a higher DT was better for condensing efficiency.

 

He worked out my flow rate was 3 times what the house needed!!

[JohnMo]

22 minutes ago, marshian said:

higher DT was better for condensing efficiency.

That really depends on your starting point. There gets a point where return temp doesn't really get much lower, especially with WC when heating at 10 degs outside. Return temp can never drop below room ambient, even if it got to ambient, rads would be huge.

 

1 hour ago, marshian said:

When the burn is finished the circuit cools down to around 4 deg below the target so 23 Deg C (this takes around 20 mins ish)

So practically you are at min return temp already. Getting a bigger dT is just increasing flow temp for no reason. If you lowered your return temp any more, your 4 degrees delta for restart would never be reached or take too long.

 

 

[marshian]

Just did some quick an dirty calculations on burn time v HDD ranges

 

At HDD

 

between 0 and 4.9 Boiler runs for 19% of the time so 11 mins in every hour (Sample 10 Days) (in reality it runs for 20 mins in two hours)

 

Between 5 and 9.9 Boiler runs for 35% of the time so 21 mins in every hour (Sample 24 Days)

 

Between 10 and 14.9 Boiler runs for 56% of the time so 33 mins in every hour (Sample 14 Days)

 

Between 15 to 20 Boiler runs for 69% of the time so 40 mins in every hour (Sample 2 Days)

 

[marshian]

14 minutes ago, JohnMo said:

That really depends on your starting point. There gets a point where return temp doesn't really get much lower, especially with WC when heating at 10 degs outside. Return temp can never drop below room ambient, even if it got to ambient, rads would be huge.

 

Exactly my starting point is pretty low already - my flow temps are only good when I'm heating 24/7 - I can get away with a very small set back but the house does need a little solar gain to recover 

 

14 minutes ago, JohnMo said:

So practically you are at min return temp already. Getting a bigger dT is just increasing flow temp for no reason. If you lowered your return temp any more, your 4 degrees delta for restart would never be reached or take too long.

 

I think it might heat for longer if the boiler responded correctly to being range rated but as it seems to hate being range rated I'm not going to try any more to work with a higher DT at the boiler.

 

House is a comfortable temp all the time so it works as it is

 

System Noise since I fitted the pressure independant TRV bodies is super quiet

 

Boiler doesn't seem to be unhappy with a higher flow rate and a 6 deg DT