New Viessmann 100-W taking longer to heat up than old Ideal Classic NF70 unit

[Agile]

There are two requirements!

A proper auto bypass valve set at about 0.3 bar which will normally be closed until most of the TRVs start to close.

Then, perhaps, a manual bypass in parallel with the auto bypass to set a SMALL amount of bypassing to aid increasing the return temperature to help the boiler in ramping up when starting from cold.

If I had a moment then I would check further on the latest Viessmans with the return sensor. But this feature does give a bit of a problem on Vaillants if the return temperature does not rise soon after start up as can happen on larger systems.

If you were to monitor the flow and return temps during start up then it would give a bit more information on what is happening.

Tony

 

[Phill Peck]

Tony is correct in that technically an auto bypass is required.

However I have found with many boilers especially on larger systems, the boiler will sit back waiting for an increase in return temperature.
Depending on the system set up this could be quite a while.
It is also possible that your pump is not up to the job. Don't assume because it was working well before that it is now.
Also resistances have changed, so the system probably requires balancing.

I often read and have discussed with engineers in merchants about this issue.
Some are blaming the boiler.

 

[Agile]

14 rads is not a small system but then its not a very large one either.

With a differential of only 10 C the pump is probably adequate.

But the answer lies in the balancing. If just one rad near the boiler is set for too much flow then the boiler will modulate back and the others will take ages to heat up.

So check the flow and return temps on each rad and that will give an idea of just how far out of balance it might be.

Maybe I will get my hotline call tomorrow!

Tony

 

[MikeAustin]

The way the Viessmann OV works, it will fire at a low rate and modulate up as the return warms. What is needed on larger OV's is a bypass almost acting as a mixing valve to provide a small amount of heat back into the return keeping the temp up to keep boiler firing.

However I have found with many boilers especially on larger systems, the boiler will sit back waiting for an increase in return temperature.
Depending on the system set up this could be quite a while.
I often read and have discussed with engineers in merchants about this issue.
Some are blaming the boiler.

Interesting couple of posts gasservice1985. My issue with a Viessmann 100C was tackled in another thread. It was very slowly reaching 60C and dropping back to 41C. Some 'positive feedback' from a bypass/mixing valve could get my boiler up to speed more quickly. Where do you suggest I look for solutions?

 

[FlyingBrick]

Hi Agile

These are the readings I was getting over the weekend. They are rather messed up in getting them on the forum. Grundfos Super Selectric 15-30 130 pump set on the middle speed as when the Ideal boiler was used and nothing changed on the bypass valve. Pump replaced 2 months ago ready for the new boiler to be fitted. Old pump was still found to be ok when dismantled.


Viessmann Vitodens 100-W open vent boiler CH only Ambient temp outside 1*C

Time min Output temp *C Dial setting LCD bar read out Return temp *C

0 12.4 6 1 12

1 31.5 6 3 13

2 38.5 6 4 15

3 39.7 6 5 fluctuating down to 1 16

4 42.0 6 5 fluctuating down to 1 18

5 40.5 6 5 fluctuating down to 1 20.4

6 40.5 6 5 fluctuating down to 1 23.0

7 37.9 6 3 Fluctuating down to 2 23.0

8 46.0 6 4 Fluctuating down to 3 26.0

9 46.9 6 4 fluctuating down to 3 27.0

11 52.0 6 5 fluctuating down to 2 29.0

15 Turned off and then re- started

16 46.3 6 1 33.0

20 60.0 6 5 fluctuating down to 2 39.0

30 66.0 6 3 47.0

35 66.0 6 2 48.0

40 66.0 6 2 50.0

45 66.0 6 2 50.0

50 65.4 6 2 50.0

55 65.4 6 2 50.1

60 65.0 6 2 50.1

90 65.4 6 1 54.1

120 65.4 6 1 54.3

150 65.4 6 1 54.7

240 70.0 6 1 59.0

 

[FlyingBrick]

Back Agile with the HW only running temp with the new boiler heating up the copper tank upstairs.
Will turn the Grundfos pump to top position 3 later on to see if this makes any useful changes.



Viessmann Vitodens 100-W open vent boiler HW only Ambient temp outside minus 1*C

Time min Output temp *C Dial setting LCD bar read out Return temp *C Cu Tank *C

0 63.8 6 3 48.0 20.0

5 66.0 6 2 50.0 35.0

10 65.4 6 2 51.0 43.2

15 65.2 6 2 51.0 45.1

20 65.4 6 1 52.0 48.5

25 65.4 6 1 54.3 51.1

30 65.4 6 1 54.0 52.0

40 65.4 6 1 54.0 54.3

60 65.4 6 1 54.0 56.0



Central heating turned off and hot water only.

Ambient outside temp minus 1*C.

Grundfos Super Selectric UPS 15-60 130 pump set on middle speed.

5 bed house with 14 rads installed and 15mm bypass pipe above 2 Honeywell valves (washing machine valve type)

 

[Agile]

What style of house?

What wall construction?

Leave pump on "2".

 

[MikeAustin]

Hi Flyingbrick. Just curious (as always) - what is the approximate length of your pipework from boiler to the furthest radiator?

 

[D_Hailsham]

Why did you restart at minute 15?

What temperature is set on the cylinder thermostat?

 

[FlyingBrick]

Hi DS_Hailsham

Did not restart at 15 minutes into the run. The boiler actually clicked off for some 20 seconds then went back onto burn all by itself and continued to heat up?
Cylinder upstairs thermostat was set to 59*C and checked with a thermocouple directly behind the Danfoss stat strapped to the copper tank for accuracy. About one third up the tank from the bottom.

Hi MikeAustin

Using just straight lines form the boiler in the utility room to the furthest radiator on the ground floor with a Bosch laser we are talking around 50 feet away.

Hi Agile

Left pump on position 2 as recommended and house is 24 years old with red brick construction on the outside and Durox brick on all the inside walls.
Unfortunately at the moment the cavity wall is empty as our architect did not stipulate putting in Rockwell cladding as the next door neighbour did when he built a copy of our house two months later (builder by trade) That's one other thing on the to do list because those awfully nice people from British Gas came around and told us that brick construction on the outside was to recessed for drilling and blowing in glass wool. Ho ho. They wanted £3800 to replace the boiler and change the gas pipework plus wiring mods. Three local chaps quoted £2100 - £2300 to do the same job.

 

[Agile]

What does "too recessed" mean in plain English?

I suppose that 24 years ago insulating the cavity may have not have yet become mandatory.

Exactly what balancing has been done? Have you taken flow and return temperature measurements on the rads?

Tony

 

[FlyingBrick]

Back Agile

British Gas measured the mortar between the Ibstock outer bricks which was 10mm below the outside brick surface. ie cement recessed by 10mm on all faces and told me that they could not go ahead and drill the bricks and blow in the glass wool. Yet other owners with the same recess of mortar between the bricks have used different cavity wall company's without being told can't do?

Had gone round the 14 rads and checked they were as close to possible to a 11* drop in temp on the output and input pipes. One or two were 9-10*C when checked.
As well as the isolation valve used on my bypass had checked the rads warming up from cold and found the new rad put in my bathroom a traditional heated Victorian chrome towel rail (8 vertical column type) has the hot inlet water going into the lock shield valve and the cooler return coming out through the thermostatic valve?
Will this be a big problem having them connected the wrong way? Just this one rad only Agile. Clever plumber.

One question for you. Would it be worth installing the outside temperature sensor to the Viessmann 100-W type WB1B open vented boiler. Have a radio thermostat from Drayton (MiStat N model: N110R) installed at the moment that normally sits around the living room. Would there be any advantage in the CH with outside temp input?

As I check up on the plumbing more little interesting problems seem to appear.

 

[MikeAustin]

Hi MikeAustin
Using just straight lines from the boiler in the utility room to the furthest radiator on the ground floor with a Bosch laser we are talking around 50 feet away.

That's probably about the same as mine then. I plotted your readings over mine to compare. On CH you reached 66C in 30 mins. I would have been quite happy with that as mine was only getting to 60C after about 4 hours. Now, after some tinkering, it reaches 72C in 28 mins. I wondered if you had similar issues to me but it does not seem so.
By the way, I also changed from an old cast iron boiler to a condensing boiler 10 years ago and the performance (in terms of heating up time and maximum heat) went down. I also had to relocate the boiler to a less convenient and less efficient position.

 

[FlyingBrick]

Hi MikeAustin

Yes like you I had fallen for the new gas saving devices pushed out in the media and found that my old 24 year old wall fitted boiler had the temp up in the rads in around 20 mins and piping hot water out of the tank. Output temp was nearly 80* with the Ideal NF70. The boiler is situated a long way from the airing tank upstairs and takes a long draw off water to get hot water downstairs in the kitchen sink, best part of nearly 1/4 a gallon before it comes out at tank temp.
Would have re-built the PCB and replaced the fan if I had known. To late now.

Have yet to drain down the system to the manual rubbish bypass valve that is stuck and will not try to move it as the seal in the isolation valve is leaking very slightly. Have like you been playing with the rads this week and stopped down a few lock shield ones around the house as a temporary fix. Also been taking temp again of the boiler in and out plus the copper tank when HW is turned on.

With just hot water on only the boiler runs with a output of up to 71.3*C 1 bar on the LCD and the return of 59.8*C. Copper tank set to 60*C and the boiler cuts out at 72.3*C which seems to be OK considering the seized bypass valve.
Now after some re balancing with CH only we get 60*C output in 20 min and 37.7*C return with 3 bars out of 5.
66 25 42.3 3
66 30 43.0 2
66.1 40 44.9 2
66.2 50 45.2 2
66.0 60 44.5 2
65.8 70 44.5 2
66.1 80 45.1 2
66.1 90 44.5 2
66.1 110 44.8 2
66.2 140 45.4 2
65.8 165 44.7 2
65.8 180 44.8 2
65.8 4hrs 44.9 2
66.2 5hrs 46.4 2

These were taken with the ambient temp outside on plus 1*C and slowly falling during the evening to minus 1*C
All 14 rads in the house were turned up to full position 5 (Westherm valves frost to 5)
No outside sensor fitted yet as I need some feedback but most were within 11-12 degree temp drop having done some tinkering with the lock shield valves.
Grundfos pump left at the middle position 2 for this little ditty.
Misses won't let me partially drain down the system until things warm up for a valve change. Grrrrrrr. As if I could make things worse!

Many thanks for showing interest in my boiler update as I am learning quickly about plumbing and some plumbers.
I service all my own cars and motorbikes as I have caught out garages so many times charging for work not done. Seems my installation might also fall into that area.

Chris

 

[FlyingBrick]

Sorry Austin once again when sent out to the forum all the figures have been rammed together.

Chris