Another Viessmann 100 problem

[Agile]

You seem to be clouding the issue by wanting to heat a hot cylinder.

can you leave the immersion off and then run the boiler on HW only and see what the flow and return temps are at the boiler?

I dont suppose you moved a valve under the boiler immediately before this problem arose did you?

Do they have plastic knobs on metal shafts? You could try taking off the knobs and see if ths shaft slots are in the expected positions.

Tony

 

[MikeAustin]

Thanks for your patience with my problem, Tony.

You seem to be clouding the issue by wanting to heat a hot cylinder.

This was already part of the system when I moved in 22 years ago. I retained it when the boiler was renewed 3 years ago. It has proven to be a useful backup!

can you leave the immersion off and then run the boiler on HW only and see what the flow and return temps are at the boiler?

Here are some plots of display temperature. The boiler is on maximum. The first is HW only (cylinder at 80C). The second is CH only (thermostat on max) with HW added after about 1 hour. Note that the temperature always drops back to 41C.
View media item 10054 View media item 10053

I dont suppose you moved a valve under the boiler immediately before this problem arose did you?

Nope. The problem seems to have arisen over a period of time.

Do they have plastic knobs on metal shafts? You could try taking off the knobs and see if ths shaft slots are in the expected positions.

It sounds like you are talking about the gas tap. There are no other valves I can see there.

 

[Agile]

My first reaction is that you boiler is set to a too high output power for the flow rate through your system.

Since you have still not provided me with any evidence relating to the flow rate, can you reduce the maximum boiler output power?

Boilers should always be range rated to suit the load. Did your installer ever do this or even mention it? Is he a nupty?

Either the flow rate is too low or the boiler output power is too high.

I dont think you have given any information on your heat load but a 22 y.o. cylinder is not going to have a very great heat input!

Whats the maximum boiler power?

Whats the current max power set at?

Tony

 

[MikeAustin]

My first reaction is that you boiler is set to a too high output power for the flow rate through your system.

The boiler temperature dial can be set at any position by the user. Maximum is 80C. There is a 'most efficient' position, which is 60C.

Since you have still not provided me with any evidence relating to the flow rate, can you reduce the maximum boiler output power?

I have no way of measuring flow rate. The pump is a Wilo Gold RS50 running at maximum. I have used the boiler with the temperature dial at both 60c and 80C.

Boilers should always be range rated to suit the load. Did your installer ever do this or even mention it? Is he a nupty?

The dial on the front is user adjustable. He suggested it should stay at the efficient position unless it was cold - just like the manual. He is not a complete idiot - he has some parts missing.

Either the flow rate is too low or the boiler output power is too high.

Can you explain why the boiler doesn't go to maximum temperature until the return gets within the working range? Or shouldn't it work this way?

I dont think you have given any information on your heat load but a 22 y.o. cylinder is not going to have a very great heat input!

The cylinder was replaced 6 weeks ago. I called in the guy to sort out the boiler, fit a new cylinder, Magnaclean and radiator valves. I have nine radiators.

Whats the maximum boiler power?

It is a Vitodens 100C, which has an output of 18KW or 24KW. I think it is the lower of the two.

Whats the current max power set at?

I don't follow. I have looked in the installation guide, the service guide and the user guide and I find no reference to this.

 

[MikeAustin]

Whats the current max power set at?

I don't follow. I have looked in the installation guide, the service guide and the user guide and I find no reference to this.

Correction: I just found it. It says:
"The gas control is fully modulating and will normally facilitate a rapid heat up at maximum output followed by a continually controlled heat output. If the system has a particularly low heating load it is possible to restrict the maximum heating output to prevent short cycling."
There is a pot one can adjust. I guess it is set at maximum because he chose the power to match the previous boiler.
The statement in the manual would suggest that the boiler should go straight to the temperature setting on the dial and remain until the return is within limits. Are you suggesting that it might indeed do this if the output were turned down?

 

[Agile]

You boiler is clearly short cycling and it heats up too quickly.

This is either to little flow or too much heat!

Set the boiler to a lower power say about 8 kW to start with and see what effect that has. Thats done in the engineerr settings NOT the front panel. A good installer would have done that in the beginning!

Whats the heat loss of yuor house. You can calc it yourself but the installer should have done it before fitting the boiler.

If you cannot see the power of your boiler then call Viess with the serial number and they can tell you.

Tony

 

[MikeAustin]

Your boiler is clearly short cycling and it heats up too quickly.

This is either to little flow or too much heat!

This does not seem logical to me. Look at the facts:

HW alone: 70C/50C after 15 mins
CH alone: 55C/40C after 50 mins
CH+HW: 63C/40C after further 15 mins

These, in this order, require increasing heat and allow greater flow.
Heat and flow seems more or less OK for HW alone - and it does not heat up too quickly. CH seems to be the problem - not reaching the same temperature and being very slow. Then why do you say it heats up too quickly when the facts indicate the opposite?

Set the boiler to a lower power say about 8 kW to start with and see what effect that has. Thats done in the engineer settings NOT the front panel. A good installer would have done that in the beginning!

Whats the heat loss of yuor house. You can calc it yourself but the installer should have done it before fitting the boiler.

If you cannot see the power of your boiler then call Viess with the serial number and they can tell you.

The point is that this system WAS adequate for 3 years. The heat loss of my house has not changed and settings on the boiler have not changed since installation. Something has gone amiss with the system somewhere.

 

[Agile]

This does not seem logical to me.

It makes no difference what you think is logical because you have no knowledge or experience of boilers. If you were as clever as you evidently think then you would have sorted it out by yourself wouldn't you?

Obviously you think that I am an ignorant plumber who left school at 14 and is very thick indeed. Odd that I have such a good success rate at fixing boilers though!

Unfortunately for you, I take umbrage with people who take that attitude and I am now going to stop giving you any further unpaid advice.

I also hope that after demonstrating that attitude towards heating professionals, no other boiler engineers on this site will help you either!

I only give free advice to people on this site when they respect my advice.

You had better call back your nupty installer!

Tony Glazier

 

[MikeAustin]

Unfortunately for you, I take umbrage with people who take that attitude and I am now going to stop giving you any further unpaid advice.

Tony, please don't take umbrage. I am grateful for your help, but you are reading way too much into my wish to understand why I should follow a particular course of action that does not make sense to me. If you could explain what is wrong with my reasoning, I would appreciate it. If you would rather not, that's OK.

 

[Agile]

Its fun to help to fix problems by email.

Its very time consuming and of no interest for me to explain simple situations to people without the basic understanding. Go on a heating course at your college if you are so interested and then work for five years at a firm which understands what they are doing. They are very few and far between.

Just do what I suggested and if it solves the problem THEN I willl tell you why!

Also tell me the heat loss of your house.

Tony

 

[MikeAustin]

Just do what I suggested and if it solves the problem THEN I willl tell you why!

OK. I have an open mind and I don't see any harm in trying. I had HW and CH on full, boiler at maximum temperature but I turned the power down to its lowest setting (8kW). I turned it back to max after 46 mins. Here are the results.
View media item 10075Not much difference, it seems.

While doing this, I did detect an ever so slight smell of gas around the boiler when I got really close. Here's an outside guess: would a hairline crack in the casing cause constant cycling?

Also tell me the heat loss of your house.

I will pass on this one, although I have done those sorts of calculations before.

 

[Agile]

The positive slope is no different!

You must have been unsuccessful in turning the boiler power down!

Could you try again. I dont have time to look up the instructions but usually you set it and then have to memorise it.

You have passed on the heat load which is an important aspect of diagnosing the problem for me.

Can you describe the property, number of rooms, wall construction, loft insulation depth etc.?

Tony

 

[MikeAustin]

The positive slope is no different!

You must have been unsuccessful in turning the boiler power down!

Could you try again. I dont have time to look up the instructions but usually you set it and then have to memorise it.

I definitely turned it down to minimum! I took pictures:
View media item 10082The boiler flame indicator remained at block one (out of a maximum of five) during this test. That confirms it. In my other tests it reached block four. It seems to me that this is more about operating temperatures than operating power.

You have passed on the heat load which is an important aspect of diagnosing the problem for me.

Can you describe the property, number of rooms, wall construction, loft insulation depth etc.?

Tony, I really think that this is not relevant for a misbehaving system that was working fine in the same house for 3 years. (I also note today's date!)

 

[Agile]

Tony, I really think that this is not relevant for a misbehaving system that was working fine in the same house for 3 years. (I also note today's date!)

What you think is irrelevant.....

You clearly....

Same as 1243hrs 31-03-09

 

[MikeAustin]

I checked the gas supply pressure - 21mb static and slightly more when running. I had a closer look at the condensate drain and it was quite mucky, so I gave it a good clean. See before and after:
View media item 10105This made no difference to the constant short cycling. I will check the flue next.