Could lack of pump overrun cause hex problems in the future?

[fronty]

Just thinking... my new boiler (a Viessmann) was installed without pump overrun, apparently it's not required because it has a stainless steel heat exchanger, but when the burner cuts out I see the temperature slowly rise (as reported by the boiler) to around 95 degrees, I assume this is the latent water temperature in the hex.

But what I am wondering is if this means the water is (close to) boiling could this eventually cause issues with kettling later on down the road, or maybe mineral deposits building up in the hex as the water is repeatedly boiled? I'm assuming Viessmann don't see it as an issue else they would enable pump overrun by default, but I just can't get my head around the temps I am seeing and have that little niggle in the back of my brain that in a few years time I'll start hearing those tell-tale kettling noises.

Does anyone have long term experience of this? I am wondering whether to get pump overrun installed anyway (at my own cost) to keep the latent temp in the hex down after the burner shuts off and safe guard the hex against potential kettling.

Maybe I am just thinking about it too much!

 

[Old Account]

Hi fronty, what type of viessmann boiler have you got?

 

[townfanjon]

Pump overrun is always favoured but I can assure you Viessmann are ok with it .

 

[chris watkins]

I have had my Viessmann 100 - W B1B for 7 years now. When I first installed it, it didn't have a pathway to allow the pump to dissipate the residual heat & I had exactly the same sort of temperature rises, that in itself was not the reason I altered it. It was the thermal shocks as the heating came on after the hot water, the sudden cool water entering the heat exchanger use to make the most worrying noises which I was not happy leaving. I have WC on mine so couldn't have them on at the same time even if I wanted to.

But what I am wondering is if this means the water is (close to) boiling could this eventually cause issues with kettling later on down the road, or maybe mineral deposits building up in the hex as the water is repeatedly boiled?

Kettling / mineral deposits should not be a problem as I presume you have a sealed system that doesn't leak so there will be no minerals to be deposited on the internal surfaces.

 

[chris watkins]

What system is it connected too?

 

[fronty]

Hi fronty, what type of viessmann boiler have you got?

Vitodens 100 WB1B

 

[fronty]

Interesting, no weird noises being experienced, but you have got me thinking now about the sudden change in temp when the heating comes on and allows cool water into a hot hex. I'm guessing the sudden change in temperature is well within what steel can handle else Viessmann would be having horrendous issues and their reputation would be in the bin.

It's not a sealed system, but I don't have any leaks. I have a header tank and now wondering if I go up into the loft if I'll see boiling water or steam coming out of the vent - maybe I should go up there some day and check?

 

[chris watkins]

Convert it to a sealed (I did). The risk of drawdown, pumping over, dirt from F&E etc. make it a no brainer for me.
What are your controls S or Y plan etc.?
Did you not take advantage of the built-in WC?

 

[Old Account]

I was under the impression that the Pump overrun on all Vitodens 100-W boilers is temperature controlled and it is optional on the open vent system. I don’t think you can harm a boiler with that

 

[fronty]

Convert it to a sealed (I did). The risk of drawdown, pumping over, dirt from F&E etc. make it a no brainer for me.
What are your controls S or Y plan etc.?
Did you not take advantage of the built-in WC?

Y plan, I didn't think WC was an option on this boiler, how would I get the hot water up to temp in the summer? The missus complains if the hot water isn't hot enough (I swear she's cold blooded, maybe she's really one of those lizards from "V"...) ;-)

 

[chris watkins]

Unless the boiler has changed in the last few years WC is built in it just needs an outside sensor kit (or resistor in a box mounted outside)
& a few wiring mods so that it knows when there is call for HW when it goes to full temp after which if heating is required it drops back to lower temps depending on the weather.
You can do it with your existing mid-position (Y) valve or if the valve if old I would replace it with a diverter valve (W) that might free ups some wiring cores & allow the pump to be wired directly from the boiler pump terminal to give over run, the open port will allow this.

If you need wiring diagrams let me know I think I still have them or speak to Veismann.

 

[Last Plumber]

I would make sure it has Pump over run. The temperature you're witnessing through residual heat is too high in my opinion.
Protect the Boiler from itself and keep things safe by having it sorted.
It can only be a good thing.

 

[decomplexity]

WC can be fitted to any Vitodens 100-W but not when the management of DHW is external to the boiler (e.g. S-plan). I have a 100W Open Vent wired as S-plan, and I gather from Viessmann that the WC (plus its DHW 'demand control box') only kicks in when DHW demand is satisfied; it then can alter the CH flow temperature up or down in line with outside temperature variations.

 

[chris watkins]

WC can be fitted to any Vitodens 100-W but not when the management of DHW is external to the boiler (e.g. S-plan). I have a 100W Open Vent wired as S-plan, and I gather from Viessmann that the WC (plus its DHW 'demand control box') only kicks in when DHW demand is satisfied; it then can alter the CH flow temperature up or down in line with outside temperature variations.

Not sure what you mean in the above, in WC system the hot water can't be heated at the same time as the heating but as this normally only take 30 mins it is no hardship. I also have an S plan & have my HW timed to come on at 5:20 with the heating not until 6:20. The HW does take priority which is why I suggested the "W" (diverter) if I was re-doing the pipework, alas I had no access & had to adapt the S plan when I fitted the 100 W, I still have a room stat & it all works fine & delivers gas savings.