Talk to me about boilers

Russinating

Russinating

Russinating

Soldato
Joined
27 Dec 2005
Posts
17,315
Location
Bristol
Never replaced a boiler before (thankfully) so am pretty blind to them as thing.

We've just finished a loft extension and we're struggling with the temperature coming out of the bathroom taps up there. The shower is more than hot enough for me, but the OH prefers them a tad hotter (what feels like scolding to me!) and it's not getting there. We also had a problem with the bath tap the other day where it just wouldn't keep a consistent heat, more often than being hot just going between varying degrees of luke warm. Heating was off at the time.

Now I don't know if this is anything to do with the boiler or anything a new boiler could help with, or just down to the distance from boiler (ground floor) to 2nd floor, insulation en-route, and colder air temp atm.

We've got 5 bedrooms, 3 toilet/bathrooms, 3 reception rooms, so about 13-15 radiators in total of varying sizes.

Our current boiler is a Viessmann Vitodens 100-W Compact WB1B 26kW, which says should be about right for that number. Water gets up to ~85c on the boiler's OSD on max temp.

So is this a boiler or an insulation issue? And would it help if I measured the output temp at the 3 bathroom taps etc (ground floor, 1st floor, 2nd floor)?
 
That’s a nice boiler if a little small for the size of the house. Where is it ? Might be struggling with the head pressure if it’s got that far to pump up.
 
You have a hot water tank ? That boiler is a regular boiler isn’t it ? Thermostat on hot water tank might be set lower than 85c
 
Simon said:
That’s a nice boiler if a little small for the size of the house. Where is it ? Might be struggling with the head pressure if it’s got that far to pump up.
Click to expand...

Boiler is in the ground floor kitchen, rear of the house. New 2nd floor bathroom is spurred off the 1st floor bathroom, both not far off being directly above give or take a few metres.

dLockers said:
Mixer tap
Click to expand...

Eh?

Simon said:
You have a hot water tank ? That boiler is a regular boiler isn’t it ? Thermostat on hot water tank might be set lower than 85c
Click to expand...

No hot water tank.
 
Do you have mains hot water then? That boiler number isnt the combi version but might be struggling with the size of the house and the additive demand of the loft.

The lack of heat might be long pipe runs especially if its all 15mm, i assume its not hot enough even if but its a 12l/min boiler at +30C heat gain so lower ambients could hurt it, you wont be hitting 85C if you pulling peak flow. i would imagine.
 
Boiler is fine for the house size based on what I have just had installed. I have had a 19/25kw Viessmann 100W fitted with an unvented cylinder and the water cylinder temp is set at 55 degrees which is way to hot to use without mixing, 85 therefore seems very high.

It sounds to me like your shower issue is potentially due to a thermal limit on the mixer, assuming it is thermostatic. Depending on your brand this can require removing the levers and adjusting an internal hex bolt to increase/decrease it. The tap issue does seem to be a bit odd, is that thermostatic as well by any chance? Newer properties require a TRV in the bath flow to prevent scalding risk, maybe during the bathroom install they put one in place which is malfunctioning?

If you need a good Viessmann specialist in Bristol then drop me a message, I can highly recommend the guy I used to fit my system.
 
Simon said:
Do you have mains hot water then? That boiler number isnt the combi version but might be struggling with the size of the house and the additive demand of the loft.

The lack of heat might be long pipe runs especially if its all 15mm, i assume its not hot enough even if but its a 12l/min boiler at +30C heat gain so lower ambients could hurt it, you wont be hitting 85C if you pulling peak flow. i would imagine.
Click to expand...

It is the combi version (see below).

Bouton Aide said:
I assumed that but didn't want to really assume anything. I had a feeling you might have done it and asking the question about the boiler.
Click to expand...

No all done by a couple of plumbers employed/contracted by the building company who did a good job.

SpeedFreak said:
Boiler is fine for the house size based on what I have just had installed. I have had a 19/25kw Viessmann 100W fitted with an unvented cylinder and the water cylinder temp is set at 55 degrees which is way to hot to use without mixing, 85 therefore seems very high.

It sounds to me like your shower issue is potentially due to a thermal limit on the mixer, assuming it is thermostatic. Depending on your brand this can require removing the levers and adjusting an internal hex bolt to increase/decrease it. The tap issue does seem to be a bit odd, is that thermostatic as well by any chance? Newer properties require a TRV in the bath flow to prevent scalding risk, maybe during the bathroom install they put one in place which is malfunctioning?

If you need a good Viessmann specialist in Bristol then drop me a message, I can highly recommend the guy I used to fit my system.
Click to expand...

The shower is thermostatic yeah, so I'll look into that. None of the taps are.

This is the boiler when just heating (Nest Thermostat, so that'll just be full demand for heat):

cqSopo4.jpg


And with the hot kitchen tap running after peaking/settling. Seems it goes between 80-87 with it firing itself up and down (the bar 'chart' at the bottom goes from 1-5 and back to 1 and so on):

do7VYbP.jpg


I'm not sure what "COMFORT" means, there's no mention of it in the manual referenced above.
 
My boiler is obviously the newest version, but I assume they are the same in principle. Viessmann have 3 levels of heating control, Normal, Reduced and Comfort. It allows you define 3 temperature categories and then set the timing blocks with the desired temperature based on your personal preference. I just do mine through the app.

On the bathtub, just ensure that the fitters haven't added a thermostatic mixing valve?:

https://www.screwfix.com/p/pegler-p...bLd9zIGbwtt4MZSoI-UaAhPJEALw_wcB&gclsrc=aw.ds

You would not necessarily know it's there but if fitted it could explain the odd behaviour.

On the shower, I assume you can remove the tap on the mixing side and there is a limit adjustment screw of some sort.
 
Russinating said:
We also had a problem with the bath tap the other day where it just wouldn't keep a consistent heat, more often than being hot just going between varying degrees of luke warm. Heating was off at the time.
Click to expand...

How quickly was the water flow? Assuming it's a combi, i've found with ours that if you want the hottest temperature then water flow needs to be reduced out of the tap. I.e. if tap is running at full flow it was struggling to make the water very hot (temperature was fine to me, but as in your case the OH wanted it hotter). Reducing the flow did make the temperature quite a bit hotter.
 
Russinating said:
And with the hot kitchen tap running after peaking/settling. Seems it goes between 80-87 with it firing itself up and down (the bar 'chart' at the bottom goes from 1-5 and back to 1 and so on):

do7VYbP.jpg


I'm not sure what "COMFORT" means, there's no mention of it in the manual referenced above.
Click to expand...

If that temperature reading is while DHW is being drawn, then it's too high!

Internally, the boiler has a water to water heat exchanger, and your boiler is showing that it has to generate 87C to give you your desired DHW temperature. So it's showing the heat exchanger is doing a poor job, and either needs cleaning or replacing.

By comparison, my 33kw Baxi runs at around 70C to give 60C DHW after stabilising for a few minutes.
F5NfomO.jpeg


The 'Comfort' setting on the boiler is a preheat feature that let's the boiler keep the DHW heat exchanger warm, so you get hot water out of the tap sooner. So less waiting time, especially good during Summer months when the boiler is 'cold'

Vaillant and Worcester Bosch boilers do the same.
 
Oh and if you're interested here's a picture from a blocked up water heat exchanger from a Viessmann combi I washed out a few years ago.
It was poor hot water performance reported by the customer.
dtkKsFr.jpeg


:D
 
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