Hot water is 37°C max

under warranty ,get installer back to sort problem .

 

At the very least we need to know make and model of boiler, type of hot water storage system, what the 'stat on that cylinder is set to, and whether the hot water was hotter before this boiler was fitted - is the boiler the only thing that was changed?

And what type of boiler did you have?

 

Just to be clear do you have immersion heaters switched on in the current setup? or have you always relied on the boiler heating up the tank?

 

Bad practice fitting a new boiler, & leaving a 15 year old cylinder in place,as for trouble, should have replaced the lot.
Old cylinder is problely seriously scaled up & restricting flow & the heating of hot water.

 

Not familiar with Vaillant boilers, do they show the water temp on the display when running?

Has it possibly been set to "summer" mode on the likes, while I agree scale could be an issue, it's not likely just to drop from hot to not overnight. Which part of the country are you in?

 

Another one not familar with the Vaillant ecoTEC plus OV 418, (like my Valliant combi's) but it has a summer mode for heating & a quick nosey at manual you can set hot water temperature, see linky below.

https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&ved=0ahUKEwiy842usMzUAhVHAsAKHZ4aBWMQFgh4MAY&url=https://www.vaillanthome.co.uk/download/ecotec-plus-open-vent-user-manual/&usg=AFQjCNEJmKbDQ5xyFJkF3SAR_NAzuEwCzw

If scaled up cylinder, could crud in it cause an obstruction in HW heat exchanger & boiler safety feature is reducing HW temperature.

Also there is a Frost setting on boiler which works with heating & hot water which is set at 30 degrees.

 

Yes

What size/ capicity is the present cylinder.

I thinks your installer totally screwed up, any decent installer would have recommended replacing the cylinder because of it's age, unless OP asked to keep orginal.

 

A limescale build up will not stop the water being heated - it may allow an initial overheat. The hot water at say 80c passes through the coil, any deposits act as a buffer and slow transfer to the stored water which will gradually rise to the nominal 65 degree cut off. However, limescale on the cylinder will act as another buffer and it will take time for the thermostat to register 65, water may well be 70 or 75 initially but equilibrium will soon occur. There is no way that the stat on the outside could register a temperature higher than that of the stored water.

Try adjusting the stat up and down a few times before returning to the normal temperature.


Think about a water softener too.

I have just opened up my cylinder for a look with the intention of replacing it as I am in a very hard water area. It has been in place for 25 years or so with a water softener running for the last 8 or 9 and when the previous cylinder was removed - it was similar to those pictures. When I looked inside - there was just a fine almost dust like coating and no layers - teh softener certainly works and over time the water has reabsorbed some of the scale hence there is none present.

 

Why?

A 450x900 has a capacity of 117 litres and a 400x1050 is 114 l so there is a small loss. Would be more sensible to stay as is or move to a 450x1050 with an extra 20% at 140 l.

 

Agree Mr P.

A 450x1050 would be the better option.

 

Best idea is rip the lot out, & replace with a combi or system boiler with unvented cylinder.

 

Unvented still have scale build up issues.

Water softeners are a must in some places.

 

This model is open vented so could be an airlock.
Do the flow and return pipes at the cylinder both get quite hot?
The cylinder could be perfectly fine.
What is the boiler stat set at? I usually set it about 68 degrees to ensure cylinder can reach 65 via the cylinder stat.
If using a normal cylinder stat ignore what the hot water stat on the boiler says as this won't be functioning.