Vaillant Issues

[MrV]

If you get your system design issues sorted and you still have issues with heating your cylinder and it continues to annoy you, you can use a valiant controller and module.
It will cost you extra of course, but one of the main advantages it can offer you, which NO other controller can, is that you can set the power rating of each mode independently. e.g. 18kw for heating and 8kw for your cylinder.
The type of replacement you may need would be a VRC392 and a vr65.

 

[mickyg]

crikey, cant add alot to whats been said except that if you have an old cylinder you may need to get it replaced, some of the really old ones had really short coils that can only absorb a few kw, which is gonna cause your boiler to cycle,

 

[ALEC1]

thats the beauty of the vaillant controls, you can down rate on hot water too..

 

[MrV]

thats the beauty of the vaillant controls, you can down rate on hot water too..

Yup. Ebus controls have this ability to do so. Can tell you how to in cc if you wish to know.

 

[Agile]

Alex should already know how to do that!

 

[DP]

Mickyg too uick for me. OP, was the cylinder replaced when boiler fitted?

Did you get boiler cycling for CH only? Are you able to fire the system for CH only without HW kicking in as well?

Would certainly recomend pump over be corrected as a matter of urgency.

 

[MrV]

Misread ya Alec. Soz there being helpful

 

[mickyg]

thats the beauty of the vaillant controls, you can down rate on hot water too..

yeh but if the coil rating is lower than the min modulation range it will still cycle

 

[MrV]

True for sure. But it will greatly improve it and enable you to reduce waste energy on the central heating too, and if your coil rating is much lower than the minimum capacity of most boilers then the cylinder seriously needs upgrading too, as you have already said.

 

[vince101]

Wow...fantastic advice. Here's an update and reply to comments.

Agile asks re. load. My first pass calc this morning is about 22kW but there's a more accurate method that I shall check tonight. (I have a 4 bed house plus conservatory). Hence you are spot on re. oversizing.

The system is still cycling on HW duty only despite derating boiler. I need to try down rating the boiler further, but I hadn't suspected the cylinder which was retained in the new booiler installation. Thanks mickyg. The original system must have been installed over 25 years ago...and assume the cylinder is the original.

I really like the idea of the vaillant controller. Thanks Mr V.

DP asks if I can fire the system for CH only. The answer is no. Its (CH +HW) or just HW.

Agile asked if I had a gate valve on the HW heating coil...and the answer is no. There used to be a 15mm gate valve between the main feed/ delivery lines but when the installer "upgraded" the pipework he then added the automatic bypass valve which I don't think is doing anything. As spudkey says...the expansion tank is now doing that! Hence I don't think I have any options open to me.

So my task over the next week is:

- Recheck total load and derate as necessary
- Get installer to relocate cold water feed/ vent to pump suction
- Enquire about new cylinder/ controls

ps. I haven't paid the invoice yet...so at least I got something right!

Vince

 

[Agile]

Wow...fantastic advice. Here's an update and reply to comments.

Agile asks re. load. My first pass calc this morning is about 22kW but there's a more accurate method that I shall check tonight. (I have a 4 bed house plus conservatory). Hence you are spot on re. oversizing.

ps. I haven't paid the invoice yet...so at least I got something right!

Vince

You have not described your house or the construction, particularly the walls.

A four bed detached house which needs more than 18 kW would usually have serious heat loss problems like solid 9" walls and inadequate insulation.

This aspect is very serious because heat loss is causing continual over purchase of now very expensive fuel.

What is your cylinder outside made of and colour? This is a combination of two aspects, insulation and heat absorbtion. It could well be that a new cylinder would be adviseable. Depending on size this would usually be £400-£500. It could also give a useful increase in efficiency as boiler cycling reduces this by perhaps 5-10% during HW only heating but no effect when CH is in use at the same time ( which is about 30% of the year ).

Tony

 

[DP]

Vince, I do not think you will get a satisfactory operation, even with setting the boiler to minimum rate (it will have to be a seasonal setting should you go down that route). Even at minimum setting, boiler heat output will far exceed what the cylinder will be able to use.

A new boiler must have two zones (i.e. HW and CH) which you should be able to fire and thermostatically control. Thus you would be able to run CH by itself and HW zone could run the boiler to heat water before CH kicks in.

Change the cylinder. You will then find the cylinder will be able to 'use' what the radiators require if you know what I mean. Fitting an unvented cylinder would allow the boiler to chuck 20 plus kw at the cylinder and cylinder would be able to make good use of the input.

I would run the boiler at max as at this time of the year, heating load and cylinder (unvented or replacement) would make full use of the heat output of the boiler.

 

[oph]

If you have a 28KW boiler then your primaries should be 28mm IMO.

Old rule of thumb on non steamers always 22mm flow and returns up to 70K BTu's (21KWish) 80K and over should be 28mm.

It does seem to be the case that some installers wildly over estimate boiler size

 

[Agile]

Oomph... this is a nice new Vaillant condensing boiler!

Your rules of thumb are a little different from what I have gathered by inspecting many existing installations.

I have always found that 22 mm was normally seen as adequate for non condensing of at least 24 kW and that 28mm was only used on 28 kW and above.

Tony

 

[vince101]

Agile has asked details of the cylinder.

There are no markings whatsoever on it. Its factory lined foam with a dirty yellow/ green colour. Thickness of foam is about 1/2". Size is 900mm tall by 450mm diameter. Looks as though comes out about 120 litres.

Agile has also asked details of the house. Its detached about 30 years old. Cavity wall, not insulated. Approx 11 metres long and 7 metre wide plus 4m * 3.5m conservatory. Double glazing. Attic has about 3" insulation on floor. Pretty exposed to wind as at top of hill. I used energy trust short calc and gives 22kW. I need to use more accurate room by room version.

Vince