Combi Boiler Hotwater query

Bulltog

Hi, I recently installed a Worcester Bosch 30 CDI combi boiler and I have a couple of queries;

First of all it has a DT20 digital timer on it and it has an option to control hotwater timings as well. I was wondering how this works. I mean I get hot water even though the hot water settings is set to off (it got four options; off, timed, once and on). It got a small storage tank in it and has an Eco mode (operated by another button) where it will keep the water in this tank hot by turning on every so often. So do you know whether this timing options on the hotwater control is to set the time for this to turn on and off at specific times?

Secondly, I moved to this from a traditional storage tank system, but after installing this system if I open a second tap in the house at the same time the pressure in the first tap goes down considerably or stops all together depending on where the tap is. Is this normal with combi boilers? I never had this problem with the conventional boiler I had.

I was advised to go for a higher power boiler and hence went for the 30 CDI and this is the situation still. The corgi plumber is saying this is because this boiler works on the pressure of the mains water pressure and hence the loss of power. But if I open more than one of the cold water tap at the same time there is no pressure loss. I am now worried whether he did a bothched job of fixing it.

Thanks for your help in advance.

penrhyn

With a combi hot water is available on demand it is not timed. I don't understand why the timer would have the option for HW, unless is common with system boiler with tanks.
The Hot water should be coming through at mains pressure, is the stop valve to your supply fully open?

macman

The flow rate from a combi will be limited to the hot water output it can produce, so two hot taps open at once will of course reduce the output at each tap. Nothing to do with water pressure-you could have extremely high pressure, but still inadequate flow rate, though a 30kw combi should do the job for most properties.
If it's bad now, it will be worse in winter with colder water coming in from the rising main. Did your RGI not explain this limitation of a combi?

daveyjp

Before installing a combi the hot water demands of the system should be fully assessed. In larger properties with lots of occupants and frequent demands for hot water in two places at the same time they are not recommended.

I assume a full assessment of your needs was carried out and this boiler was deemed suitable.

Bulltog

Thanks for the replies;

@daveyjp The reqiurements were assessed had various quotes icluding British Gas and every one suggested 30CDI boiler.

@Macman thanks for that clarification, that is what I thought as well, that it can't be fully based on the mains pressure. So am I right in understanding if we need water at two taps at the same time Combi is not ideal. Or with a 30kwh boiler should I be getting water at two taps at the same pressure.

@penrhyn My combi has a small storage tank in it though, that could be the reason. The pressure of water is fine normally, it is only affected if a second tap is opened. What you mean by stop value to your supply? Are you referring to the mains valve to my home or is there another valve to the boiler?

macman

What I am saying is that a combi has a finite flow rate. If you open more than one tap then the flow is probably going to be reduced at the last one on the feed.
It is for this reason that a combi is not suitable if for example one person wants to use a shower while another is running a bath.

Dave_save

I have exactly the same system as yourself. The 'off' setting for the hot water refers to the pre heat. When in the 'on' setting, a small quantity of water is preheated in readiness for the demand. You then don't get a long time lag until hot water is delivered. 'Timed' merely sets this preheat to activate between the times you set on the clock. If you have this in the 'off' setting, or when the timer isn't calling for pre heat, you will still get hot water, albeit at a much slower rate and build up. You can, if you wish, leave the hot water pre heat setting off. You'll get a small saving in gas usage but the downside is the long lag time in waiting for hot water.

As other posters have said, the demand for hot water will reduce the flow rate, so more open taps will produce a smaller quantity. I personally find the system adequate, although it takes a little getting used to.

Incidentally, the 'Eco' mode merely overides the hot water settings. In Eco mode, ie with the green light illuminated, the pre heat isn't activated.

daveyjp

Bulltog wrote: »

Thanks for the replies;

@daveyjp The reqiurements were assessed had various quotes icluding British Gas and every one suggested 30CDI boiler.

@Macman thanks for that clarification, that is what I thought as well, that it can't be fully based on the mains pressure. So am I right in understanding if we need water at two taps at the same time Combi is not ideal. Or with a 30kwh boiler should I be getting water at two taps at the same pressure.

@penrhyn My combi has a small storage tank in it though, that could be the reason. The pressure of water is fine normally, it is only affected if a second tap is opened. What you mean by stop value to your supply? Are you referring to the mains valve to my home or is there another valve to the boiler?

The WB produces water at about 12 litres per minute, subject to mains pressure. You will only ever get this as a max so if two taps are open the supply needs sharing, hence loss of pressure.

Its too late now, but Vaillant have a self assessment system on their website.

Bulltog

Thank you all for the quick responses. With regard to the pressure of the water, it becomes a problem only when I open a tap downstairs and another one upstairs so I presume I have to live with it. Should do proper homework next time. @DaveyJp will check out that vaillant self assessment even though it is a bit late

@dave save , thanks for the explanation about the timer for the hotwater really appreciate it.

@macman thanks for the clear explanation, my RGI while friendly didn't explain it this clearly.

unclebulgaria

erm I think you're getting the "hot water timer" confused here a little. The DT20 digital timer doesn't do anything on the hot water side when its fitted to a combi. I may be wrong but when I queried this with WB they said it was a standard timer that would fit system boilers as well as Combi's but the hot water side of the timer only works on system boilers with controls to a hot water cylinder. The green light is illuminated depending on whether you have chosen the preheat function on the plate heat exchanger (the bit that heats the hot water in your combi) or not. As for how much difference it makes, well IMO its negligible.

Dave_save

unclebulgaria wrote: »

erm I think you're getting the "hot water timer" confused here a little. The DT20 digital timer doesn't do anything on the hot water side when its fitted to a combi. I may be wrong but when I queried this with WB they said it was a standard timer that would fit system boilers as well as Combi's but the hot water side of the timer only works on system boilers with controls to a hot water cylinder. The green light is illuminated depending on whether you have chosen the preheat function on the plate heat exchanger (the bit that heats the hot water in your combi) or not. As for how much difference it makes, well IMO its negligible.

The Worcester Bosch DT20 and DT20RF (wireless room thermostat type) is compatible with Worcester's Greenstar condensing combi and system boilers. On a combi boiler, such as the OP's, you can only control the pre heat periods using the timer. What you are suggesting is that the OP ignores the facility he has for timing the pre heat function, and instead leaving pre heat permanently 'on'. I've always considered saving whatever I can, so the timer is only set for pre heat for those periods when it's likely we'll use hot water. I've had the boiler for 4 years so far. Pre heat is on for around 6 hours per day, so I'd think any savings I've made overall must be worth it, although I've never quantified it.