Can I preheat the feed to my combi boiler?

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Hi,
Only my 2nd post, but you were all so helpful with the first post I thought I'd have another go.
I was talking previously about how I have a fairly new combi boiler (Vaillant Ecotec Plus 832) and also some solar PV and had the notion of fitting a HW cylinder as well.

Since then, I've seen talk of pre-heating water to your boiler to save on gas costs. The thinking being if you have a HW store heated by say "free" solar, then you use that as the supply for the boiler. Depending on the temperature of the water, the boiler either fires up and heats it or realises no heating is required and bob's your uncle, gas saved :)

Im assuming an unvented cylinder, with everything under mains pressure. So then in the summer, the boiler might not be needed at all and in the winter more so. It all sounds very logical, and I've seen threads talking about modulated & unmodulated boilers, but I'm sure the devil is in the details. I've tried reading the installation instructions for the boiler but can't find anything that specifically allows or forbids pre-heating so I thought I'd throw it out for some help and advice.

Very much appreciate your time. One of the only problems with this forum is you see lots of ideas that make you think hmmm I wonder if I should do that ? I may end up never doing anything at this rate !

Cheers
 
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Best to call Vaillant and ask if your boiler is suitable some are some arent, what you suggest can certainly be done, but not with every boiler
 
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It took several calls and a lot of waiting, but I eventually got through to someone at Vaillant that could answer my question, and they say "no" :(
Apparently the Ecotec Plus range can only accept a cold water feed. I tried to ask why, if it was a no because it couldn't cope with the hot water or if they were just saying that it wasn't a boiler that could modulate its heating, to apply just a little or a lot, if it was just on full blast or off and therefore wouldn't save any gas anyway? No joy, oh well a man can dream .......
 

CBW

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Afaik you can range rate the heating depending on the load. As for the preheat for hot water, this can be turned on and off.
 
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I've done that exact thing with an unvented HW cylinder feeding the cold inlet to an Ideal Vogue. That was 5 years ago, and have had no problems. The reasons: the combi was cheaper than the system boiler, the unvented cylinder was oversized for the house, and the solar PV system had an Immersun attached (PV diversion into immersion heater). This Vogue combi fed just one outlet, the kitchen hot tap.

Can't speak for Vaillant though.
 
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It took several calls and a lot of waiting, but I eventually got through to someone at Vaillant that could answer my question, and they say "no" :(
Apparently the Ecotec Plus range can only accept a cold water feed. I tried to ask why, if it was a no because it couldn't cope with the hot water or if they were just saying that it wasn't a boiler that could modulate its heating, to apply just a little or a lot, if it was just on full blast or off and therefore wouldn't save any gas anyway? No joy, oh well a man can dream .......
It is to do with the boilers electronics, some boilers operate a flow switch system to detect HW demand , there are a few ways , some detect temp differentials to show hw demand, some monitor incoming temp and outgoing temp, not all boilers use the same methods
 
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I’m doing the exact same with my worcester turns out it can take pre heated upto 65C so will have a 250L direct tank heated by my solar bank, to 55C with boiler set to 53C

Also once a week tank will cycle upto 65C
 

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