Boiler schematic explanation

I've posted already about problems I seem to be having with my Ravenheat CSI85T boiler (sometimes I think I'm needing the CSI team out to it!!).

I'm still waiting on the gas engineer coming out to see it, but in preparation, I do like to know what I'm talking about.

I was wondering, if I posted up a schematic of the boiler, is there anybody that would be able to go through the workings of it with me?

In some areas, it seems relatively simple in how it works, but there is a bit that kind of confuses me and it looks like the central heating water goes into the heat exchanger for the hot water - which I don't understand.

Any help gratefully received.

Thanks,

Alison

the HW heat exchanger transfers heat from the boiler side(radiator water) to tap water, keeping the two seperate like the coil in a cylinder

So there's no way that central heating water should mix with tap water?

This is my problem - when the central heating is on and you turn on a hot tap, the water that comes through is scalding hot from the central heating. After almost a minute, it does eventually regulate down to the right temperature.

when the boiler is hot from CH you will get an amount of very hot water from the taps as the diverter valve just blocks off the rad circuit and recirculates the water in blr

That's what the gas engineer said, but the tech guy from Ravenheat said it shouldn't do that.

All I know is that is seems to be bloomin dangerous to have such hot water come through for that little time - I've lost count of the number of times I've forgotten what I'm doing and put my hand under the tap when I turn it on.

then ask him what can be done about it. I for one would like to know

The Ravenheat guy? He was very vague when I spoke to him and told me that water temperature should remain constant because the system does not mix the water from the central heating.

Looking at the schematic, I'd understand that if there wasn't a pipe coming from the three way valve back into the DHW heat exchanger!

Can I say that I hate this boiler? It wasn't my choice, by the way, to have it installed!

without that pipe from div valve to DHW heat ex the HE would never get hot!!!

I'm a girl - is that excuse enough?

I must be looking at it too basically then. My friend's boiler doesn't do what mine does but she has a different boiler.

Oh, how I wish I had a different boiler - then I wouldn't have permanent wash day red hands!!

I suspect you have a problem with the diverter valve. Perhaps when the boiler is heating the radiators the diverter valve is not fully shutting off the secondary heat exchanger port. As a result a little water (at very high temperature) is still flowing through the secondary heat exchanger (heating it to high temperature) resulting in an initial "hot slug" of domestic water.

Only the older bi-thermal heat exchangers tended to have this hazardous feature.

That could be the problem. It was one of my friends that directed me to this forum and she thought it sounded like the diverter valve (how she knew that I'll never know!!).

At least with that information, I can sound like I know a wee bit about it so that the guy that comes out doesn't try to fob me off like he did before!

Thank you.

The gas man's been. He fell out with me! Being a nosey kinda person, I was trying to have an intelligent conversation with him about the workings of a boiler and was it really normal for it to have that first slug of scalding water. I also mentioned that I'd phoned Ravenheat first before phoning him out - cos I didn't want to phone him out for nothing - and that seemed to be the straw that broke the camels back.

Basically, he informs me that every single combi boiler in the world does exactly the same thing as ours does and unless I turn the thermostat down on the central heating (which of course will turn the heating down), there's nothing I can do about it - so live with it.

And there's absolutely nothing wrong with my diverter valve (x-ray vision and he never took his hands out of his pockets).

Lovely man and all this at 8 in the morning!!

I've got an earlier boiler (Ravenheat RSF84ET), which has a very similar heat exchanger design.

I agree with your gas engineer. Ravenheat are correct in that the CH water should not ever mix with the tap hot water, but they are coiled together in the heat exchanger.

When you switch on a hot tap, the diverter valve 'short-circuits' the CH pipework, so it doesn't leave the boiler. This short loop then passes through the heat exchanger with the incoming mains cold water. The heat is transferred from the CH pipework to the hot tap pipework.

If you have the CH on, then the entire CH pipework will get hot. Therefore, when you first turn on the hot tap, some mains cold water will be super-heated by the hot CH water already inside the boiler. The CH water will then go cold, and will have to be re-heated by the burner (which fires within a second or two of you opening the hot tap). It takes a short while for the burner to get warm and start heating the CH water again... hence the hot tap going a slightly colder, then warm again.

If the CH was not on, then all of the CH pipework is cold, so you have to wait for the burner to warm up.

I'm not sure that every combi in the world works like this, but yours does. He's correct that you could reduce the CH temperature slightly (turn down the dial) but that will have the side effect you predict...

Regarding the diverter valve, you can perform two tests:

a) Turn the boiler off. Open the hot tap and listen for the microswitch clicking. Turn the tap off and on a few times. Does it click every time? Even with a fairly feeble flow? If so, the diverter's diaphragm is fine.

b) With the system stone cold, hold the CH flow pipe under the boiler. Turn a hot tap on and let it run for a few minutes. Does the CH flow pipe get hot? If so, the diverter is broken.

Hope that helps!

Perhaps I am looking at a different boiler, but Partsarena shows CSI85T as having a plate2plate HW heat exchanger. Have to agree with Gasguru that the divertervalve may not be closing the HW port fully.

Way to proove this is to take temperature readings. Am a little surprised at your heating man falling out with you. No matter how clever you are, a day never goes by when you don't learn new things. Would have been easier for him to proove you wrong and still have your custom instead of falling out with you.

The diverter is a brass section that channels all the heated (dirty) water to the radiators when boiler is heating mode. When you run a hot tap, all this dirty water is diverted to pass through a hot water heat exchanger. This unit is like a stack of sardine cans. Imagine the dirty water entering the stack in one corner, flowing through odd spaces, comes out opposite corner to be reheated. This flow of water heats the plates in the stack that contains this flow. Meanwhile, cold water enters a corner and flow through even gaps. This (clean) water flows out of the 'sardine stack' hot for use at the tap.

When the boiler is in heating mode, there will be no (well hardly any) flow through the 'sardine stack' so initial flow might be tepid but not boiling.

Suggest you have the diverter looked at.

That was a damned fine explanation, thank you. Maybe I'm a pain in the butt as far as tradesmen think cos I like to know how things work. I hate not understanding something and that's all I was wanting this morning, but the gas man lost it with me - I'm sure he called me a few choice names as he left (and I was being nice, honest).

I still think it's a flippin stupid design - but your explanation makes sense. Not knowing an awful lot about boilers, I'd have thought - two heat exchangers - one for the heating and one for the water - running independantly of each other (but I bet that's not how a boiler works!).

Thanks again.