Combi's (Accepting Pre heated Mains water)

[Juliancanoe]

how do combi manufacturers stand with the risk of scolding when using their products? My Halstead would quite happily scold you if you ran the hot tap just open a little.

Man runs tap. Boiler is offended and upset at being aroused: "You ran the hot tap... how dare you! Do not do it again or I will have to throw a fault..."

(Sorry, couldn't resist, I think you meant scald ;-)

[pete_w]

For what it's worth, this is slightly off-topic, but I had a chat about this with Vaillant when I was designing my heat dump, because I wanted to pump hot water through my shiny new efficient condensing (but conventional, open-vent) ecoTEC Plus 428 boiler, when the boiler was inactive.  Their initial answer was "oh no you mustn't" but when I got past the techsupport people to talk to someone that knew what they were talking about, the advice was that my "trad" boiler would be absolutely fine with it - that's how Vaillant do heat dumps in their own solar systems, apparently -  but I mustn't try it with their "system" or "combi" boilers, apart from damage to the inlet components, it freaks out the boiler control system, potentially leading to multiple resets and lockouts.

So I'm very happy that I stuck with a traditional plumbing layout - open vent boiler feeding (thermal store) tank and/or central heating via valves, with separately mounted valves and pump. 

Cheers
Pete

[bmike]

I'm quite interested in this detail. Can anyone tell me how does the boiler control the temperature of the water. It can't control the flow so the best it can do is control the burn. However, I would assume this can't react very quickly so the boiler starts by turning on the heat full blast getting the temperature up as fast as possible (because I assume that's a selling point) only to find just as the water approaches the right temperature it suddenly receives hot water and needs to turn the heat off very quickly. So in practice could you get sudden rise in temprature before it drops back.

For this to work it must be able to control the amount of gas burnt but when I see boilers they seem to be either on or off so applying a small rise in temprature might be difficult. Also if the gas is turned down how does that affect the efficiency? What I need is a complete guide to the internal workings of boilers -any references.



 

[martin W]

how do combi manufacturers stand with the risk of scolding when using their products? My Halstead would quite happily scold you if you ran the hot tap just open a little.

Man runs tap. Boiler is offended and upset at being aroused: "You ran the hot tap... how dare you! Do not do it again or I will have to throw a fault..."

(Sorry, couldn't resist, I think you meant scald ;-)

hmmm. I think its about time I re-read my posts before actually posting them Mind u using the spell checker occasionally wouldn't me hurt either

[martin W]

For this to work it must be able to control the amount of gas burnt but when I see boilers they seem to be either on or off so applying a small rise in temperature might be difficult. Also if the gas is turned down how does that affect the efficiency? What I need is a complete guide to the internal workings of boilers -any references.

I think this is where the boiler modulates its gas useage to the output required. Hopefully someone will be along to clarify this.

On efficiency I am sure that most boilers are most efficient when going full blast. My Halstead is 81.3% efficient on heating hot water at max gas and 74.1% efficient at heating hot water on min gas. I am assuming this is to do with the water temperature setting on the boiler.

see the attached photo - I worked out the efficiencies in excel based on the input and output of the boiler. 81.3% ties in with the SEDBUK 82% efficiency rating for this boiler.

[pete_w]

I'm quite interested in this detail. Can anyone tell me how does the boiler control the temperature of the water. It can't control the flow so the best it can do is control the burn. However, I would assume this can't react very quickly so the boiler starts by turning on the heat full blast getting the temperature up as fast as possible (because I assume that's a selling point) only to find just as the water approaches the right temperature it suddenly receives hot water and needs to turn the heat off very quickly. So in practice could you get sudden rise in temprature before it drops back.

For this to work it must be able to control the amount of gas burnt but when I see boilers they seem to be either on or off so applying a small rise in temprature might be difficult. Also if the gas is turned down how does that affect the efficiency? What I need is a complete guide to the internal workings of boilers -any references.

Can't comment on any other boilers, or indeed how they do it, but mine seems to modulate very well.  Mine is very far from "on" or "off", its output range is from something like about 6kW up to 28kW.  I got into correspondence with someone else on this forum who's had a very unhappy experience with Vaillant boilers - I suspect he had older firmware than mine, and his started and ran at 12kW for precisely one minute before modulating up or down.  This was presumably so that the control system could calibrate the flow rate of the water, so if the heated water came back round the loop to the boiler input again in less than a minute, such that the 12kW being added then took the output temperature over the target cut-off, his boiler would shut down again.  Not a happy bunny.  My Vaillant seems happy to modulate itself within a few seconds of startup, the cal is obviously much quicker.  It can produce a range of noises from an almost inaudible whisper when pottering along keeping the CH circuit at 45C with most of the TRVs shut, through to something resembling a jet aircraft in my utility room when the weather compensator says "damn me, it's below zero outside and he's just come in and asked for some heat, let's nail it at at 85C....".  The burner fan (which I take as an audible indication of burner capacity) does seem to react quite quickly (a couple of seconds or so) to small variations in incoming water temperature.

[martin W]

Just has though on using a TMV with a combi boiler - probably a stupid thought, but I'd just as well run it pass you folks....

My pre heated water was at around 38°C this morning and so combi boiler came on to heat the water up. Hot water temp on boiler is set a minimum. I ran the kitchen sink tap at max flow. I still sort of got scolded ( ) by the boiler.

Now here's the stupid thought.... What would happen if I fitted a TMV after the combi boiler and fed it with hot water supply from boiler and cold water supply is actually from the preheated boiler supply (from thermal store).  So hot water out of boiler could be = 65°C or more, Cold water (from preheat thermal store) could be at 35-45°C. Could I then mix to around 50-55°C? What would happen to the combi boiler? Would I use less gas and still not scald myself? WOuld boiler go pants up and throw a wobbly and shut down cos its gettign too hot if it can' modulate down (or modulate down enough)....

whats your thoughts on this folks...

[Justme]

I would feed the full solar temp water into the cold inlet of the boiler & fit the TMV after the boiler. That way the TMV will control both sources

OOPS now why have I just done the total opposite on my system?

Oh yeh it was to protect the plastic pipes lol.

Justme

[dhaslam]

It would need two mixing valves because presumably there is a maximum temperature for the inlet to the combi.  One problem  is that mixing luke warm hot water into the hot outlet means that the hot water is not fully sanitized.   Anyway using more water from the cylinder will lower  the temperature of the cylinder water  and eventually lower the temperature of the input to the combi.   

[KLD]

OTOH if the tank temperature isn't that high, then its losses won't be high either. You could just leave more warm water in the tank, and use cold mains to mix after the combi boiler.
The problem with mixing after the boiler could be that the flow rate through the boiler drops too far and the boiler simply switches off.

Klaus

[stephend]

There would always be some cold lag between the tank and the boiler, even if the tank was at a hot enough temperature.  So how do you prevent the boiler from firing up for a few seconds as it clears the cold lag?  Can you move the boiler's input temperature sensor onto the tank itself?

[martin W]

There would always be some cold lag between the tank and the boiler, even if the tank was at a hot enough temperature.  So how do you prevent the boiler from firing up for a few seconds as it clears the cold lag?  Can you move the boiler's input temperature sensor onto the tank itself?

Sort of... I've put a tank stat in circuit with the boiler water demand switch , it makes the boiler think that the water is not being drawn when the cylinder is above the set point (about 45°C) and stops it firing.

[martin W]

The problem with mixing after the boiler could be that the flow rate through the boiler drops too far and the boiler simply switches off.

Klaus

Thats what I thought.... maybe if I get my hands on another TMV I might try it anyway and see what happens....

[solar_cambridge]

Sort of... I've put a tank stat in circuit with the boiler water demand switch , it makes the boiler think that the water is not being drawn when the cylinder is above the set point (about 45°C) and stops it firing.

Thats a very clever solution to the dead leg problem when there is sufficient hot water!

[Water Systems]

You could try combi made by Alpha, they take preheated water up to 60c. You would also need a
diverter valve on cold feed into combi, to change flow at 60c. Alpha will not sell you their solar
diverter valve unless you have bought their solar kit.

Kenny

A normal blending valve can be piped up to be a diverter valve.  If water is below valve setpoint it is sent to the combi for heating top-up, if below setpoint it send it to the taps.

You may need a blending valve on draw-off in case DHW is higher than scaling temperatures.