How do combis control HW temp?

I have noticed the combis which have "instant" hot water (i.e. the ones that keep a small store of a few litres pre tempered) work better in this respect. As the demand falls below the burner minimum it shuts off, and they keep supplying hot water from the store. As the store cools they relight the burner to temper the store. So you can have a more or less continuous trickle of hot water if you want. The temperature fluctuates a bit, but not enough to end up being cold or needing you to turn the tap on more.

I have noticed there have been a number of threads recently concerning the "fringe" behaviours of combi boilers (i.e. non obvious pros and cons). It would be quite handy to compile a table of models that exhibit these issues since it is the sort of information that is difficult to get from data sheets.

For example: how much hot water do you need to demand to get the boiler to switch into HW mode? Some I have used are pretty insensitive such that you need to turn the tap on pretty hard for the boiler to notice. Some are daft to the other extreme (like my mum's Myson) where even a dripping tap will leave it stuck in HW mode with no heating.

What happens at minimum flow and below? What is the temperature delivery profile like (i.e. is it one that accumulates a small pre charge of scalding water)? How good/bad is the fan noise? I am sure there are plenty of other examples.

Reply to
John Rumm
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Brilliant idea! Should we invite our resident combi expert - IMM - to write it?

Reply to
Set Square

I was thinking of a list of all the things he normally fails to mention!

Reply to
John Rumm

I know, I know. I assumed you'd realise that my tongue was firmly in my cheek!

Reply to
Set Square

Hi, getting back to the original question about how combis, i

particular the Vokera Linea 24, control the temperature, what is th kind of difference between the max and min temperatures for a Linea 24 My friend and I have been trying to work out why she cannot get use he shower or her bath. the CH works fine, the downstairs tap works fine She has a very high pressure at the stop tap and this evening I hav turned the stop tap down so that we can get a reasonable flow rate an maximum temperature at the shower head. However, the temperature of th water will rise to, generally, 55 deg C then plummet to 31 deg C. th temp stays at 55 for around 1 minute. 31 is too cold for commfort Reducing the flow rate so that the min temp is acceptable makes a ma temp that is scalding. The general difference between the max and th min is 20 deg C which is impractical for a shower. Is this what the call modulating and if so shouldn't it be more in the region of 5 de C? If so then what is the probable reason for this 20 deg C range. I' really appreciate a bit of advice on the matter. thanks

-- Helen

Reply to
Helen

Can't speak from personal experience of that particular boiler, however it is important to realise that most combi boilers do *not* control temperature as such. They typically have a thermostat that sets the

*maximum* temperature, but the lower limit is only dictated by the flow rate of water through the boiler (and gas rate on those with a modulating burner).

If you set the stat to 55 say, and turn a tap on low[3] you would expect the output temperature to rise to about that level. What happens next depends on the sophistication of the boiler. One with a modulating burner (like yours) ought to turn down the power so that it matches the energy demand and you will get a constant flow of water at the temperature. If however the demand is so low that it can not modulate low enough, it will turn the burner right off. The then temp would fall until it turns it back on again - and so it would continue. A non modulating boiler will have to use this trick any time the heat demand is less than its maximum output.

With some boilers this means you get a fluctuating temperature swing that can be significant[1]. This can be bad news for showers. One solution that may help is to increase demand (perhaps by running a hot tap in a basin while showering, or changing shower head to a more "thirsty" one).

As you increase the flow rate through the boiler the temperature will fall. There is usually no minimum temperature - the more flow the lower it will get[2]

It sounds a bit like your boiler may have a fault. With a modulating burner you ought to be able to get a constant temperature out of it at most flow rates once it temperature has stabilised (even if it is too high for comfort and you need to mix cold with it)

[1] the best examples - often those with a small internal store of water

- will maintain a pretty even temperature in spite of this behaviour

[2] A small number of boilers will include a flow regulator to limit the maximum flow and hence maintain temperature [3] you may need to wait a while for the temperature to stabilise. Some boilers have a habit of delivering a slug of hot water to start with that is not a good indication of the actual temperature they will be able to maintain at the selected flow rate.
Reply to
John Rumm

I think the initial slug of hot water is more a feature of the plumbing than the boiler. If you have a significant length of well insulated pipework between the boiler and the shower and some hot water has recently been drawn off then you will initially get some residual hot water from the pipe. There's a short time lag between the water flow being detected and the boiler firing up during which some cold water gets through before the fresh supply of hot arrives, unless the boiler has it's own hot water store.

I usually spare myself the cold shock by running the washbasin hot tap until the real hot water arrives before starting the shower.

Reply to
Mike Clarke

Thanks to both Mike and John for your responses. If my friend is in thi

evening I will go and suggest that we try running the shower at the sam time as running either the bath tap or the bathroom sink tap. I mysel have a worsceter bosch 532 (I think) from BG and I think that it has little reservoir tank that is supposed to deliver instant hot water However, I still think that you get hot water, maybe left in the pip maybe from the reservoir, then I am sure that a certain amount of col comes through before the freshly heated new water comes through.

Back to my friend's boiler. A few further things that puzzle me. Thi problem has been going on for a few months and one of the things tha she did to try and get a shower or bath was to set the temperatur control at max. For a normal shower flow rate the led display wa maxing at 55 deg C and dropping to 31 deg C, regardless of the sta setting. The kitchen sink, however, at a normal sink filling rate wa maxing around 68 deg C for the same stat setting. Why is there such difference between upsatairs and downstairs? My first thought, before read John's posting, was that the water pressure coming in was too grea and that there is a difference in the bore sizes of the pipes betwee upstairs and downstairs - small bore, higher pressure, lower velocity so the upstairs would have bigger bore pipes, lower pressure and highe velocity than the kitchen and that would explain the difference betwee the kitchen and the bathroom. reducing the pressure at the stop ta allowed higher temps than 55 to be reached in the bathroom for a slo shower but the difference between the max and min cycling temps wa still around 20 deg c and a max in the 60s dropping to a min in the 40 during a time period of say, 2 to 3 minutes is too extreme. With th stat set down to a sensible setting, about 3/4 around the dial, for normal shower flow rate, the max temp is still 55 deg C. basically th max temp upstairs seems to be fairly independent of the stat setting.

Further, last Nov, when this all started, she had a complete boile failure, CH as well as DHW. A gas engineer came to her house an replaced a thermistor and the heating worked, the kitchen sink worke and the bathroom sink seemed to work (that cycles between 55 and 31) Apparently when she came to use the shower that didn't work as i should. Could it be anything to do with a thermistor or is it somethin different

-- Helen

Reply to
Helen

It does depend on the boiler... some seem to have a design feature that results in the small amount of water sitting in the secondary heat exchanger getting heated up to the CH circulation temperature should that be on. Other that maintain a small store of a few litres of heated water to give "instant" hot water may also deliver a hot surge if the tap is turned on at a high flow rate (although in these cases it will be no hotter than the hot water stat temperature).

Wise move ;-)

Reply to
John Rumm

I think John Rumm has just about covered most of the subject. It is good practice to have the cold water input restricted into the boiler so that the 'natural' open positions of most/all hot taps produces a reasonable water temperature. If you turn the mains half off then the garden hose won't be as good. Almost all combis (even the bottom end models) have modulating burners. Limescale in the DomesticHW heat exchanger can make matters much worse.

The HW slug _is_ a feature of the boiler with many models having this problem. The DHW heat exchanger get to primary (radiator) temperture in winter and when you draw off HW you get a really hot slug through.

Reply to
Ed Sirett

I think that my friend needs to get a corgi out to look at her boiler

-- Helen

Reply to
Helen

Having done battle with a Vokera Linea 24 before I have to say that I think the display in DHW mode can be a bit of a red herring.

I tried a number of things (since I was getting confusing results with the temperatures and flows) including a new temperature sensor, nothing seems to work. In the end treating the DHW exchanger with dilute hydrochloric (Screwfix Patio Cleaner) made a big difference.

Reply to
Ed Sirett

I see what you mean. I suppose I've not noticed the effect due to he long and devious route between the boiler and bathroom. I expect the long length of relatively cooler pipework takes a bit of heat out of the initial slug.

Now you mention it, I think there is a bit of this effect in the kitchen where the tap is a bit closer to the boiler but not enough to be a problem, probably because the scantily insulated hot pipe runs under the suspended floor for a significant distance - a matter to be rectified when we re-fit the kitchen.

Reply to
Mike Clarke

Not all boilers exhibit this problem anyway... it seems to be a quirk of design that some do.

Reply to
John Rumm

To not get this feature the boiler would have to, at least, not have the following features: a) The use of the DHW heatXer as a bypass for the central heating. b) The DHW heatXer being part of the primary heat exchanger. (e.g. Ferrolli, Pott. Lynx, Saunier Duval)

This will cut down the number of suitable models by a few bit.

Reply to
Ed Sirett

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