C/H Boiler & Shower Temperature Issues

[Richard Faulkner]

Hi,

I have just had a flat refurbished, to include a new C/H system with a Worcester Junior 24i combi boiler, (not greenstar), and a shower cubicle with a thermostatic shower and sliding doors.

There are 2 problems and I am not sure whether I am being fobbed off by the plumber/builder. I'll either fix them myself if simple enough, or I will be armed to argue the toss.

1) Having tested the shower, I found that I could not get the water temperature to remain stable and hot enough. The plumber said that this was because the water was reaching a high temperature set by the boiler thermostat, the boiler cuts off, and waits too long before igniting again. He suggested turning down the thermostat on the boiler to prevent the water getting so hot and, either not cutting out, or reducing the time it waits before igniting again.

This doesnt make sense to me as I thought the boiler thermostat controlled the temperature to the rads only, and not the hot water, so altering the setting should make no difference.

I understood that hot water was supplied at a temperature dictated by the flow, and the shower thermostat should control the output at the shower head.

Following from this - the shower thermostat has a "safety" button which can be pressed to allow the temperature to be increased. I am presuming that this "safe" temperature can be altered by adjusting something in the temperature adjuster. Is this the case, and is it generally straightforward.

2) The shower cubicle has a fixed glass panel with another glass panel which slides behind it. Along the vertical end of each panel is a plastic trim with a protrusion which is obviously designed to prevent water getting between the gap. The sliding door does not close far enough for the two protrusions to meet, (which I am guessing would stop any water getting through), nor do they protrude far enough to meet the opposing glass, thus providing a seal.

Whilst I am sure it will be rare, there are some angles at which water can find its' way through the gap to run onto the bathroom floor. Knowing my luck, I will have a tenant who finds these angles and floods the bathroom and the flat below.

Any ideas on either, or both, of the above?

Many Thanks

[John Rumm]

Not all combi boilers are the same in this respect. I don't have any experience with your boiler and so can't comment specifically on it. However there are some points you ought to be aware of.

Most combi boilers will have a maximum temperature thermostat for their hot water generation. On ones like mine (Ideal Isar) it is a variable control independent of the CH water temperature. On others it may be a fixed limit. It is unlikely to be much over 65 degrees in any case. This dictates the maximum temperature. At low flows the water temp should not exceed this. As you raise the flow eventually you reach the point at which the boiler is unable to maintain the temperature and it starts to fall (save for the few boilers that have a flow regulator). The minimum temperature will be a function of incoming cold water temperature and flow rate.

If you demand hot water at a rate low enough then the combi will not be able to modulate the gas rate low enough to keep the water temp under the limit set by the stat. In this case it will have to turn off the burner. Note that with a modern modulating boiler (especially a low output one like yours) this would probably equate to a pretty low flow rate. What happens next depends a bit on the boiler design. Ones that have a internal store of pre tempered water (for "instant" hot water) will often be able to cycle the burner while maintaining a reasonably constant output temperature by re-tempering the store for time to time. However ones without this will give alternate bursts of hot and cold water.

One test to try to see if this is the problem is to use more flow on the shower. If you are already using all that you can get, then see what happens if you also run the hot tap on the basin at the same time. This will increase the demand for hot water and keep the boiler firing continuously. If this does turn out to be a solution to the problem then changing the shower head to a more "thirsty" one may fix it.

Are you sure the shower has a thermostatic control. Also is it one where you can select temperature and flow rate independently? If it is/does then be aware that you can also get strange interactions between a combi boiler and a shower of this type. Often they work just fine together, however you can get problems if the boiler is supplying water close to its maximum rate. The shower may try to correct a dip in temperature by increasing the hot flow. However this may result in the hot water getting colder (i.e. boiler can't heat the higher flow to the same temperature as before). This can create feedback cycle that gives results like you are seeing. It can also happen in this way if your cold main is not up to the task. The shower may try to compensate for a hot output by increasing cold flow, which can starve the boiler of water hence reducing its flow and thus raising its (and hence the showers) output temperature still further.

(to an extent you can see similar problems with manual showers, except you have to learn how to "tune out" in interactions in temperature and flow rate. Needless to say if anyone / thing else demands hot water elsewhere in the house at the same time, all bets are off! ;-)

Ideally it ought. It works well on mine, however the neighbours (with an ageing less sophisticated combi) gave up trying to use the shower years ago due to similar problems you describe).

On some you can. The bar mixer I fitted to my new shower room came with instructions on doing this. Yours may have similar. If not immediately obvious see if there is anything on the makers web site.

[Hugh]

Just a couple of thoughts: Builders (well the one's I come across) are notorious for not reading instructions! Did yours fit a flow restrictor on the cold inlet to the shower mixer? with a combi-boiler there should be one or the shower runs too cool. Also, was the boiler checked? It's just possible that the output of hot water needs 'stabilising' - it shouldn't!, but worth checking by a gas fitter in case the gas valve needs adjustment, or there's some other problem causing a low flow rate/heat output. Did you invest in a cheap mixer? It's a false economy to try to save money on things like this - in various rented properties, I've had a Mira mixer last for around 15 years, but three of the Wicke's (although now taken over by Mira) have all needed servicing!

My own shower has been running cool for the past few months, but I think it's the diverter valve in the Worcester boiler (it's around 8 years old) - must get around to changing it.

Hugh

[Dave Plowman (News)]

Even on the most basic water heater, this would only happen if the cold flow through it was inadequate, and with a boiler called a 'junior' this is rather unlikely.

It will have a spec for how much water it can heat in litres per minute. With a starting temperature for the cold and a maximum temp for the hot. This is an easy DIY job to check. A simple thermometer, bucket and watch.

The boiler user instructions should tell all about this?

[John]

This is just possible if the shower flow rate is below the boilers minimum modulation point. An easy check is to run a hot tap slowly at the same time as the shower is running. If the increased throughput of hot water keeps the boiler alight and the temperature stable you know where to start to sort the problem. It is possible that the plumber didn't flush the pipes before connecting the shower and an inlet filter is clogged with crud/flux/debris so causing a flow restriction.

Depends on the boiler some have a tapwater thermostat as well

Within the limits of physics (in this reality)

Wankers are Wankers and you can only try to get a tenant with at least half a functioning brain cell

[Richard Faulkner]

In message , John writes

Then I am that Wanker! It happened whilst I was testing it, which is how I found the problem. Obviously, now I know it can happen, I can avoid it - but even the best tenants wont care about the property like I do.

Thanks for the advice on the boiler and shower - and thanks to everyone else who has responded - I'll have a think about it and try a few things, before getting the builder in.

[andrewpreece]

Viz your last point, why bother fiddling around with the 'safety' button set point? It is set to blood heat I believe, which doesn't give a very invigorating shower, so I just depress it and override it to get the temperature up around

43 degrees. I set my combi so it doesn't produce hot water at more than about 53 degrees ( as hot as you can hold your hand in ), so that I can't get scalded by the hot tap. This requires a quarterly adjustment of the combi DHW temperature control, but that's not so onerous. I have annotated the control knob with the seasons so I know just where to set it for the correct temperature. If you set the DHW temperature hotter than about 50 degrees, the flow rate will decrease anyway, since combi flowrates are quoted for a delta T of 35 Celsius.

Andy.

[PC Paul]

The several showers I've installed all let you set the safety point up yourself as you assembled the control lever. I've always set them to 'bloody hot' but not scalding.

Is yours just badly set up? Change it...

[Christian McArdle]

Is the shower valve suitable for a combi boiler?

Combi boilers have an issue in that once you draw more than a trickle, the output temperature at the tap varies with the flow rate, responding several tens of seconds after the flow rate change.

In order for a thermostatic valve to work with this, it should basically attempt to draw a constant supply of hot water and just vary the cold. This means that its hot supply stays at an approximately constant temperature, making it possible to get a stable temp.

If your mixer is not designed for combis, it might vary the hot supply to maintain temp, which has the effect of changing the combi output temp, causing massive instability in temperature regulation.

BTW, a 24kW combi is not very powerful. If you have a fairly freeflowing shower head, the temperature will drop whatever you do, as you are using more water than can be heated. You may need to change the head for one with smaller holes, so it doesn't overload the combi.

Alternatively, if you have a head with too small holes (such as one designed for an electric shower), you might draw less water (and hence heat) than the boiler is capable of providing without shutting down. Then a larger head is required to give the combi something to chew on.

Christian.

[Richard Faulkner]

In message , Christian McArdle writes

It seems to be - I've swapped it with one from another flat and it works ok in the other flat, but the new one has the same problem in the problem flat - if that makes sense

So it seems to be some kind of problem at the boiler end.

I'm going to try and have a shower in it in the morning, and see how it goes.

It works OK for Central heating, and for heating water to the kitchen and bathroom sinks - but constant heat doesnt matter so much in these.

[Dave Plowman (News)]

Is the hot water flow ok from other taps in the bathroom? If so, this indicates a problem with the shower. Even, possibly, the hot and cold feeds swapped if a thermostatic type.

[Ed Sirett]

Is this flat higher than the others and has less water pressure/flow?

[OldBill]

[SNIP]...

A similar problem I had in a property was fixed like this. What actually happened was I phoned the thermo-shower makers/importers. When I told 'em I was using it on a combi they told me how to increase the shower valve's hot flow by adjustment of a large slot on the back of the valve body. Of course we already had the flow restrictor in the mains fed cold side.

[Richard Faulkner]

It's Flat 5 at the top of the building, but Flat 4 is also at the top of the building, and the shower in there works fine (Flat 4 is fed from a Worcester 24i Combi - not a Greenstar or a Junior), and now has the original thermostat from Flat 5. I'm pretty sure that the water flow is high enough - the cold water is powerful enough to have to turn it down to stop it splashing out of a reasonably deep kitchen sink.

I think the plumber is going to test the water pressure drop, or something tomorrow morning - I'll see if we can check that the hot and cold arent swapped, and if we can increase the hot water flow in the thermostat.

Thanks again!!

[Ed Sirett]

I've met the swapped pipework problem. You either get stone cold or full hot and nothing in between.

[Richard Faulkner]

Checked tonight and it's not swapped pipework - the correct side of the shower thing gets hot when it's running.

[PC Paul]

Interesting - I'm in a rented house for now and the shower is a thermo valve on a combi (the one that keeps heating till the safety cutout pops and kills the gas.. getting fixed this week!).

The shower has always been full on to get it hot, then back it off 1/4" and it's pretty much cold. Not much room for comfortable adjustment. I assumed it was a knackered themostatic valve but I suppose it could have *always* been like that. Not easy to change now though.

Although since it's a round valve with pipes to each side I suppose I could just turn it over? hmm....

[Richard Faulkner]

Plumbers been and seems to think it is overheating because:

Water gets to temperature in shower and thermostat calls for less hot water.

Boiler continues to heat less water at same rate/power, thus overheating.

He puts it down to either a gas valve operating too slowly, or an electrical sensor not operating quickly enough.

He's going to get Worcester Bosch out under warranty - so we'll see what happens.