thermostatic shower and (supposedly modulating) combi boiler

[jbert]

Hi there.

I've just had a new shower fitted, a 'Mira Extra', which (I think) is a thermostatic one, i.e. it adjusts the hot and cold flow to get the set temperature.

I've got a combi boiler (a Halstead Finest Gold) which the brochure says is a modulating boiler (i.e. one which reduces the gas flow when the hot water flow is reduced, so you shouldn't get a spike in temp when turning down a hot tap).

As far as I understand things, this should all work. However, the problem is that the shower drops the pressure down to a trickle over a minute or two, exactly as if it was pulling less and less water from the hot.

By turning the sink taps (when the shower isn't running) I notice that the hot water temp does vary a lot - the water is very hot when it's a trickle, which would I guess explain why the thermostatic shower is having trouble keeping up. Does this mean the boiler is playing up? Do they 'stop modulating' over time?

Do you think it's worth playing around with the different settings (hot water temp from boiler, thermostatic setting on shower, anything else?) or should I try and get the shower replaced with a manual? Is there anything obvious to check on the boiler? (pressure gauge etc?)

The shower starts off great (good pressure and temp) but then the pressure dies away, presumably as the thermostatic valve fights the boiler

Lastly, should I be asking this in a different part of the forum?

Thanks in advance for any ideas or suggestions.

[al-yeti]

i thought because the tap is on so slow the water has a very small flow rate through the boiler hence why its very hot and varying,

also thought that some thermostatic showers can not be run direct from the boiler.

thought i would post this as no one else has replied and i might be talkin nonsense

[gas4you]

If you run the shower as normal, can you see the boiler flame modulating down? Does hw seem ok on all other hot taps?

Is your cold supply to bath mains pressure or has it been left on a tank feed for some unknown reason?

__________________
Dave
DRL Gas Services

[Agile]

There are two things you can do.

If you have a proper model turn the DHW temp knob on the boiler down from maximum to about half way and see what that does.

Otherwise run a hot water tap on half at about 3.5 li/min and that should make itr work OK although its a wasteful way of doing it.

This assumes the boiler is properly set up. As you have never had it properly serviced it may just be out of adjustment. The gas valve needs to be properly set for min and max. Not many people seem to know how to do that.

Tony

[gas4you]

Agile wrote:

Not many people seem to know how to do that. Tony

Tony, are you Michael, sorry, Sir Micheal Caine in disguise

I must be naive I thought minimum and maximum settings were what we set correctly in our sleep

__________________
Dave
DRL Gas Services

[jbert]

Thanks very much to everyone for the ideas.

There isn't a visible gas flame on the boiler, so I can't see if it's modulating down or not.

You might be on to something with the cold supply. Cold pressure is normally very good (e.g. bathroom sink taps) but the bath cold pressure is very low (just a trickle) It looks like a pressure reducing valve was fitted at the same time as the shower. I guess that might explain it. The shower starts at a decent temp and pressure (because the hot water is still warming up?) but once the hot gets too hot, the pressure is dropped because the cold pressure is low?

I'll try some more experimentation with the boiler hot water temperature (and having a tap running). If nothing else it'll give me more info on what's going on. I'll also see if the pressure reducing valve is adjustable and see if that helps.

Lastly, am I right in thinking that modulating boilers can "stop modulating" and that is fixable (and I should get that looked at on the next service for the boiler)?

Thanks again folks.

[Agile]

Gas4 U and I do set up gas valves properly but few RGis know how to do it properly. Only yesterday I met a nice guy who has been in the industry for about 8 years who had messed up that adjustment.

If there is a pressure reducing valve on the cold then thats the first point of interest! It may have a filter on the inlet which is blocked !!!

It might have drifted its adjustment and could be increased a little AFTER any filter has been checked.

Tony

[ukboilerman]

jbert wrote:

Hi there. I've just had a new shower fitted, a 'Mira Extra', which (I think) is a thermostatic one, i.e. it adjusts the hot and cold flow to get the set temperature. I've got a combi boiler (a Halstead Finest[/url] Gold) which the brochure says is a modulating boiler (i.e. one which reduces the gas flow when the hot water flow is reduced, so you shouldn't get a spike in temp when turning down a hot tap). As far as I understand things, this should all work. However, the problem is that the shower drops the pressure down to a trickle over a minute or two, exactly as if it was pulling less and less water from the hot. By turning the sink taps (when the shower isn't running) I notice that the hot water temp does vary a lot - the water is very hot when it's a trickle, which would I guess explain why the thermostatic shower is having trouble keeping up. Does this mean the boiler is playing up? Do they 'stop modulating' over time? Do you think it's worth playing around with the different settings (hot water temp from boiler, thermostatic setting on shower, anything else?) or should I try and get the shower replaced with a manual? Is there anything obvious to check on the boiler? (pressure gauge etc?) The shower starts off great (good pressure and temp) but then the pressure dies away, presumably as the thermostatic valve fights the boiler Lastly, should I be asking this in a different part of the forum? Thanks in advance for any ideas or suggestions.

That is a big question!

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