Hi, just had a bathroom refurb and the thermostatic shower we have put in (Mira Realm) outputs a minute or so of hot water and then the water is only lukewarm for the duration of the shower (this is with no other draw on the water). We have a Worcester Bosch 30 he combi boiler rated at 30Kw (seems to be 10yrs old) with a hot water flow rate of just over 13L/min. We had the boiler serviced and it is operating within its parameters. Is the likely cause that the boiler can’t output enough hot water on demand to keep up with the flow rate of the shower? It does have a rain head on it. If so, what would be the best way to confirm this? And if we can confirm that the hot water rate is inadequate, would the best solution be to install a 40Kw boiler? Any help would be great!
Do you have constant hot water at all the other outlets? If you do it would be worth you contacting Mira as its a brand new shower, get their engineer out to assess the installation and the shower. You have a warranty on the shower, use it before going to all the expense of changing the bolier. Have you checked with the shower installation instructions that it is compatable with a combi, there are showers that are not. Finally your asking about hot water rates, you have all the literature have a read of boiler flow rate and required rates for the shower. 13l/min is a good rate and should suffice.
Hi Did you have thermostatic shower before? Is there a blending valve under bath before shower supplies? What is the shower valve min flow spec, some need 22mm supplies. It could be flat plate heat exchanger blocked! The shower will restrict the flow of hot water out of the boiler which can overheat the pipe stat which then shuts off, Can you run other outlets for same period of time at roughly the same flow rate?
Hi guys, thanks for responding. I'll answer your questions to the best of my ability. - The hot water at the bath and sink taps in the bathroom fluctuates, as in it starts hot and then runs cool, then heats up again. We normally have to sort of half open the taps for optimum temperature. - The shower instruction book states that it is combi boiler compatible. It does say that there is a 12L/min limiter that can be added to high pressure systems. - I'll check the shower instructions for flow rate and perform some flow rate tests over the weekend. - We do have another thermostatic shower downstairs which we never use, this also suffers from the same problem now I think about it! - The previous shower was mixer hose from the taps and the temp was fine (unless someone drew on the water elsewhere). - Not sure about the blending valve or flow spec, I can ask the installer unless I can get info from the booklet that came with the shower. - Re the heat exchanger, we had the boiler serviced under British Gas contract last week and the guy said it was operating fine, with the possibility that it was perhaps using less gas than it should, but he said this may have been an error at the meter. I didn't know what to make of that. Would a blocked heat exchanger have shown up in a regular service?
Its almost certainly your plate heat exchanger on the boiler thats at fault. It either wants whipping out and decalling or whipping out and replacing. If you have intermittent DHW then yes he should have advised you during the service, provided you mentioned it as a fault to the BG engineer. If it was using less gas than it should he should have adjusted the gas valve to get the correct readings on his analyser. Sounds like he was in a rush.
Thanks again. I'll read up on plate heat exchangers and see if we can get someone out again to check it over.
Sorry for the double post, but can I ask a couple more questions: If the heat exchanger is at fault, would you expect to hear the boiler fire up and then cut out when the water goes from cold to hot and back? Also, where does the initial flow of hot water come from? Thanks
Yes the boiler does fire, that is where the initial hot water comes from, because of the restriction inside the plate heat exchanger it basically overheats and the DHW thermistor sends a signal back to the board telling it to shut down, it will reignight once the thermistor senses a temperature drop. This normally happens on and off as long as there is a demand.
This seems to be your problem, shower seems to be designed fo a max 12 ltr/min. You state your boiler is supplying 13ltrs/min. Reduce the flow to the shower and you will probably have it sorted. Remember pressure and flow are 2 separate things. Get your flow rate checked.
But he still has an issue with his hot water which is nothing to do with the shower or flow/pressure problems, he has a partially blocked plate heat exchanger. Once that is resolved there will be no problems with the shower.
Noofty I tend to think something is restricting the hot water flow through the shower. Is there any restrictor on the shower hose (like a water saving device for instance)?Have you got the correct shower head (the wrong type can make a huge difference)?Check the shower hose for a blockage( is the diameter of the hose large enough)?
Hi again, thanks for the replies, I really appreciate it. Big Dunk: I'm going to check the flow of the shower this afternoon. I'll report back. Dave: While the shower is running lukewarm water the boiler is still firing, would this go against there being an issue with the heat exchanger? Crowsfoot: there is no restricting device and the shower head is the right one for the model (Mira Realm). It's quite a wide rain head. Shower hose is also brand new.
Just posting back in here as we've more or less sorted the issue and I find people never post solutions! We had Mira send out a number of flow limiters and tried them on the showers. Reducing the flow down to 8l/min results in hot water from the shower.
Thanks for letting us know, it's a learning curve at times even for plumbers with 30 years experience! Interesting that is was the direct opposite to what I thought the problem could be!
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