TMV/water flow boiler Query

[BobbyJoe]

Hi everyone, I contacted the boiler company about the DHW temperature control as there is no LED to indicate the temp so have used a digital thermometer to see how hot the water is as sometimes when I run any tap or the shower the water temp changes cooler/hotter when the tap is running.

The reply from the company regarding DHW temp was as follows:

"The unfortunate influence is, the flow of water, the greater the flow ( bath tap ) the cooler the water will be. The safest way to ensure that the water is produced at a specific temperature, which is not exceeded, is by having a thermostatic mixing valve fitted, which has a temperature setting facility."

So my question is it best to fit a TMV, are you able to adjust the temp on the TMV or is it permanently set on a specific setting?​

Where does the TMV need to be fitted, for example in the bathroom and kitchen and on each water outlet pipe such as basin, bathtub, shower and kitchen sink.​

The only valves I have at the moment which I can see in the bathroom and kitchen are isolation valves on the pipework.​

If it helps these are the specifics of the Vokera Easi Heat Plus 25c combi boiler which the engineer provided last year upon installing.
Gas Rate: 24.3 kw
Gas inlet pressure - 20mbar
Cold water inlet temp: 6 cel
Hot water outlet : 45 cel
Water flow rate: 9 I/min

Any advice would be great as I just want the DHW temp to not fluctuate when I run the bathtub or basin as the kitchen tap seems to be fine not sure if it has a flow regulator valve installed somewhere, thank you.

 

[steadyon]

1. You don't NEED a TMV. In domestic premises they are optional, except on new installations. Useful and a safety improvement if you have young children or elderly people in the home.
2. TMVs are designed to be fitted to a single outlet, so in theory you would have to fit separate ones for bath, shower, basin and sink.
3. As Vokera have told you in a roundabout way, the temperature of the hot water from a combi boiler depends on the flow rate of the cold water going through the boiler's secondary heat exchanger. Manufacturers normally quote a flow rate at which the boiler will raise the temperature of the incoming cold by 35 degrees. Thus, if they quote 9 lpm, then if the incoming cold is at 15 degrees (UK summer?), the boiler will supply up to 9 lpm at 50 degrees. If you try and draw more than 9 lpm, the temperature of the water will fall as the boiler can't heat that much flowing through it fast enough.
4. You could try limiting the flow through outlets by:
4a. Timing how long it takes to fill a 9 litre container (bucket with a brick in it?) and trying to remember which setting is about 9 lpm and limiting its use to that setting by eye. Not as tricky as it sounds.
4b. Fitting a flow limiter (e.g. BES 17527 plus 17531 (8 lpm)) to the cold inlet to the boiler.
4c. Fitting TMV(s) as suggested. Expensive option, and not always easy to achieve in limited spaces.

 

[BobbyJoe]

What kind of flow limiter is best as I would prefer to get one which is reliable and sturdy, any advice would be great thank you.

 

[steadyon]

I'd go for the BES one per 4b in my previous post.

 

[John.g]

Would a TMV on the combi hot water outlet control the temperature?, I can't see how a flow limiter will control it or maybe use a combination of both if you didn't want the water to go lower than say 45C in the winter with 6C cold water, you would set the flow limit to 9 LPM and the TMV to 45C to maintain that temperature as the user demand falls below 9 LPM and also as the cold water rises towards 15C in the summer as mentioned above.

 

[king of pipes]

Just fit a pressure reducing valve on the cold water inlet to the boiler set to 9litres a minute and turn the hotwater setting on the boiler up or down to suit the seasonal weather conditions, fitting a blender will limit the hotwater tempreture further but could possibly causs the boiler to cycle on off on again because it cannot dissipate the heat built up in the plate heat exchanger, kop

 

[John.g]

The user manual states that the DHW temperature can be controlled between 37C & 60C (didn't realise this) so as you say a PRV set to limit the flow rate to 9 LPM should give reasonable DHW setpoint temperature control as long as the DHW flowrate is between 2 & 9 LPM and should not be running hotter/colder with a tap running as stated in the first post, as long as the flow rate is at a minimum of 2 to 3 LPM.
There IS a flowrate restrictor fitted (8 LPM) so something doesn't seem to add up here.
"2 FLOW-RATE RESTRICTOR The boiler is supplied with the following flow restrictor. Easi-Heat Plus 25C: 8 - litres flow restrictor Easi-Heat Plus 29C: 10 - litres flow restrictor."