thermostatic shower not working as expected

[monkeysballs]

Hi,
i have a situation where my shower(boiler) isn't working as expected

The shower is a flova go click valve (few years old but on whole house softner) and boiler is a viessmann 200 combi (brand new)

Im trying to run the DHW at 38c as it doesn't need to be any hotter (please no legionella talk)

Yet im getting over and undershoot when using the shower (especially the first person in) when the handle is sat at the indicated 38 and i have tested the flow (after adjusting it it is 38c )

any thoughts why it might be or anything to try

 

[Madrab]

Mixer shower are by their very nature, more accurate when they are mixing hot and cold to achieve the desired temp, if you are setting the HW delivery @ 38Deg then the thermostatic valve may struggle to work properly as it wouldn't actual be mixing. The TMV2/anti scald protection may also be interfering with it, with most shower with this protection they actually require a flow of cold water to work properly

Try turning the HW up to 50 deg to test it and see if the temp stablises @ 38Deg, if not the shower may also need calibrated.

 

[monkeysballs]

what part is actually doing the temp sensing in the shower valve?
my theory was lower water temp lower gas usage but at the moment the scream of the wife at 6:30 as it swings isnt much fun
burner turned on 07:47:10 as did pump burner turned of 7:59:10 pump 08:01:40

 

[Johntheo5]

You do need a dirrerential between the incoming HW and the required mixed temperature, to give a stable mixed flow temperature, so if the DHW (combi) temp is set to 50C, and a required mixed (shower) temp of 38C from a now 18C cold mains will mean, for a 15LPM shower flow that the combi will supply 9.4LPM at 50C & the shower cold mains will supply 5.6LPM at 18C to give that 15LPM at 38C and requires a boiler output of 21kw.

Photo below of a TMV, they all work in a similar fashion , the piston valve contais the sensing (copper) element (senses the mixed flow temperature) inside in which is a liquid that pushes a little hydraulic piston inside in it and moves the piston valve against the spring pressure to give the correct mixing, that temperature adjusting screw in the top is effectively adjusted in a shower by the temperature control. (or something like that)

 

[monkeysballs]

yeah i just found a bristan installation instruction that says hot needs to be at least 10c above blend temp so i need to set it to at least 48

cheers will need to adjust the temp setting of the handle again now

 

[Johntheo5]

Was the shower still running when the DHW temp rose from ~ 37C to 40C in one minute, 07:58 to 07:59 hrs?.

 

[monkeysballs]

no that was it heating up after shower shut off but burner was still running and pump


this was when i set it to 50c oops last one is pump on also ignore the yellow temp thats not actually connected to the pipe yet

 

[Johntheo5]

Can you read the boiler's primary flow and return temperatures when DHW only on, like for the shower, at 50C DHW temperature?

 

[monkeysballs]

ask and ye shall get
just that period and day so far
for ch flow and return temps

 

[Johntheo5]

I dont think the flow and return temperature readings are actually read (properly) while on DHW only as the primary water is diverted to the plate heat exchanger so probably no flow(s) at the probes under these conditions. if the DHW temp was set to 50C, then the DHW flow as calculated in post #4 will be 9.4LPM (to give a shower flow 15LPM at 38C), we don't know what the boiler primary flowrate is but if 15LPM is assumed then the dT flow/return will be 20C, again if we assume the boiler primary flow temp at 75C then the return will have to be 55C.

 

[monkeysballs]

my ch is running a flow of 35c @ 0c external temp and im getting about 10-12deg drop on return

edit to add obviously return depends on the ambient internal temp as im so close ambient

 

[monkeysballs]

update today much much better! except the wife took a 10 minute shower

mine was much more reasonable

I may try dropping it 1c a day untill i start getting issues then back it off to try save a bit more

 

[Johntheo5]

The DHW temp is quite stable, dont know what the temperature range on the shower is but say its 35C to 43C then you could set the shower to 43C and the DHW temp to 38C (think you can go as low as 35C?) and see how stable the shower temp stays as the TMV should have the cold water shut off, if, for any reason the DHW rises above 43C then the TMV should stop the temp rising above 45C which, while not very pleasant, wont actuall burn anybody?, the problem with this is if there is a HW demand or HW reduction from elsewhere then the boiler will not respond instantly whereas when the TMV if in control with 50C DHW will react almost instantly