Why does DHW temp differ in kitchen and bathroom?

[BobbyJoe]

Hi everyone, I have a question about domestic hot water via a combi boiler and how it differs in temperature in the kitchen and bathroom. (The DHW temp gauge on the combi boiler is in the middle-all incoming water into the flat is mains fed)

In the kitchen when I turn the mixer tap all the way round to hot, the water is hot but not scalding which is fine for washing dishes etc and I don't need to mix cold water but when I turn the mixer taps in the bathtub and basin all the way round to hot, the water is boiling that I need to turn the tap back towards cold in order to use the water. (Both kitchen and bathroom have mixer taps)

So I would like to know why the hot temperature differs in the kitchen and bathroom when hot water taps are fully turned all the way round to hot.

Any advice or information would be appreciated so that I have a better understanding of how things work, thank you.

 

[Chuck]

I'm assuming that the flow rate (litres/minute) from the kitchen tap is not higher than from the bathroom taps. If so, the the most likely reason is that the kitchen tap is a thermostatic design that mixes some cold water in with the hot even when set to the 'hot' position to prevent people scalding themselves. This probably means it's been installed more recently than the bathroom one, which apparently don't have this safety feature.

It's unusual to get 'scalding hot' water from a combi when filling a bath so it could just be that the pipes are undersized, the flow rate is low, so the combi can raise the temperature of the water more in that case.

If you're really interested, measure the flow rate from each tap (using a jug and stopwatch) and the temperature (using a thermometer) and then post your measurements, the model of combi you have and photos of your taps here. But be warned, you'll be unleashing the geeks!

 

[BobbyJoe]

Thanks for the reply, all taps are new and the flow rate from the kitchen tap is not higher than the bathroom ones. I put the DHW temp on the boiler on high and the kitchen tap was then boiling hot, had to mix cold with it so then is it possible it doesn't have a thermostatic design to it?

 

[Chuck]

I put the DHW temp on the boiler on high and the kitchen tap was then boiling hot, had to mix cold with it so then is it possible it doesn't have a thermostatic design to it?

Okay, that seems to rule out my thermostatic kitchen tap guess.

What model of combi do you have? How far (in length of pipe) are the kitchen and bathroom from where it is installled?

 

[BobbyJoe]

I have a new Vokera Easi Heat 25c combi boiler (less than 12 months old) and the distance from the kitchen tap to the bathtub/basin taps it is about 7 metres.

 

[SJB060685]

Sure there's not a thermostatic mixing valve on bathroom supply

 

[BobbyJoe]

What does a TMV look like and where would it be likely installed? I only have a thermostatic shower.

Update: Just looked at TMV pics online and there isn't one on the bathtub pipes, basin pipes and I can't see it under the kitchen sink either.

 

[Harvest Fields]

Can you post a picture of the bathroom taps and kitchen taps. Or test the flow rate from each. You may only have 6 l/m at the kitchen? but dependant on design 15 at the bath? (All speculation obviously). If the boiler can only heat 10 l/m you will have to reduce the flow to 10 l/m on the bath for it to get to its maximum temp. Hope this makes sense.

 

[king of pipes]

Chaps the op has good hotwater in the bathroom so it's the kitchen tap causing the issue here, it just needs a flow restrictor installed on the hot supply to the tap in the kitchen or simply don't turn the tap on fully when using hotwater at the sink . Regards kop

 

[Harvest Fields]

Chaps the op has good hotwater in the bathroom so it's the kitchen tap causing the issue here, it just needs a flow restrictor installed on the hot supply to the tap in the kitchen or simply don't turn the tap on fully when using hotwater at the sink . Regards kop

My method still works mate but I had the locations the wrong way around.

 

[BobbyJoe]

Did some testing and here are the findings, despite the results the kitchen hot water does seem cooler than the bathroom basin hot water however it turns out there isn't a difference after all. I placed the thermometer under the tap once the water was running to check the temperature and no cold water was mixed with the hot water.

The combi boiler is based in the kitchen and is about 7 metres away from the bathroom pipework.
Do the results make sense and seem about right in regards of temps and water flow rate in the kitchen and bathroom, the combi boiler DHW water is set at around 41 degrees and I believe the maximum is 60 degrees, is it best just to leave it as it is or put the DHW temp up or does it not matter?

Any advice or insight would be much appreciated thank you once again. 🙂

Tested filling a 1 litre jug of hot and cold water individually
All taps were opened fully for full flow of water & no cold water was mixed with the hot water

Bathroom basin Hot tap: 6 seconds to fill 1 litre jug: 42 degrees Celsius ​

Bathroom basin Cold tap: 4.5 seconds to fill 1 litre jug​

​

Bathtub Hot tap: 6 seconds to fill 1 litre jug: 41 degrees Celsius ​

Bathtub Cold tap: 4.5 seconds to fill 1 litre jug​

​

Kitchen Hot Water tap: 7 seconds to fill 1 litre jug: Temp 40.2 degrees Celsius​

Kitchen Cold tap: 4.5 seconds to fill 1 litre jug​

 

[Chuck]

A fill time of 4.5s / litre is a flow rate of 13.3 litre/minute. Since this is the same everywhere it is probably limited by the incoming service pipe, or your stopcock may not be fully open.

You have 25kW boiler. At the measured flow rates, it should be able to deliver a temperature rise of 35.7°C (10.2 lpm) to the bathroom taps and 41.7°C (8.6 lpm) to the kitchen tap. So, in a cold winter you may have to throttle the bath tap a bit in order to run a nice hot bath.

Personally, I'd prefer to have the water source a bit hotter and would set the boiler stat at 50°C.

Your sensitivity to temperature (how hot water feels) is very high around 40°C. It's quite possible that to you 42°C feels 'hot' and 40°C feels 'lukewarm' although from a physics point of view they are very similar.

I suspect that the one or two degree difference you have measured probably not significant. It could be the order in which you took the measurements, the temperarature of the jug, or the path the pipework took, e.g. if the kitchen pipework has an uninsulated section running past an outside wall or through concrete, etc.

I don't think you have anything to worry about, just enjoy the nice warm baths.