water pressure through a combi

[chrisw99]

I've just had my first combi boiler installed, always had gravity before, so have to come to grips with a different way of operating. Particularly with the DHW.

My first question.. Now that the hot water is mains powered, should the flow rate at the hot bath tap be identical to the cold tap, or does going through the heat exchanger in the boiler reduce it a lot? My cold water comes out very very fast and quick, the hot water not as much.

Also noticed the flow rate from the hot water tap seems to vary at different times of the day, is this now down to being mains powered and the mains supply can be lower pressure in the mornings when everyone is using it, or is the boiler being clever and altering the flow itself based on incoming water temperature to reach the desired setting?

 

[terryplumb]

All part of what you get with a combi .and all outlets from the mains. You will probably also notice if you turn several taps on at once the flow is grim.

 

[chrisw99]

All part of what you get with a combi .and all outlets from the mains. You will probably also notice if you turn several taps on at once the flow is grim.

Ok, so second question... why?

From what I've read, the heat exchanger simply heats the water passing through it, so I would expect it to pass through at full mains pressure if the tap is open full whack (obviously not heating up as much), so where in the system is the flow rate to the hot tap actually being reduced? I'm guessing the heat exchanger bore is probably much less than the 15mm feeding it?

 

[terryplumb]

The domestic hot water heat exchanger ,containing many narrow water ways where the cold water runs through,sandwiched in between just as many where the hot water( heated by the primary heat exchanger / burner) runs through ,creates quite a bit of resistance to the flow . the boiler itself does not have the function of controlling the flow of cold water through ,that's done when you open a tap fully / partially. Hope this helps ,regards Terry.

 

[chrisw99]

Yep that's great thanks. Just wanted to check I haven't ended up with a blockage in the hot water pipes to the bath.

Another question... I had one occurrence (it was my son shouting "why's there no hot water") where the bathroom sink hot tap just was running cold. Left it a couple of minutes and it wasn't getting warm at all. Turned off the tap, waited 5 secs, turned it back on and it went hot.

Tried lots and lots of times and it always works for me, so what could my son have done which caused the hot water tap to be running cold. Does seem too much of a coincidence that it was the one time he used the tap.

 

[terryplumb]

If it only happened the once I wouldn't worry about it ,especially on the one tap. Is it a mixer tap or two separate taps ?

 

[abbadon]

If you open a tap very high flow, the boiler may never catch up, open it a bit less, and the boiler will overcome thermal lag quicker. So it's likely you are being more frugal and at same time getting g hot water quicker.

 

[chrisw99]

If it only happened the once I wouldn't worry about it ,especially on the one tap. Is it a mixer tap or two separate taps ?

Separate taps

 

[terryplumb]

And is the flow from this tap sometimes very poor ,and sometimes not ? You mentioned this was the case with bath tap .could have a problem with taps ,especially if oldish and the screw down type ( rather than 1/4 turn to open / close ). When changing to whole house pressured ,from old gravity system ,everything that was on the gravity fed side is now under three times the pressure ( roughly ,maybe more ). Flow and pressure are two different things. Some old taps can go problematic when changed over ,and the valves don't work as good ,giving intermittent flow.

 

[chrisw99]

And is the flow from this tap sometimes very poor ,and sometimes not ? You mentioned this was the case with bath tap .could have a problem with taps ,especially if oldish and the screw down type ( rather than 1/4 turn to open / close ). When changing to whole house pressured ,from old gravity system ,everything that was on the gravity fed side is now under three times the pressure ( roughly ,maybe more ). Flow and pressure are two different things. Some old taps can go problematic when changed over ,and the valves don't work as good ,giving intermittent flow.

The basin taps are relatively new, quarter turn type with a spray type nozzle. Flow from the hot water tap is now very good. Too good in fact, the basin is small so if the tap is fully open water splashes absolutely everywhere.

 

[terryplumb]

So not likely a tap issue ,unless son deliberately only opens hot a bit so he won't get splashed on !! Not enough flow to activate boiler maybe.

 

[chrisw99]

So not likely a tap issue ,unless son deliberately only opens hot a bit so he won't get splashed on !! Not enough flow to activate boiler maybe.

Maybe, but then I went in to check and opened it fully and it stayed ice cold for a couple of minutes until I stopped it, waited 5 secs, and restarted.

Like you say, only happened the once and the boiler had only been installed 5 hours at that point so may just be teething problems. I had been playing around with the temperature settings to try and find the optimal all evening. Currently got it set at 70 for the heating and 51 for the hot water. Might turn the hot water down a bit tonight as it seemed a bit too hot this morning.

 

[Madrab]

the boiler itself does not have the function of controlling the flow of cold water through

Not sure I'd agree with that one unless I have picked up the reply the wrong way? Many boilers have flow restrictors contained within the cold inlet that actively reduce the flow into the plate to ensure it can deliver the documented hot water flow rate @ a 35deg rise.

 

[terryplumb]

In the op's first post he was asking if the boiler was altering the flow itself based on incoming water temperature to achieve the desired setting ( of hot water from tap) as his flow at hot taps seemed to vary. The boiler does not have such a control function . a flow restrictor does not provide this function .think you have taken my post out of context madrab.regards terry.

 

[Madrab]

Ah yup, didn't get to the point where temperature was mentioned @terryplumb ... apols