Combi boiler hot water issues and questions

[AnthonyL]

The Baxi 105HE combi boiler is right at the back of the rear utility room extension of a bungalow, approx 15m from the kitchen which is at the front of the house and where the mains comes in.

The central heating is fine (touch wood).

Problem 1 =======

It is a real pain getting hot water to the kitchen. I calculate there's 2.3l of water sitting in the pipes which has to be pushed out before any hot starts to appear. And then 2.3l of hot being wasted in the pipe when turning the tap off. Would you agree with these figures and any suggestions?

Problem 2 =======

It is very hard to get the flow right either to the kitchen or to the bath (~9m from the boiler). Too fast a flow and the water is cold (see later) and too slow a flow and the water again is cold. Try to dribble it and the water might be hot but by the time the bath is filled at that rate the bath is still cold. Any suggestions?

Problem 3 =======

This is a more recent phenonemon but the shower, which is in the utility room hence near the boiler is generally fine but every now and again runs cold in the middle of a shower. Only occasionally but it causes a scream from SWMBO. Any suggestions?

Problem 4 =======

Turning on the cold water tap in the kitchen seems to deprive the boiler of its hot water flow and causes it to run cold. Any suggestions?*

Question 1 ========

I am not understanding exactly what the temperature LEDs are telling me. If I understand correctly the hot water is heated via a secondary exchanger so the flame heated hot water is just going round and round. Now are the LED's telling me the temperature of that water or the water temp as it being delivered?

I wonder if I have the hot water thermostat set at optimum. The dial is at "14 mins past". The LED shows 80deg. What happens if I set the thermostat too high?

• These bungalows, built in the 1960's, originally had their boilers in the attic and thus fairly central to the delivery points (kitchen and bathroom). Cold water was via a storage tank (still there for cold in the bathroom). A separate tank was used for hot water and I believe that it may have been heated both by the boiler and an immersion heater. An Italian previous owner gave rise to the current installation.

Thanks

[John Rumm]

The further away it is, the longer you will have to wait, and the more water you need to draw off. Insulating the pipe run may help a little - save it going cold quite so fast after last use.

The only real options for fixing it are providing a source of hot water closer to the point of use. Perhaps a small under sink unvented cylinder (which could be heated electrically or by the CH side of the boiler). Alternatively a "on demand" electric heater just for the kitchen sink.

The 105 appears to be a 30kW boiler... rated to give a flow rate of DHW at 14lpm with a 30 deg C rise, falling to a bit over 10 lpm for a 40 deg rise.

If you draw water through it faster than that, it won't be able to achieve the same temperature rise. Some boilers include temperature sensitive flow regulation - so that they can throttle the rate of delivery to maintain a set temp. A Quick look at the specs on this one does not suggest it has this. Hence in colder parts of the year you may want to turn down the inlet cold water tap a little to limit the flow rate through it to something it can get hot enough give its power and the incoming water temperature.

Ear muffs? :-)

It might be the shower is not using enough hot water.

At low flow rates the water temp will start to rise, and the boiler will modulate its burner power to try and stay under the preset max DHW temp. However there will come a point that it can't keep under this limit without cycling the burner off. (it can modulate down to about 11kW IIUC). So with a 40 degree temp rise that would imply a minimum flow rate of: 11,000 x 60 / 4200 / 40 = ~4 lpm

So checking the head for scaling etc, or even changing the shower head for a more "thirsty" on might help.

Is this new or has that always been the case?

Try throttling the cold flow rate to the tap with its service valve to see if it helps. If it does then fitting a PRV inline with it may make for a better (i.e. less noisy) fix.

Yup its a traditional twin HX setup. A differential pressure flow switch senses when the hot water is required. That switches a diversion valve that directs the pump flow round the short circuit inside the boiler that includes the plate HX rather than the external CH circuit.

What does the user manual say?

At low flow rates you get scalded.

How much does it annoy you? Enough to fork out the significant cost to fix it properly?

[Fredxx]

For 15mm pipework I calculate just over 1l per m of pipe.

I have an undersink water heater. That does solve the dead leg of cold water.

9m is remarkably long. Find a shorter route? Move boiler?

I would check flow rate into the property and at a hot tap. The boiler may have a spec on pressure drop across the heat exchanger and you should be able to find the same for 15mm copper pipe.

80C is very hot for hot water delivery.

[AnthonyL]

Which leaves a calculation of cost (assuming gas is still going to be cheaper despite the waste) v convenience.

I suspect that I simply had the thermostat set too low. Time will tell.

See 2 above. The couple of showers we've had since upping the thermostat have been fine. Shower has one of those free flow savers from Severn Trent, limiting to 9l/min. All else is in good condition. The mixer tap is one of those big round dial type that hits a stop before being fully on, then press the button and get really hot. Seems to predate google.

Not sure that I can find a service valve.

It doesn't as far as I can see. The LEDs also double up as fault indicators which are well explained.

I've turned it up to "20 past" - see earlier items.

The comment was more that some decent thought had gone into the original design, though I think this is one of the few areas where the council insisted on gravity feed cold water in the '60s. A downside is that the "rust proof" galvanised iron tank is showing rust at the bottom. I also need to go around the hot water taps which originally also would have been gravity fed but are now on the boiler/mains pressure and leak unless really really tightened, so I'm guessing they've got low pressure washers.

[tabbypurr]

one way to solve this is provide a microbore pipe back from outlet to DHW cylinder and add a minimal pump. Put it on a timer: when it runs you get instant HW. Lag the pipe run well.

assuming it's only turning to high flow that does that, throttle the max flow.

NT