Combi hot water flow

david29dpo

Been think of fitting one for a while but have a question about DHW flow and temperature. A friend has one and when he fills the bath, with the tap on full, the water is not very hot but if he turns the tap down a bit, the water get hotter. This makes sense of course but i would have thought the boiler should regulate the flow according to the DHW setting knob on the boiler. Does he have a faulty boiler or is this normal? If it normal whats the point of the DHW knob on the boiler?

Pssst

I wonder what make and model your friend has??

If you visit various boiler manufs websites,they will quote hot water flow specs in litres raised by a certain temp.

Note the term "raised". It obviously means the amount by which the temp is increased and not the end temp.

So if in winter your incoming temp is 2 c and the boiler spec says it raises it by 50c then you will get 52c max.

I wonder if your friend has an adequate gas supply to his boiler,also if the boiler is performing to spec/needs attention?

Filling baths is,in truth,often the Achilles heel of many a combi. Showering is the modern way you see but sometimes you like a good soak?

What sort /size of house do you have?
Perhaps a combi isnt the best solution for you?

Perhaps you need a HE system boiler of a pressurised HW cylinder?
Whats wrong with your current setup?

david29dpo

I think its a lower end of the market boiler so perhaps it dont regulate the flow, just wondered if all combis are like that. I would have thought if you say have the DHW knob set to 50c, the boiler would regulate the flow so you get water at 50c, no matter have much you open the tap.

rosalie-lavender

We have a combi boiler and I fill my bath with the hot tap full on and have no problems with the water temperature. We have mixer taps and I find if I put both taps on together the cold tends to overpower the hot so I fill cold first then hot.

Be careful fitting a combi if you are in a hard water area. Our last combi boiler kept scaling up and we have had to have a salt based water softener fitted to stop the boiler problems caused by scale.

Pssst

david29dpo wrote: »

I think its a lower end of the market boiler so perhaps it dont regulate the flow, just wondered if all combis are like that. I would have thought if you say have the DHW knob set to 50c, the boiler would regulate the flow so you get water at 50c, no matter have much you open the tap.

It depends on make/model of boiler and how sophisticated (or not.it is).

The basic equation is Q=Mc (T1-T2)
Where Q is quantity of energy required
M=mass (of water)
c=specific heat capacity
and T1-T2 is temp difference


So if Q is fixed i.e maximum burn capacity of gas burner and specific heat capacity is a constant,the only two variables are temp difference and volume.

The important thing with combis is to make sure its fed with the gas and water supplies as specified.

david29dpo

But does any boiler regulate the flow?

EliteHeat

Some boilers limit the flow to stop what you have described from happening. For large volumes of water the best way is to have the maximum temperature set and then to regulate the actual temperature by adjusting the flow of water.

You can normally have a cup of tea while a combi is filling a bath

david29dpo

Thanks but why do you have to regulate the tap if the boiler does it?

EliteHeat

The boiler regulates the maximum raise in temperature for a given flow rate. For instance a 24kW combi will raise the temperature of incoming water 35 degrees at a flow rate of 9.8 litres per minute maximum. You can choose to lower this if you like if the boiler has a hw temp knob.

However, it cannot exceed this design spec under any circumstances, so if you increased the flow of water through the boiler to double its maximum rate, the increase in temperature would be halved.

P.S. The best and definitive answer was given by Pssst in post #5

david29dpo

Thanks, I understand all that but do any boilers regulate the flow to the temp set on the boiler?

blackfive

David29dpo are you getting confused between the control for the boiler thermostat and the control for the hot water temperature on these combi boilers?

The boiler thermostat control is for how hot the boiler heats up the central heating side of the boiler and the boiler will then cut out when the water for the central heating is up to the set temperature but the pump will continue circulatiing the water if the room thermostat is calling for heat.

The control for the hot water on the other hand specifically controls the heat input into the water for the hot taps. However the faster the flow the lower the temperature will be for obvious reasons.

Combi boilers are good for low occupancy but not for high occupancy as won't cope with the water demands of say, a family with teenage daughters all wanting to wash up, bath/shower and maybe using the washing machine at the same time. A conventional system using storage for hot water can cope with peak demands far better.