Combi boiler : Hot water

[transatlantic]

I have moved into a property that has a combi oil boiler. Having been in a hot water tank based system before, I was used to getting hot water as soon as you turn on the tap (assuming it was still hot at that point). However, with this new property I find I have to wait a good 30s or more before I get hot water? Having googled a bit, this seems to be normal. Some suggested reasons for the delay being :

- Takes a little time for the boiler to be triggered that the hot water pipe is being used (not sure about that taking 30s?)
- The pipes from the boiler to the tap itself will have water in them from the last use that has cooled down, which needs to be passed before water directly heated by the boiler gets through
- Perhaps I just I have a slow system in general.

The second point seems the most likely cause to me. I have a 50ft Xhose pipe, and after turning off at the tap, it always amazes me how long it takes for all that water to drain out for it to shrink again. Assuming my pipe work is roughly as long as that, the 30s makes complete sense. Anyone know if my thinking is correct here?

I'm not really bothered about the delay, it's just a little annoying. What I am concerned about though is, being on a water meter, the wasted water I use waiting for the hot water to come through. I guess I'll just have to get into the habit of only using hot water when I actually need it (washing up, taking a bath)

Just wondering how good other peoples combi systems are?

Note : the boiler is in the garage (right next to house though). So I guess it has a noticeably longer distance to travel than most in house systems.

 

[MattRoberts]

My boiler isn't combi, it's standard, and it's also housed in the garage (along with the hot water tank). I have the same thing - it takes a good 30 seconds for the hot water to come through due to the distance in the pipes

 

[Monkey Mark]

The main reason is usually a combination if the first two reasons you posted.

There's a small delay on triggering to prevent nuisance triggering from a drip or unecessary triggering from, lets say, someone turning the wrong tap by accident.

It then has to travel from the boiler.

You'll usually find the water heats up faster (and therefore less waste) if you run the hot tap slow to start with (less cold water being introduced as it's trying to heat) and usually warms up quicker if the heating is on at the same time.

It's the nature of the beast I'm afraid. In theory they are more energy efficient as they only heat when needed. Though immersion heaters are making a comeback in ennergy efficient new builds.

 

[Phil Pascoe]

I have personal experience of only four combi systems, and they have all been crepe. It'll be brilliant not having hot water when the electricity is rationed - and there is a good chance that it will be, sooner or later.

 

[Sheffield Tony]

I lived for 6 years with a "Glow worm" combi. it had about as much power as a glow worm. Trouble is it detects the flow when you turn on the tap, then starts heating - not the tapwater itself directly - but the water in a heat exchanger that then heats the tap water. This can be sped up as suggested by opening the tap to a slow flow. But on my system, the old style rubber tap washers would respond to the warm water finally arriving by expanding slightly, thus reducing the flow below the threshold at which the boiler triggered, so it turns off and goes cold again ! Running a bath was quite a challenge.

I will never have a combi again.

If you really want instant hot water, there are systems where the last few feet of pipe before the tap has an electric, thermostatic heating cable wrapped round it, with a thick insulating jacket over it. If it is a handwash basin, often that is enough hot water.

 

[transatlantic]

I'm not fussed about the hot water aspect, just the water wastage.

The suggestion of turning on the tap slowly... Not sure how that would help, as you still need to pass all the water that is in the pipes before the hot water comes through, so wouldn't that just take longer?

 

[flh801978]

I've had 2 combo boilers in my house and installed a few for friends and daughters in their houses
My first one was a Myson Midas which was great when it was working installed in 1990 and retired in 2013 .it had a jacketed hot water storage device which stored a quantity of hot water and cycled to keep this up to temp so the only delay was the pipe run which if you place the boiler as close to where you need the HW may be ok
The Myson though had 3 circuit boards and one heat exchanger in its life but it did last 23 years before I decided that another £100 was too much to spend
I replaced this with a Baxi Duo-Tec on recommendation by my local plumb centre ...its been great
2.5 gallons a minute with a 20 degree raise
In summer we have to turn it down rather than filling a bath with hot and cold in winter we just fill with hot
The boiler is so simple inside very little to go wrong
On the other hand the boilers I like most are valiant ones

I haven't had much to do with oil fired boilers but would have thought that start up times of a combi was greater than with gas boilers

Ian

 

[Monkey Mark]

I'm not fussed about the hot water aspect, just the water wastage.

The suggestion of turning on the tap slowly... Not sure how that would help, as you still need to pass all the water that is in the pipes before the hot water comes through, so wouldn't that just take longer?

Aa mentioned, the water is not heated directly but via a heat exchanger. Think of it as a heated jacket that the tap water flows through. Tgey usually rause the tap waters temp up from its incoming temp by roughly 30 degrees (hence colder hot water in tge winter).

When the tap is turned on the boiler starts to heat the heat exchanger. The heat from this passes to the tap water as it travels through the exchanger, the tap water acts like a heat sink soaking up the heat from the exchanger.

If the tap water passes through fast then the same amount of heat is absorbed, but spread out tgrough a larger quantity of water therefore a lower increase in overall temp.

Slower means same heat exchanged through a smaller amount of water so greater increased temp overall.

Turning the tap on full pulls lots of cold water tgrough the exchanger as it's trying to heat up effectively slowing the process. It's like turning on a soldering iron then sitting it infront of a fan.

 

[transatlantic]

I'm not fussed about the hot water aspect, just the water wastage.

The suggestion of turning on the tap slowly... Not sure how that would help, as you still need to pass all the water that is in the pipes before the hot water comes through, so wouldn't that just take longer?

Aa mentioned, the water is not heated directly but via a heat exchanger. Think of it as a heated jacket that the tap water flows through. Tgey usually rause the tap waters temp up from its incoming temp by roughly 30 degrees (hence colder hot water in tge winter).

When the tap is turned on the boiler starts to heat the heat exchanger. The heat from this passes to the tap water as it travels through the exchanger, the tap water acts like a heat sink soaking up the heat from the exchanger.

If the tap water passes through fast then the same amount of heat is absorbed, but spread out tgrough a larger quantity of water therefore a lower increase in overall temp.

Slower means same heat exchanged through a smaller amount of water so greater increased temp overall.

Turning the tap on full pulls lots of cold water tgrough the exchanger as it's trying to heat up effectively slowing the process. It's like turning on a soldering iron then sitting it infront of a fan.

Sorry if I'm being dense, but if I have the boiler which contains the heat exchanger and then a long pipe all the way to the tap, then surely, any *old* cold water after the exchanger, up to the tap, has to first come out of the tap before *new* hot water passed through the exchanger can be felt? Obviously water after the exchanger might have a little heat passed on from convection, but wouldn't it still largely be cold?

So why would turning on the tap slowly help here? as that "old" cold water still needs to be moved before the "new" hot water can come through?

 

[geoffshep]

it's a bit of both - even if you had no pipe run at all between the boiler and the tap, it would still take time (and water) for the running water to get up to temp, so a slower flow, but enough to fire up the boiler, will waste less water - plus you've got the cold stuff in the pipe to get rid of.

Some combis have a reservoir of heated water in a tank in the casing - this provides more instant hot water (after you have flushed your long pipe) but means that the boiler is firing up constantly to keep the small tankful hot.

 

[RogerS]

Yup. Not only do you waste water in the flow while waiting for the hot water to arrive but you also waste the heat in the water in the pipe between the combi and tap after you've turned the tap off. It's called a 'dead leg'. This site .

Our kitchen tap dead leg wasted 3 litres of water each time we wanted to use hot water in the sink. And 3 litres of hot water left in the pipes. We're not on a combi but have a normal CH system plus immersion. So I installed a very well insulated secondary return with a special pump that keeps circulating hot water from the hot water tank down to the kitchen and back to the tank. It's thermostat controlled. Now our hot water is pretty much instantaneous.

 

[woodpig]

The last place I worked was very keen on point of use water heaters and installed them all over the site. Instant hot water with lower energy and water bills was the reason quoted.

 

[Monkey Mark]

I'm not fussed about the hot water aspect, just the water wastage.

The suggestion of turning on the tap slowly... Not sure how that would help, as you still need to pass all the water that is in the pipes before the hot water comes through, so wouldn't that just take longer?

Aa mentioned, the water is not heated directly but via a heat exchanger. Think of it as a heated jacket that the tap water flows through. Tgey usually rause the tap waters temp up from its incoming temp by roughly 30 degrees (hence colder hot water in tge winter).

When the tap is turned on the boiler starts to heat the heat exchanger. The heat from this passes to the tap water as it travels through the exchanger, the tap water acts like a heat sink soaking up the heat from the exchanger.

If the tap water passes through fast then the same amount of heat is absorbed, but spread out tgrough a larger quantity of water therefore a lower increase in overall temp.

Slower means same heat exchanged through a smaller amount of water so greater increased temp overall.

Turning the tap on full pulls lots of cold water tgrough the exchanger as it's trying to heat up effectively slowing the process. It's like turning on a soldering iron then sitting it infront of a fan.

Sorry if I'm being dense, but if I have the boiler which contains the heat exchanger and then a long pipe all the way to the tap, then surely, any *old* cold water after the exchanger, up to the tap, has to first come out of the tap before *new* hot water passed through the exchanger can be felt? Obviously water after the exchanger might have a little heat passed on from convection, but wouldn't it still largely be cold?

So why would turning on the tap slowly help here? as that "old" cold water still needs to be moved before the "new" hot water can come through?

Yes, you would still have that cold water to flush first. But then there should be less waste thereafter.