Combi - time for HW to reach taps
- Thread starter Alfie25
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Usually just the flow, chances are the bigger one has a bigger heat exchanger to warm up.
So does that mean it could actually take longer for a bigger combi to supply hot water to the taps?
Boiler size determines hot water flow rate. 24 kW boiler delivers 9.5 l of water per minute. Pipe that connects your hot tap to the boiler- assume it is cold. When you run a hot tap. 9.5 l of water will start moving through this pipe to your tap (assuming your boiler is 24kw power). This takes time- longer the line more waiting for you. If you have a boiler that does not have preheat, well you just have to have patients of a woman ( my wife's words)
Fits a bigger boiler. Delivery of hot water better so water reaches the tap quicker
If major hassle, fit an unvented cylinder to existing and have pump return, you will have hot water for use when you run a hot tap
Fits a bigger boiler. Delivery of hot water better so water reaches the tap quicker
If major hassle, fit an unvented cylinder to existing and have pump return, you will have hot water for use when you run a hot tap
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Boiler size determines hot water flow rate. 24 kW boiler delivers 9.5 l of water per minute. Pipe that connects your hot tap to the boiler- assume it is cold. When you run a hot tap. 9.5 l of water will start moving through this pipe to your tap (assuming your boiler is 24kw power). This takes time- longer the line more waiting for you. If you have a boiler that does not have preheat, well you just have to have patients of a woman ( my wife's words)
Fits a bigger boiler. Delivery of hot water better so water reaches the tap quicker
If major hassle, fit an unvented cylinder to existing and have pump return, you will have hot water for use when you run a hot tap
Fits a bigger boiler. Delivery of hot water better so water reaches the tap quicker
If major hassle, fit an unvented cylinder to existing and have pump return, you will have hot water for use when you run a hot tap
I'm going combi rather than unvented, just trying to figure out if its best to go bigger or not.
I rarely have the hot tap opened full bore so the difference between 28kW (11.4 litres per min @35 degree rise) versus 32kw (13.1 litres per min @35 degree rise) isn't that important to me, however I'd happily pay more if the bigger boiler will get the HW to the tap quicker. Assuming both boiler do or dont have preheat, does the bigger boiler heat it quicker so get it to your tap quicker?
I rarely have the hot tap opened full bore so the difference between 28kW (11.4 litres per min @35 degree rise) versus 32kw (13.1 litres per min @35 degree rise) isn't that important to me, however I'd happily pay more if the bigger boiler will get the HW to the tap quicker. Assuming both boiler do or dont have preheat, does the bigger boiler heat it quicker so get it to your tap quicker?
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Chances are if you are not turning the tap full on it may well heat up a smidge faster
The only element that determines how quickly heated water reaches the taps is the length and diameter of the pipe run (this standing water has to be drawn off before you will receive heated water at the tap! so a well designed system is essential keep pipe runs as short as possible 15mm pipe is preferable, unfortunately many combis are tagged onto existing systems where the original pipe work is retained often 22mm in diameter and this arrangement does not always follow the shortest route.
The Kw output of a boiler will determine the temperature rise/flow rate it will not improve the time taken to reach taps your dynamic flow rate should be measured before selecting a boiler with a Kw rating which is capable of heating the flow rate available
Ricky
The Kw output of a boiler will determine the temperature rise/flow rate it will not improve the time taken to reach taps your dynamic flow rate should be measured before selecting a boiler with a Kw rating which is capable of heating the flow rate available
Ricky
If you don't tune the boiler on fully then that will take longer for the hot water to reach the taps.
But until the boiler has become then its better to just take say 3 li/min.
So the best way is to turn the tap on full, time how long it takes for the outlet pipe of the boiler to get warm. This will be about 6-12 seconds.
So to get hot water quickest, just turn the tap on the 3 li/min to start the boiler and then AFTER the measured time delay, turn the tap on to FULL !
Got it?
Tony
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Sorry ricky, i have to disagree with that.
For example, say the hot water pipe volume between the boiler and the tap is 4 litres. 2 identical boilers but different sizes.
A boiler capable of 8lpm would take 30secs for the hw to get to the tap.
Therefore a boiler capable of 16lpm would take 15 secs to get to the tap.
(Obviously this is in theory, and presuming that these flow rates are actually reached at the tap and the times are from the moment the hot water exits the boiler rather than the moment the tap is turned on) but a bigger combi will be quicker to get hot water to the tap
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True, i completely agree. but i was just replying to the statement that a bigger boiler would not reduce waiting time at the tap, which is clearly b*llocks.
I made sure i was careful to point out that the flow rate at the tap would need to be correct
Op may only turn tap on at half rate for a basin, but i suspect if he has a bath, then he would turn the tap on at full rate, meaning a bigger combi delivers hw quicker than a smaller combi
TCC - I don't think I expressed what point I was trying to make too well.
The OP was questioning the length of time it took before warmed water arrived at the tap - I was trying to make clear that it is the plumbing/pipework runs that determine the time it takes to clear the standing cold water and the importance of selecting a boiler of adequate output to heat the available flow rate - hope that makes my point clearer
The OP was questioning the length of time it took before warmed water arrived at the tap - I was trying to make clear that it is the plumbing/pipework runs that determine the time it takes to clear the standing cold water and the importance of selecting a boiler of adequate output to heat the available flow rate - hope that makes my point clearer
But that is not totally true either!
Most combi boilers have an inbuilt flow restrictor to ensure the flow does not become greater that the boiler can heat.
So in many cases the flow rate is higher with a larger combi and so the cold slug of water in the pipes will be cleared sooner.
But engaging pre heat if available will significantly reduce the waiting time!
Tony
Most combi boilers have an inbuilt flow restrictor to ensure the flow does not become greater that the boiler can heat.
So in many cases the flow rate is higher with a larger combi and so the cold slug of water in the pipes will be cleared sooner.
But engaging pre heat if available will significantly reduce the waiting time!
Tony
Thanks chaps but I'm still none the wiser. TCCHeating, your theory does make sense with the 8lpm vs 16lpm boilers however its the time taken to reach the tap, not the theoretical volume that can be achieved that I'm looking at.
Let me say I have really good water pressure. It's higher than in any other house I've tried (although maybe their stopcock wasn't fully open like mine is). I haven't measured it, but if I turned the kitchen sink/bathrom basin cold tap on full, the splash back would soak me. That's why in reality I'd have the tap maybe half way to 3 quarters. I don't use the bath that much as I've got an electric shower which is fine for me. So, that's why I was asking the question on whether to go for a 32 over say a 28kW boiler.
The extra expense doesn't bother me, and I'm rarely going to be exploiting the maximum litres per minute, however if the larger boiler would get HW to the hot tap quicker then I'd rather go for that. I was assuming all other features were the same (e.g. same brand of boiler, preheat/no preheat, same length pipe run).
Tony - your last post does seem to say that a bigger boiler (with higher flow rate) will deliver HW quicker although Boilerman seems to say it makes no difference!
Let me say I have really good water pressure. It's higher than in any other house I've tried (although maybe their stopcock wasn't fully open like mine is). I haven't measured it, but if I turned the kitchen sink/bathrom basin cold tap on full, the splash back would soak me. That's why in reality I'd have the tap maybe half way to 3 quarters. I don't use the bath that much as I've got an electric shower which is fine for me. So, that's why I was asking the question on whether to go for a 32 over say a 28kW boiler.
The extra expense doesn't bother me, and I'm rarely going to be exploiting the maximum litres per minute, however if the larger boiler would get HW to the hot tap quicker then I'd rather go for that. I was assuming all other features were the same (e.g. same brand of boiler, preheat/no preheat, same length pipe run).
Tony - your last post does seem to say that a bigger boiler (with higher flow rate) will deliver HW quicker although Boilerman seems to say it makes no difference!
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