I have recently had a gravity fed hot water system changed to a lpg combi boiler and wonder if it is normal for the hot water temperature to fluctuate. After turning on the hot tap it soon becomes very warm, but then gets very cool before warming up again. This is not associated with other people using water in the house. We have a high mains pressure, with a pressure control valve fitted on the incoming supply to the house. As it stands it is a waste of hot water because you have to wait for it to settle before getting into the shower, or risk a sudden change from hot to cold. Same with the washing up, you can have started filling the sink with hot water, when next thing it is filling with cold. We have the water temperature set at 50C, although the installer set it at 45C. If you think that this is weird, I will call the plumber who fitted it.
I thought that this fluctuation was one of the disadvantages of Combi Boilers. The boiler holds a head of hot water when you open the tap or shower this hot water is pushed out by the cold mains pressure. The boiler will cut in when the cold mains has dropped the boiler temperature to its switch on point. The result can be a flow of hot water to the tap followed by cold water then hot comes back on again. I am not a plumber, just a user and having had a Combi fitted in my last home with these problems, I refused to allow my plumber to upgrade my new homes boiler with A Combi. It cost me more because it did require 2 header tanks and a cylinder but I don't get wastage at switch on, and I wonder if there are any reports as to the costs in operating the different types of boilers.
No Carrian that's not how it works, a combi works like this, you turn a hot tap on and the boiler kicks into life and immediatly starts to heat the water, now depending how far a way you boiler is to that sink/shower/bath is will take a few seconds to get hot, once the water has got there it will stay at that temp, or SHOULD do, the storage tank lessens the time it takes to get to the sink/shower/bath but it costs you money as the boiler is cycling to keep that water hot, not every combi has a storage tank,
Jitender, my hot water boiler is a the gas boiler module of a Daikin Altherma hybrid heat pump. This is a condensing boiler with a domestic hot water heat output of 7.6-32.7kW. Supplying hot water has priority over heating. Since the system is tankless, the heat pump is not involved in domestic hot water production, only in Central Heating.
Hi Helga. Whether it's an issue might depend on the timings of the hot/cold fluctuation. Eg - if your central heating was on, then your boiler innards and system water will already be hot. So, when you turn on a hot tap, you might immediately benefit from this initial hot flow for a few seconds - only for it to be followed by cold water whilst the DHW heating actually gets going. Also, some combis - like mine - have an optional 'pre-heat' feature. With it 'on', the boiler will fire up for a few seconds every now and then to keep the water inside nicely piping so's you'll get hot water delivery as soon as you turn on a hot tap. With it off, you will notice that the hot water delivery will take a few seconds extra to arrive. But, as I mentioned before, if the CH is already on, you also notice a more immediate DHW delivery. So, could you tell us what the timings are, please? Ie - when you turn on the hot tap, for how many seconds do you get hot water before it runs temporarily cold? Then how long does the cold run for before it goes hot again? Have a looksee through the boiler's instructions - any mention of a water pre-heat?
Looking at the manual I think my boiler does have a "domestic hot water comfort function". A visit to the boiler (in attic), then switch it off, then examine display, should tell whether it is on because as I understand it, this is the same as "Frost Protection", which would be displayed. Thank you DA for those hints.
A combi is not very good at maintaining a constant hot water temp. The temp will vary +/- 5 degrees or so which is why you should always have a thermostatic shower fitted with a combi. But the temp variation reported here is excessive and could well be a boiler fault. If you already have a thermostatic shower somewhere get your plumber to check it. It could be letting cold water across to the hot side and creating the problem.
"domestic hot water comfort function" is the same as "Frost Protection"? Blimey. Can you tell me - after it has run hot then cold, does it then go hot as it should and STAY like that? Or does it keep on 'cycling'? If the latter, then there's almost def a boiler fault, I'd guess.
Hello DA, it does stay fairly steady once it has settled down. I got a response from Daikin, via my installer. This is what they said, "due to the unique construction of the unit if the system is in space heating and the boiler is or has been running the water in the heat exchanger will become warm, so when the tap is used the water will be cold until the "dead leg" has ran through, you will then get the water that has been sat in the heat exchanger which has been warmed to a temperature, once the tap has turned the boiler will fire into DHW mode but as you can imagine there will be some lag before useful heat is added to the water so this is when the customer then fills the hot water run cold again, once the boiler is fully operational the water will be at set point. This is pretty much what you have all explained to me. I have gone on to ask him, how he explains a pulse of very hot water in the settling down period. Glad its Friday says I should change my shower to a thermostatic one. I think I see the point there, and so another project is born.
That makes sense. Even tho' my combi does have this 'pre-heat' function, it still gives a more instant hot water response when the central heating has also been on - the boiler is toasting inside, and I get the benefit. Mind you, this is really noticeable only with the kitchen sink which is by far the closest tap - with bathroom taps, there is always that cold dead leg. If your boiler does 'settle down' and provide continuous hot water at a fairly stable temp, then I guess it's all functioning ok. Personally I wouldn't go to the expense of a thermostatic shower unless you really want to - just like a bathroom tap takes some time to run hot, so will your shower in any case. So either reach in and turn it on and then jump in after 30 seconds, or climb in, point the darned thing away from your bod for the same time...
You should always fit a thermostatic shower especially with combis. Not expensive, £50 +vat at the bottom end and make for a much nicer shower all round.
Except I have one and it doesn't work... Why? 'Cos it needs a hot flow at 60o min - which I don't particularly want.
If it doesn't work it must be faulty! I normally fit Bristan ones which work really well, I expect the cheapy Tritons work ok as well to be fair. You're right DA, a minimum of 60o is no good, quite a few combis cannot achieve that anyway. Strip it down, give it a service or replace the cartridge. Should be ok then.
Thanks GtF. It never worked from new, tbh. And when I read the instructions and it mentioned the minimum 60O hot supply, I guessed that was why. It was probably designed for stored-water systems?
Another Daikin adviser has just emailed me to explain how to switch on the "comfort function - eco", which I have done. This is supposed to keep a reservoir of warm water to reduce the time lag between turning on the hot tap and getting warm water. The "eco" bit, depends upon it's memory. It works out the times I am not likely to use hot water e.g. night time, and does away with the reservoir during those times. He says that I can now use a lower hot water temperature, so I turned it back down to 45C. All well, and thanks all for inspiring me to ask Daikin the right questions.
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