Help understanding why I'm not getting enough Hot Water for a full bath

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mjgreen81

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Hi all,

I moved into a new house earlier this year, so this is the first winter. The house is approx 25 years old and has the original (system) boiler with HW cylinder (serviced each year).

When we first moved in we had plenty of HW. However, since winter has come along and we have had the CH on all day we barely have enough HW to fill the bath a quarter of the way before it goes cold. We don't seem to have this problem when having showers and can easily have a 10-15 minute shower without it going cold (we only use the bath for my 1 year old daughter).

My understanding is that the way our system works is that the HW supplied to the radiators is via the cylinder (therefore using the same supply as the HW to the taps). Therefore my presumption is that the CH is using the majority of the HW and therefore we don't have enough to fill the bath. However as the CH & HW is programmed to be on from 6AM - 9.30PM at the moment, I am not sure what I can do to create more HW.

My old house had a regular boiler where the CH was heated directly via the boiler and only the HW was stored in the cylinder.

I have tried turning up the thermostat on the cylinder, but this doesn't seem to make any difference. For reference, the initial amount of HW that comes out of the tap is lovely and hot (the same as when we have a shower or fill a sink up for washing up).

The only other thing I have not adjusted is the the temperature on the boiler. It is currently set on level 4 (of 6). I dont really think there is any point in adjusting this if the water seems plenty hot enough (there is just not enough of it when having a bath).

There are some images of the system here if it helps:









Am I doing something wrong here in terms of how I have the system programmed? Any suggestions and thoughts as to what is going on would be appreciated.

Thank you
 
It seems you may have a thermal store, so not my forte, but something is starving the cylinder of heat. Try just having the hot water on, or perhaps at different times?
 
What are the dimensions of your cylinder (ignoring the cold tank on top)?
 
There are many factors, one is in hard water areas the tank capacity can be reduced. The Irish can heat up the whole tank with the Willis system, but for rest of UK there is a cold bit at the bottom, it depends on height of hot coil.

Some tanks work the reverse to the traditional and the DHW goes through the hot coil which allows a higher pressure with out pressuring the tank, if tank is at cold water pressure it needs testing every year.

Some systems use the tank so multi heat sources can combine, solar, solid fuel, LPG etc, but this is expensive so unlikely if not multi fuel.

But tank filled with lime is most likely problem.
 
mjgreen81 said:
we barely have enough HW to fill the bath a quarter of the way before it goes cold. We don't seem to have this problem when having showers and can easily have a 10-15 minute shower without it going cold
Click to expand...

That seems strange, because a 15 minute shower will use a lot of hot water. Especially if it is at mains pressure (thermal store). Is the shower definitely fed through the cylinder? Could it be an electric shower?

Have you tried turning the central heating off whilst you run the bath?
 
As said by higher persons than me "recollections may vary"
You could try turning on the immersion heater for an hour before bath-time and see if that helps.
 
mjgreen81 said:
My understanding is that the way our system works is that the HW supplied to the radiators is via the cylinder (therefore using the same supply as the HW to the taps). Therefore my presumption is that the CH is using the majority of the HW and therefore we don't have enough to fill the bath. However as the CH & HW is programmed to be on from 6AM - 9.30PM at the moment, I am not sure what I can do to create more HW.
Click to expand...

Apols if you know this already!

If your cylinder actually is a thermal store, then that's not quite how it works. For your hot water, the cylinder works like a combi boiler. The cylinder is full of very hot water. Cold mains water passes through a heat exchanger in the cylinder, and the hot water in the cylinder heats it instantly, like a combi. Then the heated mains water goes to the hot taps. Obviously, this cools down the water in the cylinder, and after a period there's not enough heat left in the cylinder to keep warming the water from the mains. I'm not sure if this will help, but hopefully it might help you work through a solution.
 
I'm not a thermal store expert, but I have worked on the controls of a couple of them.

Those I saw all stated that the boiler thermostat be set to maximum.

Capture.JPG


The below is from a Gledhill Boilermate manual, so may, or may not be applicable to yours.

Capture1.JPG


Also, it was recommended that boiler should be left 'on' permanently 24/7 because when there was no demand the thermal store would be charged up and ready to go instantly, the idea being that if there is no requirement for heat, the controls would shut the boiler down until there was a demand. Although users reported that due to heat loss, the boiler would fire up occasionally for a few minutes to top it up. Sometimes in the middle of the night. In one case I added a dual channel timeswitch and used the hot water channel and set it to switch the boiler off between 11pm and 6am so that it didn't wake the sleeping household.

Capture2.JPG


In your case, as the thermal store is now being used to provide CH as well as HW I'm imagining in the winter it isn't storing enough heat to do both.
 
Turn off your central heating for an hour before you use the bath so all heating capabilities are directed to your hot water tank, also turn the boiler stat up to 6. I like a really full quite hot bath and have to do this when I use central heating in cold weather. The extra coldness of any cold water added to the mix will also reduce the effectiveness of the hot water added to the bath. As said if you have an immersion heater put that on for an hour as well. Having said all that why would you want to fill a bath more than a quarter full to bathe a 1 year old child?
 
Hi everyone,

First of all let me say thank you for all the replies. They have been extremely helpful in helping me understand my system and what might be going on.

@JonathanM - I had not fully understood how the HW is heated via the cylinder on a thermal store. Your explanation was very helpful and I think may be pointing me towards a possible resolution....

@cross thread and @stem - You reminded me that over the summer I turned down the thermostat on the cylinder as I thought the previous owners had it too high and were likely wasting energy (the HW was very hot so I was happy to reduce the temperature). I cant remember what it was previously set as, but I set it to around 65c. It sounds like I was misunderstanding how the thermal store operated so it needed to be set high.

I have now set it to 70c, lets see if this makes a difference :). I have also increased the boiler heat from 4/6 to 5/6. As mentioned previously, this is not a setting I have touched since moving in so I am a bit wary about setting to max 6/6 in case it wastes energy (please correct me if I am not thinking about this logically and I would be better setting it to max.

I would like to get a better understanding about whether I am best to set the boiler to be on 24/7 for a thermal store? I understand the logic about not letting the tank cool overnight, but with he cost of energy nowadays I would love to know whether it would be more cost effective to have it on 24/7 or just 6am - 10pm as I do currently.

@Bouy to answer your question regarding why I would want more than a quarter full bath for my baby.....I probably didnt explain that at the point it runs out of hot water (probably less than a quarter full in reality) it has slowly been getting colder and the bath temp is less than 35c. I would also like the possibility of having a bath myself at some point if I fancy it.

On a side note - I tried a little experiment when running the bath last night. Instead of using the bath tap, I used the over- bath shower instead. It took a bit longer to fill but the water was warmer than from the tap below. Before anyone asks, it definitely is not an electric shower ;). Can anyone think of a reason this might be the case, or is this a tangent not worth exploring at the moment while I fiddle with other things?

Thanks again for all the help, it has been very useful.
 
If the heated water comes from the same source so there's no way shower outlet could be warmer than the bath tap.
 
The shower flow will be less than the tap so the hot water will be pushed out more slowly and will keep warmer longer as inrush cold mixing will be slower...This all points to a clagged up tank...do you live in a hard water area ?
 
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