Small volume of hot water

[Tony_P]

I’ve a conventional gas boiler and unvented water tank which is probably about 500l; it’s two mètres tall and about a meter across.

I get good hot water pressure at a good temperature (it’s set at 60c) but only enough to fill about 10cm of the bath before it runs cold.

Does anyone have any idea what could be causing this? I had a plumber visit who suggested that the main volume of water in the tank isn’t what’s being supplied to the taps, but rather there’s a coil inside the main tank through which the hot water runs to get heated. Is this possible, or likely?

The house is a five story Victorian terrace with five bathrooms, so a system that only supplies such a small amount if hot water seems strange. The boiler’s a 45kw unit, so well capable of providing enough.

The boiler’s in a mews block at the end of the garden with a flat above, so conceivably there was a smaller capacity setup for the flat which someone has then switched for the main house, but does that seem possible?

 

[Deleted member 120897]

The plumber is likely talking nonsense. The coil for the cylinder is what’s classed as an indirect heat exchanger. Sounds like it’s not being heated correctly, either for the duration of the timed period, or a faulty motorised valve. However it’s unvented, which I’m not au fait with, so could be something in that arrangement.

Are you able to confirm what type you have? If you’re unsure, post a photo of cylinder cupboard.

 

[Gasmk1]

If the cylinder is that big it would be a lot more than 500 litres approx 6000 litres if am correct are you sure it’s not a thermal store.

 

[Tony_P]

The plumber is likely talking nonsense. The coil for the cylinder is what’s classed as an indirect heat exchanger. Sounds like it’s not being heated correctly, either for the duration of the timed period, or a faulty motorised valve. However it’s unvented, which I’m not au fait with, so could be something in that arrangement.

Are you able to confirm what type you have? If you’re unsure, post a photo of cylinder cupboard.

 

[Tony_P]

Thanks for the reply; here’s the tank and boiler.

 

[Tony_P]

If the cylinder is that big it would be a lot more than 500 litres approx 6000 litres if am correct are you sure it’s not a thermal store.

No, I'm not certain, sorry.

It's qute a complicated system, there are two heating zones in the main house, where radiators are controlled by thermostats and a Horstmann controller in the kitchen (which also controls the hot water); a separate inderfloor heating system (which seems at the moment to have a broken valve so never comes on) , and then a flat above the garage with its own radiators and hot water.

There have been a string of issues thathave proven hard to fix, and the current water issue is only one of them. Another at the moment is that the heating zones in the main house seem to only respond to the control commands sporadically. Sometimes they remain full-on despite the theromstat set to zero and the controller being turned off, and at other times they don't come on for a couple of hours after the heat has been asked for.

It's all quite frustrating.

 

[Chuck]

If the cylinder is that big it would be a lot more than 500 litres approx 6000 litres if am correct

You've missed the factor of 4 in 'h*pi*(d^2)/4' so it's closer to 1500 litres.

The likely problem is that the cylinder is being under-heated so only a small fraction gets up to temperature. For how long is the hot water programmed to heat the cylinder each day? (It'll take roughly 4 hours at 45kW to heat 1500 litres of water from 10°C to 60°C.)

From your description, it seems like you need a heating engineer to review your controls and programming.

 

[John.g]

The temperature gauge 2/3rds up on the cylinder is showing 48c, if the cylinder is being heated by a coil or a bottom mounted immersion then its more than likely that the whole tank is at 48C which would fill a good few baths, It takes 66kwh to heat 1500 litres from 10c to 48C or 87kwh to heat to 60C so a 20kw coil will take 3.3 hrs or 4.4 hrs and a 5kw immersion 13.2 hrs or 17.4hrs.

The op should check that temperature gauge when the water runs cold into the bath.
A 1500 litre HW cylinder does sound more like a heat store so just maybe there is coil supplying the HW? but it looks like a secondary circ pump in the photo, if so, then a HW cylinder.

 

[Tony_P]

You've missed the factor of 4 in 'h*pi*(d^2)/4' so it's closer to 1500 litres.

The likely problem is that the cylinder is being under-heated so only a small fraction gets up to temperature. For how long is the hot water programmed to heat the cylinder each day? (It'll take roughly 4 hours at 45kW to heat 1500 litres of water from 10°C to 60°C.)

From your description, it seems like you need a heating engineer to review your controls and programming.

It’s on 24/7 at the moment while I’m trying to get to the bottom of the issue, but still won’t put out enough hot water for a bath.

 

[Tony_P]

The temperature gauge 2/3rds up on the cylinder is showing 48c, if the cylinder is being heated by a coil or a bottom mounted immersion then its more than likely that the whole tank is at 48C which would fill a good few baths, It takes 66kwh to heat 1500 litres from 10c to 48C or 87kwh to heat to 60C so a 20kw coil will take 3.3 hrs or 4.4 hrs and a 5kw immersion 13.2 hrs or 17.4hrs.

The op should check that temperature gauge when the water runs cold into the bath.
A 1500 litre HW cylinder does sound more like a heat store so just maybe there is coil supplying the HW? but it looks like a secondary circ pump in the photo, if so, then a HW cylinder.

It’s reading 65; I’d turned it up from 60 to seventy earlier today to allow the bath to be warm enough even after I’d run it for long enough to cover my legs.

There’s no immersion heater on it, only the gas boiler.

I’ll check the water temperature when it runs hot and then when it’s started to run cold later and report back.

 

[Gasmk1]

any photos of system so we know what we are looking at

 

[Tony_P]

any photos of system so we know what we are looking at

Yes, as per my post at 10:58, thanks.

 

[John.g]

Is the black item adjacent to the HW cylinder a heat exchanger?.

 

[Gasmk1]

Is the black item adjacent to the HW cylinder a heat exchanger?.

Low loss header

 

[Tony_P]

Is the black item adjacent to the HW cylinder a heat exchanger?.

I think it is, yes.

 

[John.g]

Presume pump(s) heads are important after looking at this?.

 

[Jerry]

I spot 3 pumps but no zone valves?

 

[Tony_P]

I spot 3 pumps but no zone valves?

The zone valves are in the basement of the main house.

 

[John.g]

The boiler circ pump feeds one "side" of the LLH, there will be another pump to distribute the water to the different zones presumably, can you give details of this including make/model/settings and check that it's actually running when HW cyl coil motorized valve is opened.

 

[Tony_P]

The boiler circ pump feeds one "side" of the LLH, there will be another pump to distribute the water to the different zones presumably, can you give details of this including make/model/settings and check that it's actually running when HW cyl coil motorized valve is opened.

Hopefully these are the right ones.

 

[ShaunCorbs]

Needs a commercial engineer to have a look and diagnose

 

[Tony_P]

Needs a commercial engineer to have a look and diagnose

Yes, I had one this morning that I am not sure was correct in his diagnosis, so I'd hoped someone here may know.

I'm struggling to find anyone capable of diagnosing and fixing the issues.

 

[John.g]

If the cylinder coil is supplied from the secondary side of the LLH then the hot water temperature can vary a lot depending on the ratio of primary/secondary flows, the greater the secondary flow rate the colder the secondary flow temp due to dilution from bypass from the secondary return.
The coil may be supplied from the primary (boiler) side?, you could check this by looking at where the coil flow and return are teed into the system.
Did you carry out the bath test yet? and how many rads in the system?

 

[ShaunCorbs]

Yes, I had one this morning that I am not sure was correct in his diagnosis, so I'd hoped someone here may know.

I'm struggling to find anyone capable of diagnosing and fixing the issues.

What was his diagnosis and was he a commercial gas / plumber ?