Hot Water Cylinder Questions

mmmmikey

michaels wrote: »

Isn't there a pump in the tank to heat exchanger circulation?

Hi - no, there is no pump at all. The heat exchanger is a simple coil inside the cylinder and circulation is by mains water pressure. It's the simplicity of the design that I find particularly appealing.

pile-o-stone

michaels wrote: »

Unless you have the store at a decent temperature how are you able to top up/hot up a bath?

It's the same as any other hot water tank, if the water in the tank is cold then you have to put the boiler (or immersion heater) on to warm it up. In our tank, the HW feed is high up in the tank at a similar level as the feed in from the gas boiler. It doesn't take too long to warm up. Stratification keeps the hot water at the top of the cylinder.

The advantage the heat exchanger has over an internal coil is that the exchanger takes its feed from the top of the tank where the water is hottest. A coil starts at the bottom of the tank and spirals its way up to the top. If only the top of the tank is warm, the water flowing through the internal coil doesn't spend very long in the hottest part of the cylinder before it comes out of the coil and off to the taps. You therefore need a greater volume of the tank to be warm in order to heat your water with a coil than you do with a heat exchanger.

michaels

mmmmikey wrote: »

Hi - no, there is no pump at all. The heat exchanger is a simple coil inside the cylinder and circulation is by mains water pressure. It's the simplicity of the design that I find particularly appealing.

OK, different system to the one I installed about 15 years ago where the exchange was outside the tank and the tank hot water was pumped through it.

pile-o-stone wrote: »

It's the same as any other hot water tank, if the water in the tank is cold then you have to put the boiler (or immersion heater) on to warm it up. In our tank, the HW feed is high up in the tank at a similar level as the feed in from the gas boiler. It doesn't take too long to warm up. Stratification keeps the hot water at the top of the cylinder.

The advantage the heat exchanger has over an internal coil is that the exchanger takes its feed from the top of the tank where the water is hottest. A coil starts at the bottom of the tank and spirals its way up to the top. If only the top of the tank is warm, the water flowing through the internal coil doesn't spend very long in the hottest part of the cylinder before it comes out of the coil and off to the taps. You therefore need a greater volume of the tank to be warm in order to heat your water with a coil than you do with a heat exchanger.

I see, rereading it is the return that you are keeping cool rather than the overall tank water temperature.

I seem to remember the system I had tried to keep the return temp up to avoid mixing so that if heat was being drawn from the tank at the top (to the exchanger) then the boiler would keep the top of the tank warm by having as warm a flow as possible but then this was pre condensing boiler days. I seem to remember there were thermostatic valves on the boiler circuit and the hot supply to the taps. Part of the reason we choose it was because DPS did us a system with a cylinder mounted on its side which we were then able to fit in the eaves when we converted the loft.

mmmmikey

pile-o-stone wrote: »

The advantage the heat exchanger has over an internal coil is that the exchanger takes its feed from the top of the tank where the water is hottest.

Thanks for the info, not something I had considered. I have still decided to go for the Advance Appliances thermal store but your comments have tipped the balance towards buying the larger 210 litrre cylinder which I have just ordered.


Plan is now delivery on Tuesday, DIY installation on Wednesday, drying out the house and arranging plumber on Thursday, getting the job done properly on Friday and redecorating the week after.



(On a serious note as the whole thing is pre-plumbed installation doesn't look any harder than for a washing machine - hot and cold connections to plumbing that is already there and an overflow pipe.)

pile-o-stone

michaels wrote: »

I see, rereading it is the return that you are keeping cool rather than the overall tank water temperature.

Ah, I understand your original comment now. Yes, the feed of hot water from the gas boiler goes to the top of the tank and the return of water from the tank to the gas boiler comes from the bottom of the tank, which is typically much cooler. This is slightly complicated in our tank because we actually have a return feed from the bottom and middle of the tank that goes to a mixer valve which then goes to the boiler. This is so the return water temperature is at an optimal temperature for condensing.

Condensing boilers only really condense properly (thus extracting the waste heat from the flue gases) at a water return temperature of 54C to 56C. Our return mixer valve is set to 55C.

Interestingly, the use of thermostatic valves on radiators often means that the return temperature to the boiler is higher than 54/56C and so prevents the boiler from condensing. So much for TRVs saving money!

With a thermal store, the feed to and from the radiators is separate to the feed to and from the gas boiler so a hot return from TRV radiators doesn't impact the return to the boiler. Well, until the water at the bottom of the thermal store is above 56C, but in that case the top of the thermal store would be hot enough that the boiler turns off. The water in the rads would circulate through the thermal store until its temperature reduces to the point where the boiler turns on again (by which time the bottom of the store is back under 56C and condensing can continue).

Phew, I hope that makes sense

mmmmikey

Some (limited) data and observations to round this post off now the thermal store has been installed and working for a couple of weeks....


1. No significant difference in energy usage between the old cylinder and the new thermal store - 5.3kWh per day for the cylinder, 5.2kWh per day for the store.
2. This probably doesn't mean a great deal because I suspect I'm now using more hot water due to the mains pressure shower and I stupidly didn't measure how much water I was using before. So I suspect that I'm probably losing less now through heat losses but using more because of the shower but unfortunately I can't quantify this. Also, I'm maintaining store at 75degrees which is hotter than needed so not very efficient, but this does leave me with hot water after a cloudy day - see next point.

3. One benefit is that the store is big enough to cope with quite big solar pv variations this time of year - e.g. 2.48kWh one day and 8.24kWh the next is plenty to keep it hot enough without paying for electricity. With the old cylinder I'd still (just) be using about 2kWh E7 overnight to make sure I have hot water in the evening (better to do this than risk a peak rate boost on cloudy days). Maybe represents a saving of 120kWh off-peak electricty a year but at 5.85p per kWh not anything to get excited about.
4. Very easy to install which should also mean very easy to maintain (not expecting limescale issues as installed water softener as part of same project)
5. Was losing an additional 1.2kWh per day before I lagged the pipes in the vicinity of the thermal store, which I didn't do straight away due to lack of confidence in my plumbing. So well worth doing this!

6. Hot water temperature much more consistent due to mixer valve on coil output, and shower flow excellent without the noise and cost of a pump.



Happy to answer any questions anyone has, but really just posting this here for future reference if anyone finds this thread through searching.


Thanks again for the various advice / comments that gave me the confidence to do this.

Coastalwatch

Hi mmmmikey, really interesting thread as I've been provaricating over which of two options to install to best utilise the excess solar we enjoy here. I'd originally considered a direct unvented system until I discovered the complexities of installation and annual servicing. Upon reading of your experience I'm now swaying toward the thermal store which appears not to require the two aforementioned issues.
I assume you passed on the option of an isolation kit, preferring to fit an overflow instead!

Something I'm struggling with is heat loss stated at only 68 Watts, other products quote over/around 1 kWh/ day so am wondering if 68 Watts is per hour!
I notice you've sensibly fitted a mixer valve on the coil output, which presumably also requires a feed from cold mains also?

Many thanks in anticipation of your reply.

zeupater

Coastalwatch wrote: »

... Something I'm struggling with is heat loss stated at only 68 Watts, other products quote over/around 1 kWh/ day so am wondering if 68 Watts is per hour!...

Hi

Wh is a measure of energy, with W being power ... a 68W load would consume 68Wh in 1 hour , so 1.6kWh per day (68*24/1000) ... same applies to standing thermal losses etc.

HTH
Z

mmmmikey

Coastalwatch wrote: »

Hi mmmmikey, really interesting thread as I've been provaricating over which of two options to install to best utilise the excess solar we enjoy here. I'd originally considered a direct unvented system until I discovered the complexities of installation and annual servicing. Upon reading of your experience I'm now swaying toward the thermal store which appears not to require the two aforementioned issues.
I assume you passed on the option of an isolation kit, preferring to fit an overflow instead!

Something I'm struggling with is heat loss stated at only 68 Watts, other products quote over/around 1 kWh/ day so am wondering if 68 Watts is per hour!
I notice you've sensibly fitted a mixer valve on the coil output, which presumably also requires a feed from cold mains also?

Many thanks in anticipation of your reply.

Hi - see Z's post re: heat loss.



Yes, I opted for an overflow rather than the kit as I wanted to keep it is simple as possible. Having said that, the overflow is a regulatory thing rather than a real, practical requirement. The feed to the header tank is normally shut off so no danger of a float valve dripping and overflowing. The only time it could overflow is when you're standing in front of it servicing it once a year so the risk is minimal. So the instructions actual suggest just capping off the overflow and removing the handle from the fill valve to stop someone accidentally turning it on (and even this wouldn't necessarily be an issue).


The hot/cold blender valve is pre-fitted to the unit, so you literally just have to connect the cold-in and hot-out pipes (and an overflow pipe is you want to).


If you're in a hard water area, the installation instructions recommend softening the water. Not sure how necessary this is, but I decided to fit a water softener as part of the same project. Can't believe the difference it makes showering in softened water and how much easier it is to clean the bathroom

Coastalwatch

Thanks mmmmikey and Z, looks like I'll be heading in that direction then. Just have to decide whether to fit one in the "old" immersion cupboard or up in the loft. I suspect the 160 litre version will have to suffice as the 210 litre appears too large for either the cupboard or fitting through the loft hatch.
I'd rather keep any heat loss within usable living space as opposed to the atmosphere, so guess the cupboard may end up as it's final resting place!

I assume the expansion vessel to be in addition to the diameter of the unit stated on the data sheet!