Central heating idea

[thinfourth]

In our house we have an occasional issue where we run out of hot water.

We shouldn't as we have a 200litre unvented cylinder which is fine.

It is heated by an electric immersion and a dedicated circuit from the oil fired boiler.

The immersion is fine and the boiler is fine

however

The tank is feed from our own well via an electric water pump which gives an utterly excellent pressure and flow. So excellent that we once bought a cheap and nasty showerhead and it split down the middle.

I like long hot showers, the wife likes long hot showers, the wife uses huge amounts of hot water in the kitchen at times.

So about once a month one of us ends up with a cold shower as the hot water runs out.

This is a issue with the users not the system


I am considering modifying the system

for about £50 you can buy a small stainless steel plate heat exchanger

What i am considering is taking the feed to the heating coil and breaking into it before it enters the tank.

Fit the heat exchanger into this line and then pipe the tank outlet into it and then onto the house.


Then of an evening when the heating coil is on and we are drawing hot water if it drops in temperature then it will get a boost from the quite warm return leg going to the boiler.

So i have a sort of combi boiler effect going on.

Is there a reason this won't work?


The snags i can see is this is unregulated temperature wise so you could potentially get hot water at the boiler circuit temperature instead of the hot water tank temp. The shower is thermostatic so it can handle this.


Or i could use it to warm up the water entering the tank which would solve the overheat issue as once the tank comes up to temp it will shut off the heating circuit via the motorised valve.

 

[tlovatt]

Would you not end up taking heat from the hot water tank to heat up the return leg and prevent the boiler working properly

 

[WorkshopChris]

I would imagine that heat extractor may be quite restrictive in respect of the flow rates obtainable.

 

[julianf]

Id say its a bad idea. I dont entirely understand what you are planning, but it sounds like youre taking heat from the CH and trying to put it into the DHW?

If that's the case, just do it with electronics. Keep the tank hot using zone valves, prioritising the tank.

 

[waddycall]

There won’t be enough energy there to do much to your how water temp. All you’ll do is cool down your heating circuit.

 

[julianf]

If your boiler is set up to be running in condensing mode, then the return leg will be no hotter than 54c anyhow.
If your tank is down to, say, 55c, all that a fphe is going to do is lower the temp further.

Just have a stat in the tank that closes off the CH zone valve if the tank drops too low. Thats how oil combi boilers work - they just shut down the CH flow when there is demand on the DHW side of things.

 

[gaz1]

id be careful with that way of thinking you will get burnt by the hot water coming out of it

both me and @Kayos will warn you against doing so

 

[hotponyshoes]

It's a waste of time mate.

My setup sounds exactly the same as yours.
If we do the washing up and run the hot tap for a bit then we can't both have a long really hot shower.
Turning on the immersion an hour before showering helped a bit

It turns out my boiler to hotwater tank temp was a bit on the low side.

Also, my immersion element was only 12" long and in the top of the tank. I changed this for a 24". It also looks like there is a hole in the side of the tank to take a 2nd immersion if required?

Just changing the immersion element seems to have sorted out my issues (providing it gets turned on in advance.

Either that or just drag the other half in the shower with you!

 

[julianf]

id be careful with that way of thinking you will get burnt by the hot water coming out of it

both me and @Kayos will warn you against doing so

To be clear - my suggestion was not to use the FPHE at all - just to maintain tank temp by priority, using its exiting (probably) high recovery coil.

Anything using a FPHE should have a TMV after it - just like combi boilers are set up.

 

[julianf]

Can someone tell me why using the immersion element is ever a good idea? I mean if you don't have solar, or economy 7?

Electricity is more expensive for heat than both oil and gas. Your boiler is probably about 5x the kw output of an immersion.

I've never understood it. I guess maybe if you're just wanting a tiny bit of hot water stratified at the top of the tank it may be efficient, but for any other reason, why not just use the boiler?

Loads of people use immersions, so I think I must be missing something?

(We have an immersion in our tank - I've never turned it on. I'd have to wire it up before I could turn it on.... : )

 

[voipio]

The central heating programmer should be set to the hot water being on when you're having showers, so that the boiler is already reheating the tank as cold water flows into the bottom. If ithe programmer is set for hot water heating on, check that the cylinder thermostat is working properly. The hysteresis on the thermostats often starts to increase before they expire completely, resulting in the water in the tank needing to be quite cool before the thermostat switches back on.

 

[normspanners]

Electricity is more expensive for heat than both oil and gas. Your boiler is probably about 5x the kw output of an immersion.

Hum But immersion is 100% efficient or nearly anyway, what heat goes in goes into the water, a new gas boiler is 88% for direct and less if indirect ----I believe, but I'm sure someone will come along and have the exact figures, but from my findings nowadays most plumbers dont bother setting systems up properly and they are not running very efficiently anyway. with the water returning at far from the correct temp of 55 deg or below then condensing doesnt take place.
I hasten to add I am not a heating engineer but A lot of properties I have been too visiting , and people have had problems, the systems are a shambles and seemingly speaking with several plumbers they simply dont know.
One place had had a massive extension basically another house tagged on the end of their system and had the biggest combi boiler fitted, the extension never really got even warm the first room got a warmish rad and that was it !
They had several plumbers in too quote and the guys said the main runs needed changing for 28mm, another two pumps etc etc and were wanting thousands.
I said did they set up the system when they fitted the boiler? HUH no, they just left it as was, I said I will have a quick look, (as I was fixing the car that day) and sure enough every lockshield was wide open !! the first room had a10ft K2 in it and the room stat in there !, so I did a quick run around and did a few adjustments and had the extension warm, I said it needs doing properly but I need my thermometer.
so TEN minutes later and the system was working I went back and did some more fine tuning and it has worked for several years since. Without the thousands they wanted to modify .

 

[gaz1]

Can someone tell me why using the immersion element is ever a good idea? I mean if you don't have solar, or economy 7?

Electricity is more expensive for heat than both oil and gas. Your boiler is probably about 5x the kw output of an immersion.

the reason why its there is down to design implement it for those that need to use it for those that dont then its there as a just incase should really be wired in so you can use the electric when the boiler fails

fyi the 200 tank is never red hot as the lower at the bottom of the tank never reaches the middle or top of the tank temp therefore the last 30 litres is not at the true tanks heat ratio

to create a better system then either you change your habbits or add another tank or bigger tank to the system

there is another system that heats the cold incoming on hot going through the tank before it gets to your taps the original tank is heated much higher that what you have in your immersion heater that is set much lower

 

[normspanners]

I have my immersion set to max temp 80 deg iirc so I get enough hot water for 5 showers, one after the other especially with the boiler on also I was contemplating fitting a second element at the bottom of the tank but have now managed to get another larger tank with two elements to be fitted ASP . The shower is a scald safe low voltage pumped in shower cubical unit.

 

[fizzy]

Hum But immersion is 100% efficient or nearly anyway, what heat goes in goes into the water, a new gas boiler is 88% for direct and less if indirect ----I believe, but I'm sure someone will come along and have the exact figures, but from my findings nowadays most plumbers dont bother setting systems up properly and they are not running very efficiently anyway. with the water returning at far from the correct temp of 55 deg or below then condensing doesnt take place.
I hasten to add I am not a heating engineer but A lot of properties I have been too visiting , and people have had problems, the systems are a shambles and seemingly speaking with several plumbers they simply dont know.
One place had had a massive extension basically another house tagged on the end of their system and had the biggest combi boiler fitted, the extension never really got even warm the first room got a warmish rad and that was it !
They had several plumbers in too quote and the guys said the main runs needed changing for 28mm, another two pumps etc etc and were wanting thousands.
I said did they set up the system when they fitted the boiler? HUH no, they just left it as was, I said I will have a quick look, (as I was fixing the car that day) and sure enough every lockshield was wide open !! the first room had a10ft K2 in it and the room stat in there !, so I did a quick run around and did a few adjustments and had the extension warm, I said it needs doing properly but I need my thermometer.
so TEN minutes later and the system was working I went back and did some more fine tuning and it has worked for several years since. Without the thousands they wanted to modify .

Yes an immersion heater is almost 100% efficient. But electricity costs about 4x as much as gas or oil.
The best system is what I would have liked in my old farmhouse. Oil central heating with a giant heat store connected to my huge woodburner.
In my current house I have an oil fired combi boiler that also has a small hot water tank built into it. Instant hot water without the boiler kicking in and out every few seconds. If you use up the water in the tank then it kicks in.

 

[knighty]

I'm with thinfourth on this

I've looked at this quite a bit recently for work (where we use a lot of hot water), I'm moving a few things around and have 3 new 300 litre hot water tanks to go in... but a bit of googling finds the coils in them are only good for 14kw heat transfer, and that's with a decent temperature difference between the mains water / central heating water

instead of fitting the fphe to the feed out of the tank you could fit it to the feed into the tank - then there's no chance of the water getting too hot and burning you (because the tank start will turn off the heat to the tank once it's up to temp?)

have the heating water flow in reverse to the mains water, so the heating water coming out of the tank heats the water going into it


these heat exchangers... https://www.ebay.co.uk/itm/192347917796

come out of some sort of boiler add on system... they're made for heating mains water with central heating water - and they're pretty big, the bigger the better in my opinion :-)

 

[thinfourth]

I'm with thinfourth on this

I've looked at this quite a bit recently for work (where we use a lot of hot water), I'm moving a few things around and have 3 new 300 litre hot water tanks to go in... but a bit of googling finds the coils in them are only good for 14kw heat transfer, and that's with a decent temperature difference between the mains water / central heating water

instead of fitting the fphe to the feed out of the tank you could fit it to the feed into the tank - then there's no chance of the water getting too hot and burning you (because the tank start will turn off the heat to the tank once it's up to temp?)

have the heating water flow in reverse to the mains water, so the heating water coming out of the tank heats the water going into it


these heat exchangers... https://www.ebay.co.uk/itm/192347917796

come out of some sort of boiler add on system... they're made for heating mains water with central heating water - and they're pretty big, the bigger the better in my opinion :-)

I am thinking that taking the heat from the return leg from the coil and putting into into the cold feed


This way i reduce the temp going into the boiler which will increase its thermal efficiency as it is a condensing grant boiler. Its an oversized boiler for the house so I can't see it not be able to keep up. Even if it can't the worst is the happens is the radiators will be a bit colder than normal.

As to the safety factor.

The temp switch in the tank will shut off the feed from the boiler when it reaches the correct temp using the zone valve.

Lets say this fails

Then the max temperature the tank can reach is 80C as this is the output of the boiler. The temp/pressure safety valve on the tank shouldn't activate at this temperature.

The addition of the heat exchanger cannot increase the tank pressure or temperature above the same temperature and pressure that the heating coil can reach on its own.

to create a better system then either you change your habbits or add another tank or bigger tank to the system

there is another system that heats the cold incoming on hot going through the tank before it gets to your taps the original tank is heated much higher that what you have in your immersion heater that is set much lower

No room for a bigger tank

And it might be easier to put an extension on the house than to change our love of a long hot shower or to stop the wife using tons of hot water in the kitchen

 

[Wendelspanswick]

Hi, I have a Plate Heat Exchanger on my thermal store. It's designed so that hot water from the thermal store is pumped through the PHE and the controller measures the temperature of the exiting cold feed and adjusts the pump speed to achieve 70°C.
The problem I see with your solution is you have no way of controlling the flow of hot water through the PHE apart from the boiler pump, I doubt whether you'd achieve a high enough flow rate through the PHE to adequately heat the cold feed especially as you plan to fit it post hot water tank.

PS
Just lurking now!

 

[normspanners]

HEHE a few weeks ago I went in the shower and it was VERY hot, scalding so I tried to adjust it down and it was making no difference now I thought that is strange coz it has an anti scald, solenoid valve thats weird it must have stuck so there I am in my birthday overalls with screwdriver etc off came the cover of the pump housing to check out whats wrong screwed the valve all the way to the cold and it was still scalding hum seems like its plumbed up wrong, its been there 20 yrs so cant be ?? so I clamped off the silicone hose for the hot and it was still ridiculous, so clamped off the cold and it obviously stopped ?? suddenly had this horrible feeling, AH!!! shouted to the wife get the bidet and bath turned on cold , she thought I had gone mad, it was the eureka moment when I realized I had a 60/70 gals tank full ish of absolutely boiling water in a plastic tank in the loft !! ---------as well as a hot tank full. She was shouting the taps are the working the wrong way whats happening
WOW how lucky that we found out and hadnt left it !!
Off to the shed for a spare stat, not in birthday overalls BTW
Any views on safety clamp-on trip stat on the vent pipe ? it would have to stay tripped though once tripped, otherwise its a new complete immersion ?

 

[armalites]

I'm struggling to understand how you have the immersion and boiler running and you are running out of hot water. Surely you just need to run the water cycle on the boiler for longer. Our boiler does the hot water and central heating and the immersion has onlyu every been used when we didn't have a boiler up and running. There are 4 of us in the house all having showers and we don't have a massive tank. If it looks like theres going to be a rush then I just put the water on advance.