Heating systems with pressurised hot water tanks

Please educate me STW.

Our boiler is on its last legs (currently locked out awaiting repairs). It’s a combi but several people I’ve spoken to suggest that our house would be better with a pressurised hot water tank (based on radiator/shower/bath count). We have the room next to the current boiler for a tank but I don’t know where to start it what questions to ask. Ideally the current boiler can be fixed and work long enough for a properly thought out replacement system. There are a number of reasons that a tank sounds good to us not least being able to run multiple showers at once, having an immersion as backup water heating (or during cheap electric periods), bring able to have an ideally DC second immersion to use excess solar PV.

So where do you start?

Is the boiler element still a combi, just with a tank in the loop? It is there a special boiler.

Could a tank be added later.

What details should be considered for system design?

Can you have tanks with triple immersion (elec, excess pc and maybe solar water tubes).

I’m sure someone will be along to suggest a heat pump however I strongly suspect they will be no good for this house without substantial changes (external insulation, under floor insulation etc). The current 30kw heating boiler has a hard job keeping the house warm in sub zero days.

Just swapped to this from a combi my self.

I had a gledhill stainless + solar fitted.

I have 4sqm of solar thermal  panels on the roof and just use the immersion for when there isn’t enough solar thermal output or is excess solar PV but the thermal panels can’t get hot enough.

You can get a tank with multiple coils but I didn’t see the need for my boiler to be on the loop.  Triple immersions don’t tend to be on <300l tanks. Currently I’m still running a combi boiler and just feeding my utility room from it to keep the diverter working. -you’d normally fit a system boiler but I ain’t changing what works currently 

Can still do night immersion /boost immersion and excess PV on a twin immersion system. 

I have 250l for 4 people with one bathroom. Gives us a days hot water and has done three baths in one evening in the past.

The tank goes to 77c from solar thermal/60c on immersions and outputs at 45c through a thermostatic mixer to all outlets  -so is real terms much more than 250l

Most important thing is calculate your need.last thing you want is to run out. Second last thing you want is to be heating excess every day. Especially as you need to hit 60c in the whole tank weekly to keep legionella at bay.

Also. Don’t underestimate the space needed dependant on tank. The prvs/tundish/drain off pipework/ large expansion vessel and pipe work all need a home my tanks 6ft tall as well.

Go to gledhill and download the manual for one of their tanks. They have useful PID diagrams for various set ups in the back so you can work out what your installers talking about.

Building control notificable work following g3.

Oh and I wouldn’t go expecting masses of difference between a good size combi and a pressured tank.

We had a 28kw grant oil combi and the flow off both is comparible……

Where the tank comes into its own is when another outlet is turned on there is no drop.

Where the tank comes into its own is when another outlet is turned on there is no drop.

we are on a pressurised system and the pressure drops when you turn another tap on.

Personally I prefer combi boilers. If it wasn’t such a faff to take out our pressurised system I would do

If you want to go to a tank then you will switch from a combi to a “system” boiler. You will need a three port diverter valve that will automatically switch between the heating and hot water depending on the tank temperature. The whole system will prioritise the hot water tank but it should heat up from cold in about 45 mins.

Combi systems are generally for smaller houses and flats and struggle when you start having multiple showers and baths that are used simultaneously unless they are massively oversized.

Thanks that’s great detail, thanks. We are also 4, but 3 showers (and two soon to be teenage girls). The shower water flow is fine at the moment, untill another outlet is used. Being able to use multiple outlets would be a major plus point for our usage.

So in your system hot water is only heated by the thermal panels or by the immersion? Is the 4sqm enough in the winter or does the immersion take the load then? I had thought that the solar PV divert would need it’s own immersion coil but if we had thermal panels as well then I’d imagine at the times there was lots of excess PV the thermal panels would easily handle it.

The current boiler is in a cupboard off the loft space (inside the insulated walls). Although it goes under the roof slope it’s a big space so not worried about not having room in there, other than perhaps height but I understand there are different shaped tanks. There is also a similar space next to it that could if needed be used.

Oh and first thing to do if your trying to make you house warmer before insulation etc is to make sure all your doors and windows are properly sealed. You’d be surprised how much heat is lost to cold air leaking in.

So in your system hot water is only heated by the thermal panels or by the immersion? Is the 4sqm enough in the winter or does the immersion take the load then? I had thought that the solar PV divert would need it’s own immersion coil but if we had thermal panels as well then I’d imagine at the times there was lots of excess PV the thermal panels would easily handle it.

Doesn’t need it own immersion. You’ll need something like an eddy but it can do standard immersion duties also.

I have solar thermal as my first heat up. Even at the moment with poor sun hours and cold air I’m getting to 30 most days off the sun and occasionally 45-50.

I top that up with immersion on night time cheap rate

There’s no excess PV at the moment as I also have a battery

I would have needed another coil for my boiler to be added in -but as I’m on oil I thought given I know what my solar PV output is across the year and that my solar thermal would likely mirror that -id only be using the immersion 3 months of the year – and the cost of the extra coil and plumbing to connect in the oil boiler -buys alot of electric especially when I generate (over the year)  much more PV than I use.

Solar thermal -1sqm per 50l is the rule of thumb so technically speaking I don’t have enough but you also don’t want to be baking your Glycol in the summer sun. Those things can reach 200+ if the system gets to peak heat and fluid goes static.

We have a mains-fed hot water pressure system- two teenage girls and us two, it works fine – really strong pressure that only really drops if both showers are on and someone else use the toilet or taps, but never really a big deal. Only once or twice have we run out of hot water completely, usually because we’ve used showers outside of scheduled heating times.

we did get a huge HW tank though -it’s about 5ft high so it stores a huge amount of water.

overall I am so happy we decided to do this – it cost loads and our girl were just five yrs old at the time, but we’d have spent more getting it upgraded as they got older.