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.

If flow is of concern then use an accumulator as well.

also consider buying a heat pump cylinder so if you ever go down that route the cylinder will be in place, slightly more expensive but a lot cheaper than buying twice.

I would never specify a combi for a high demand house. Also no back up if boiler breaks no hot water!

Your boiler may not be at fault for keeping house warm, but more likely system design. Probably 22mm pipework from boiler if it is like most combi installs. As a tule of thumb that can only accommodate about half the output from your boiler. That could also apply to the whole system as it probably reduces down too soon. Radiator size could be wrong, balancing of the system, so many things rather than the boiler.

And yes you could have a heat pump regardless, if the load is 30kw (which I doubt it is) it doesn’t matter how you deliver it, what does matter is correct system design regardless of energy source. Running costs should be similar with correct design, even cheaper with some of the energy companies offering 15pence per kw tariff for heat pump use.

A popular mis conception that heat pumps only work in well insulated houses, the load is the load, how you deliver it is critical with a heat pump. Otherwise the heat pump we have installed in a castle wouldn’t work, or the one in a church, or a listed building…..

Went from an old combi to a system boiler and unvented cylinder, would never go back. Best shower ever.

make sure all your doors and windows are properly sealed

They are other then one window of concern, a single glazed 100 year old stained glass window. That has secondary glazing but needs attention as the leading is starting to go. Getting that safely removed and encapsulated into a triple glazed unit is a big job which I’m looking into. We’ve also put in lots of window coverings than made a significant difference (e.g. insulating hexagon blinds).

Also no back up if boiler breaks no hot water!

This is exactly where we are at the moment and one goal of any replacement system is to no be in the position of no hot water and no heating. Very glad it is mild at present.

Probably 22mm pipework from boiler if it is like most combi installs.

I’m not 100% sure of this as we didn’t have it installed but the pipework I can see where it comes down from upstairs into the kitchen is larger than this. Worth investigating though. I suspect the hallway radiator is undersized which may well contribute. The hall and landing are decent room sized spaces, there is a single radiator in the hall and then to top it off my office is in a room in the loft. This is hatched through the landing ceiling with a sliding stairway so the heat does as heat does and rises into my office.

Apparently prior to the current combi there were actually 2 boilers fitted, one for upstairs and one for downstairs.

I like the idea of a heat pump and the idea of ditching gas entirely. If the current boiler can be repaired and buys us some time then I’ll at least get someone to come look and assess what would be required. I’ve a feeling though that the current boiler may be uneconomic to repair and its going to be a case of getting something quicker than I’d like. We do not want a to shotgun purchase a combi though. Part of the problem is we don’t have a trusted local company. The last one let us down badly when the boiler (in previous house) failed. Ended up getting BOXT to replace that as we had a young baby, it was sub zero and the company had a 3 week lead time vs next day from BOXT. (Anyone in/around Southampton recommend a company?).

Probably 22mm pipework from boiler if it is like most combi installs.

Found the invoice in our purchase paper work. You are correct it is 22mm.

We’ve done this – we had a standard vented hot water tank and cold water tank in the loft. Moved to a system boiler with a pressurised cylinder which meant we could lose the shower pump for the en-suite and the huge water tank in the loft.

What we’ve found is pressure is great for one shower / filling a bath – but it drops a lot if you try and use a 2nd shower or tap.

This is because our water inlet to the house is an old size and ideally they need a bigger size to maintain high water pressure into the system. Our pipe goes under a drive and under the house so not viable to change.

Ours is just a 30kw boiler (no solar stuff) and we ended up with this setup in our airing cupboard – 210 litre tank:

If you have the space then a system boiler and pressurised hot water tank is the way to go.

I think our unvented cylinder in the cellar is 250L. Works well. Combi-based showers are always a relative disappointment.

Combis seem like a great idea ‘only heat the water you need when you need it’ but are generally even more oversized for purpose than system boilers. This can mean that for central heating they are more inefficient than necessary. For hot water you are burning a lot of gas to heat the flowing water quickly.

A system boiler can be optimally sized for heating the house. Heating a fairly fixed volume of hot water can be done with modest power. And as some have pointed out you have the immersion backup. And with cheap electricity or solar you might be able to heat the water cheaper than gas in some circumstances.

IDK if it is still the case that combis do either hot water heating or central heating at any one time. Under this assumption, running a bath may result in the house cooling down for that time.

in either circumstance choose your boiler wisely. Some are pretty shocking in their materials and construction.

Still, if I were replacing my boiler today I’d definitely replace it with a heat pump. But that’s a different thread.

we recently extended and was recommended to change our setup.

First some simple explanations in case you didn’t know. Apologies if its obvious

Combi boiler. turns on/off only when needed making it great for smaller houses. So when your heating is required, it pumps water round the circuit. When you demand hot water from the taps, the closed heating circuit is turned off, and cold water flows from the mains into the boiler to be instantly warmed up. Thus if you turn on two taps its trying to instanteously heat two taps which is hard for the boiler rated to manage.

regular Boiler = has a cold water tank in the loft as a gravity feed into a hot water tank. (not really something i’d install in a new install)

System boiler – has one heating circuit through the boiler but you use valves to divert the hot water to any number of heating circuits (eg upstairs, downstairs etc) plus one extra one through the pressurised hot water tank. there’s a coil in the tank that heats all the water in there. You programme when you want the boiler to warm that water. this hot water does have parasitic losses and often has an immersion backup to manage these smaller losses. in some you fit another immersion heater to take solar electricity direct and heat the water that way before sending the rest back to the grid.
Cold water feeds straight back in as you draw hot water keeping the system pressured. No matter how quickly you draw off the hot water (from multiple taps) the pressure or flow of hotwater is not really affected as you’ve pre-warmed it all up. downside being that once you’ve used it all, you ahve to wait for the boiler to warm up a tank of cold water up before you cna drain more while a combi would just keep on going.

WE swapped to a system boiler and 250litre tank due to the size of the house and part of me wonders if we’d have just been ok with a very large combi. we don’t run that many taps at the same time, and while the HW tank is massive so we don’t really run out of water, we do have to pay for the HW being kept warm.

if you’re plumbing, personally it’d run 22mm to all showers and baths to manage high flow but 15mm or smaller for all sinks to encourage fast flow from cold to hot. our kitchen tap takes an age to get hot water out due to the low flow through the hoses at the last section and the large volume of cold water between the tap and the tank.