Replacing system boiler and water pump with combi boiler on mains water

33forfree · 07-03-2025 01:20PM #1

Recently bought a house in D16 with a gas boiler, a hot press tank, a cold water tank in the loft, and a noisy pump. Unpumped mains water is only to the two cold taps downstairs and I think the downstairs toilet. Basically all bog standard for an Irish house as I've been told, and several people mentioned the loft tank is a legal requirement but an actual plumber told me that law is no longer in force.

We would like to replace it all with a combi boiler directly hooked up to mains water, and no pump. The plumber I mentioned above said mains pressure is often too low for that, but he thinks our street would be alright. Neighbours no help with advice though, they have just got used to the immersion.

Would the first step be testing the mains pressure, I suppose under the street drain cover outside our front gate?

Then if we have enough pressure to be working with, is this still a stupid idea for some other reason? I'd like a setup like in the UK where most houses have no pump, cold tank or hot press and the boiler just activates on demand, but I know their mains pressure is often higher.

Apart from installing the boiler itself, and disconnecting the tanks and pump, would we be looking at changes to radiators or other pipework to get it all set up properly? We have a shower upstairs and we'd like it fed directly from the boiler (currently fed from immersion tank/pumped cold water, and the hot water runs out too fast).

Then thinking longer term, is there any other option besides a gas boiler for hot water on demand and no pump noise? Without breaking the bank of course. I've heard that heat pumps are a lot more expensive and can't really tick either box.

Calahonda52 · 07-03-2025 03:35PM

where is the pump and what does it pressurise?

Can plumber verify the new rules?

Downside of you idea is that pressure may often be too low and once they start repairing the mains, widespread disruption is expected

Lenar3556 · 07-03-2025 10:28PM

Advice from plumber about no requirement for cold water storage is incorrect. It is required by building regulations - although there is very unlikely to be any enforcement of these in your own house.

Aside from the above, it’s not a great idea, principally because of the variability in mains pressure.

I would be thinking along the lines of;

New water tank(s) in attic.
New larger capacity hot water cylinder.
No noisy pump - all gravity and use a pumped power shower such as a Triton Novel SR or Aqualisa Aquastream. (And or electric shower)

No particular requirement to interfere with radiator installation for above.

Gas boiler / oil boiler is still the most practical solution. This can heat your cylinder too (and keep it hot 24/7.

33forfree · 08-04-2025 01:44PM

https://www.boards.ie/discussion/comment/123241574#Comment_123241574

Pump is upstairs in the hot press, which is in the kids bedroom. Not the best place for something making that much noise.

Now we've been here another month, it definitely just starts when the (upstairs) bathroom shower or hot and cold taps are turned on. The toilets, the cold water loft tank, the immersion cylinder and all the downstairs taps are either running by gravity from the tank or cylinder upstairs, or directly from the mains.

The pump often doesn't stop for a while after water is turned off, so I guess that would be limescale getting stuck in the valves?

33forfree · 08-04-2025 02:00PM

https://www.boards.ie/discussion/comment/123242851#Comment_123242851

I take your point about no promises for mains water presssure, with global warming getting worse. Our water would be from the Blessington reservoir and I've heard about it getting low sometimes.

Now thinking we should keep the loft tank for cold water, but pump never made much sense in the first place since the taps and shower still run without it, and it doesn't fill the loft tank. Shower pressure is lower with the pump off but not so bad we can't use it. I think the pump is wearing out so it might need to go quite soon.

Still not sure about a new system boiler when that finally goes, when we have to run it at least 2 hours a day to cover morning and evening showers, but a combi might run on demand for as little as half an hour a day in summer. I haven't worked out what that's costing us yet but it seems like a massive waste of gas.

I should have mentioned the immersion element is dead, so the cylinder is just working as a big thermos flask for hot water from the boiler. Plumber I mentioned earlier said there's a big risk of cracking the cylinder if we get it replaced, and labour costs would be high anyway because the press is cramped for working on it, so it's a better idea to install a bigger new cylinder.

Not too keen on getting a bigger immersion if it's only for a year or two but have to see how long hot water lasts, especially if we get a power shower like you suggested. Can combi boilers work with immersion cylinders (instead of running the hot water directly to the shower from the boiler) or that's not compatible somehow?

I don't follow why you would change the loft tank, can you explain? And is it worth getting a lid for those? Ours is uncovered at the moment.

Anyway thanks for detailed reply, cheers

Lenar3556 · 08-04-2025 07:29PM

Yes, a combi boiler can be used with a cylinder. This would generally only be done if the combi was unable to provide the required hot water flow rates, and the cylinder then acts as a buffer. Generally the benefit of a combi is you don’t need a cylinder.

Power showers (if switched on full) use a lot of water, and will typically be emptying faster than the mains can fill it, so a second tank in the attic provides better capacity to serve them. If the tank is there a while I’d also change the current one, there are relatively inexpensive and can deform and leak with age.

33forfree · 08-04-2025 10:19PM

https://www.boards.ie/discussion/comment/123338954#Comment_123338954

If I can get the same sort of flow through a combi and upstairs to the shower as my old house in the UK had when I lived there, I wouldn't need a power shower. Just need to be sure about the flow rate before installing the boiler.

The mains flow rate is likely to be more powerful than what I get from my loft tank though, no? Which brings me back to the building regs and futureproofing problem. Is there a building regs requirement to have a hot water cylinder?

The house is 15 years old and as far as I know the loft tank is original. Nothing obviously wrong with it at a glance. Made of black plastic and it's about the length of a bathtub but deeper.

Lenar3556 · 08-04-2025 10:25PM

Yes, it is likely to be greater than a gravity feed from an attic tank, but it’s uncertain and may change in the future.

No building regs requirement for a hot water cylinder / storage.

15 years wouldn’t be too bad. They are about €100.

Why the need to change to a combi boiler?

33forfree · 09-04-2025 02:43PM

https://www.boards.ie/discussion/comment/123339613#Comment_123339613

I'm keen on a combi to avoid either running out of hot water if several people need a shower at about the same time, or having to waste a lot of energy heating it in advance if I'm not sure exactly when we will need it. We have different schedules on different days of the week, and sometimes we have houseguests, so hot water on demand with no cylinder would be more practical.

If a combi would be asking for trouble then I guess the fallback is an electric shower that heats its own water, fed from the loft tank, but I've had to use those before and only got a sad little dribble. Maybe they improved a bit though.

If I was going to change the loft tank would I need part of the roof to come off and the new one lifted in with a crane, or are they collapsible? What we have now wouldn't fit through the loft hatch in the ceiling.

Lenar3556 · 09-04-2025 03:27PM

Don’t rule out the hot water cylinder, most of the new ones are very well insulated and will come with declared losses per 24hours. The cost of keeping a cylinder at 60 degrees for 24 hours a day is actually quite small. Typically within the 1-2KW per 24hours - So in the region of €50-€70 per year. Plus bear in mind that this heat is lost to the house, so in winter at least it’s not a true loss.

There is nothing wrong with your combi boiler proposal, except the risks arising from being directly connected to the mains. Is it a worthwhile investment though? Is space at a premium such that getting rid of a cylinder will be very advantageous? Typically replacement water tanks pass through attic trap door openings. Particularly the long coffin types. Is the opening particularly small?

33forfree · 09-04-2025 09:26PM

https://www.boards.ie/discussion/comment/123341424#Comment_123341424

Not ruling it out but if I get an electric shower (looks like Triton T90 SR is the one to beat?) the cylinder would only be feeding three sink taps, and we've got a dishwasher that just takes cold mains water. Doesn't seem worth a new cylinder for that.

I'll need to go up at the weekend and measure the loft tank properly. You're probably right it would fit. Anyway messing up the ceiling to make a bigger hole is better than taking the roof of.

Coming round to decision now. We don't really need super high pressure showers, but we don't ever want the shower to run cold. So I can get an electric shower to replace the unpowered one we have now (fed by the hot press pump), turn off the power to the pump and make do with gravity for the sink taps (I already tested that water still flows, just a bit slower), and program the boiler to heat the cylinder for half an hour every morning just to have hot water from the taps. Apart from that the boiler would only be for radiators in the winter.

How about connecting the new shower to mains cold water if that has higher pressure right now, but with a valve to switch over to the loft tank if need be? Maybe trying to be too clever.

Then longer term, ie when the gas boiler dies, replace it with a ground source heat pump using a borehole. I know that's a fairly massive upfront cost but hopefully we have a few years to save up. We had the boiler looked at and were told we won the lotto it's in such great shape 🤣

Does that make sense? And if you're in the trade yourself, what would you charge for the shower installation?

Back Home · 10-04-2025 12:19PM

Fit a combi you will never look back.

Lenar3556 · 10-04-2025 07:49PM

If you are happy with the performance of an instantaneous electric shower, than that is certainly an economic solution, both in terms of running costs and initial installation. Mains or tank fed will make no difference in performance.

The bottleneck is in the rate at which it can heat water instantaneously. Mains fed electric is a different type of shower, so a changeover isn’t viable. (Nor beneficial)

Cost - it depends on how easy it is to get a dedicate cold water feed (if there isn’t one there already) and cable to your fuse board. At a guess maybe €1300 + VAT. Yes the Triton T90SR would be the best of them.

Note that the electrical installation must be capable of powering the shower safely, so this would need to be assessed by an electrical contractor in the first instance.

t5max · 10-04-2025 08:46PM

Hi,

Combi boilers can work well when installed properly.

That said - I would 100% advise that you do not directly connect a mains water supply to one

There are numerous reasons why the Irish Water By-Laws do not allow the direct connection of a mains water feed to a heat source including the potential for contamination and pressure problems

Where you need to connect hot and cold water supplies to shower mixer valves , the differential pressures between a mains fed combi and possible just the available head pressure from a cold water storage tank will likely cause problems

Do it the right way with a booster pump boosting both hot and cold water supplies and walk away from any plumber who’s willing to direct connect mains to a combi boiler or an elec shower if you have a suitable power supply available

Back Home · 11-04-2025 01:08PM

https://www.boards.ie/discussion/comment/123345295#Comment_123345295

Are you a experienced Plumber?

pjdarcy · 11-04-2025 01:15PM

I'm not a plumber but I've had a combi boiler connected to the mains since 2018 with no issues.

Back Home · 12-04-2025 09:49AM

https://www.boards.ie/discussion/comment/123347119#Comment_123347119

Then why advise against connecting to the mains if it's working for you?

pjdarcy · 12-04-2025 11:01AM

You must have me confused with another poster. I never advised against connecting to the mains

Lenar3556 · 12-04-2025 12:39PM

It is quite common to have combi boilers connected to mains water supplies in Ireland, and in many cases these provide satisfactory performance. Indeed in the UK it is the norm.

Note however that such arrangements don’t comply with building regulations in Ireland, and care needs to be taken to avoid differential pressures affecting thermostatic showers. Before going down this road, you would also need to be confident that the mains water supply can reliably provide stable pressure. That’s something that can vary from case to case and time to time.

Combi boilers can be a good solution in certain situations, but in many cases they bring nothing extra to the table and in these instances should be avoided as they are more expensive, and have additional maintenance requirements and points of failure than a conventional system.

Back Home · 12-04-2025 01:22PM

https://www.boards.ie/discussion/comment/123349526#Comment_123349526

Apologies

t5max · 12-04-2025 06:45PM

hi

https://www.boards.ie/discussion/comment/123347098#Comment_123347098

Hi, I'm a consultant engineer in mech + elec building services.

Been designing, specifying and project managing residential and commerical projects of all sizes for many years.

Have seen and been involved with rectifying plenty of poorly installed systems over the years.

Back Home · 13-04-2025 11:53AM

https://www.boards.ie/discussion/comment/123350801#Comment_123350801

Okey dokey 👍️

Sam Russell · 02-06-2025 07:47PM

I have had a submersible pump installed in the attic tank, with an expansion tank so pump does not turn on for a toilet flush. Basically, the pump pressures the cold water system that feeds all taps except the cold water on the ground floor. It works at about 3 bar, until a tap is opened which cause the pressure to drop and pump turns on. When the tap is closed, the pump runs for a few seconds to build the pressure. The pump is located in the attic, and is quite quiet, but not silent. Of course, neither is the boiler.

It has been installed for 15 years but the pump has now failed, so will be replaced. It cannot be replaced like for like because The original model is no longer available. So will be replaced with an equivalent solution. The expansion tank failed a while ago, but now the whole system is to be replaced.

Advantages of the pump is that a shower can operate anywhere, even in the attic. Gravity feed is inadequate for a shower or filling a bath.

OP = why not fit a new quiet pump next to the water tank? Cost should be less than €500, plus labour. My new system will cost about €400.

Lenar3556 · 02-06-2025 10:25PM

https://www.boards.ie/discussion/comment/123509348#Comment_123509348

That can be a good solution in certain cases, particularly with an attic conversion like you describe. It does require an unvented hot water cylinder however, and the associated expansion and safety controls.

With suitably sized pipe work there should be no issue in filling a bath on gravity alone.

The shower will need to be pumped to provide any kind of decent performance, but this can be achieved with a standalone power shower such as Triton Novel SR or Aquastream.

It’s not usually necessary to pump the whole thing.

Sam Russell · 03-06-2025 09:29AM

https://www.boards.ie/discussion/comment/123509735#Comment_123509735

Problem with such a pump is the likelihood of a hidden leak. My system was checked with a high pressure before hidden pipes were sealed. It can fill a bath in half the time of a gravity feed solution, and provide a good pressure for the shower wherever it is located.

Other than that, the pressure from both hot and cold are equal, and non-return valves are installed to stop hot water returning to the tank due to expansion.

I think it is a good solution, but it needs to be installed well.

Lenar3556 · 03-06-2025 12:03PM

https://www.boards.ie/discussion/comment/123510356#Comment_123510356

I don’t understand what you mean by a hidden leak? The pump is on the wall within the shower in the open air. No pipe work in the house is pressurised.

There is certainly nothing wrong with what you propose - it’s just considerably more expensive to install, vs a traditional system and has more points of failure, maintenance, noise & energy use. Well designed domestic hot and cold supplies to toilets, baths and basins don’t typically need to be pumped.

You could run 1” pipe work on gravity to a bath if you want an exceptionally fast fill rate. But in most cases 3/4” is fine.

Sam Russell · 03-06-2025 05:02PM

https://www.boards.ie/discussion/comment/123510934#Comment_123510934

The hidden leaks refers to a fully pressurized domestic system. A pressure test should be done before closing up the pipe work. Obviously, a pump only feeding the shower does not have the same problem.

If there are multiple showers, or showers in a converted attic, then a fully powered system would be justified.

Higher cost? Not sure about that.

03-06-2025 06:32PM

https://www.boards.ie/discussion/2058399997/replacing-system-boiler-and-water-pump-with-combi-boiler-on-mains-water

I'd avoid you "actual plumber".

Building regulations state cold water tank is required for every dwelling. It should be able to hold sufficient water to last 24 hours. The only thing allowed to be connected to the mains water supply is the cold water tap in the kitchen and the feed for your cold water tank. Correct way (& best practice) is to feed the combi boiler from the cold water tank. The cold water coming from the tank for everything in the house and the feed from the tank to the combi should be run through a pump

Any thermostatic showers require equal pressure on the hot & cold. This is why both should be pumped. Any power shower you might have, Triton, mira, Aqualisa etc have maximum inlet water pressure. This means if you are keeping these showers them they will need pressure reduction valves fitted.

I don't do boilers. I specialize in showers myself. If your plumber who does do boilers hasn't given you as much information as I have then I'd be looking for a different plumber

03-06-2025 06:41PM

In one post you say that you are happy with the pressure without the pump. Gravity pressure. In the post above you say you are happy with electric shower pressure. Electric shower pressure is greater than the gravity pressure without a pump. At least on the upstairs bathroom. A ground floor shower should give you around the same as an electric shower pressure

Lenar3556 · 03-06-2025 07:53PM

https://www.boards.ie/discussion/comment/123511988#Comment_123511988

Any pipework installation should be pressure tested after installation, regardless of what system is involved.

There are greater install, running and maintenance costs in the arrangement you propose, which run into several thousand euro.

Booster pump in attic supply and install costs.

2. Unvented hot water cylinder - Over twice the price of a vented, plus extra install costs for 2 x blow offs, expansion vessel, and pressure reducing valve.

3. Maintenance of the cylinder and controls. Manufacturers recommend annual service.

4. Maintenance of the pump.

5. Electrical Running cost of the pump.

Sam Russell · 03-06-2025 08:09PM

Well, it can be expensive I suppose, but I already have it installed.

Replacing system boiler and water pump with combi boiler on mains water

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