Replacing immersion cylinder with Condensing boiler, is it possible?

[ericmark]

Last house I removed the cylinder as it gave me more room, in fact enough to make the airing cupboard into a room after moving a wall and taking some space off the landing. However this house the cylinder was there and now with solar panels in summer we use the immersion heater to give us free domestic hot water.

As part of the solar system I have a unit called an iboost+ which controls the hot water, and also reports how much power used. Last 7 days 2.44 kWh, the big question is how much energy is used by the gas instant heat boiler to heat the heat exchanger, it clearly changes boiler to boiler, but the whole idea that gas is cheaper than electric assumes the boiler is 90% efficient which it may be with a 3 hour burn, but a 2 minute burn unlikely, it is likely cheaper to fill a bath with gas boiler, but to wash ones hands, unlikely. And if you have tried filling a bath with a instant heat gas boiler, oh it is super slow.

I was told many times it was cheaper to heat DHW with gas or oil, once in a position to check the claims, found it was not true, while CH running yes very likely boiler already hot, in summer no it's not.

 

[Burnerman]

In a small nutshell Fiona any installer will check if your water supply and volume is satisfactory and likewise your gas provision.
After that the flue installation has to be assessed, followed by the condensate drain route and if that's all good you can go ahead.....you have the pros and cons now.
Good luck with your plans!
John

 

[Madrab]

Valid points. But with respect I’ve repeatedly said I prefer a combi boiler. I am cabin crew so away from home for consecutive periods and live alone. When I return home I like a soak in the bath and need to hand wash a fair amount of clothes, I find I run out of hot water and have to wait for the cylinder to refill/heat up. I find this really inconvenient hence my preference for a combi.

The convenience and instant nature of a combi has its advantages and especially for someone with a busy lifestyle like yours. Combi's do have their limitations though. Filling a bath takes a long time unless the is one of the largest and realistically, on a standard HW system with standard outlets then only one hot outlet can be used at a time, this is just the nature of how combis work.
If you have the budget to swap the system out and if that's your preference, the change over is relatively straightforward. Depending on the size of boiler that was there before you will probably also need a gas supply pipe upgrade. Water supply would need to be considered and that question needs to be asked of the potential installer, some just open the tap and say, yeah it's fine. FYI, That's not good enough, it needs to be properly gauged ending up with dynamic pressure and flow figures.

I was told many times it was cheaper to heat DHW with gas or oil, once in a position to check the claims, found it was not true, while CH running yes very likely boiler already hot, in summer no it's not.

Not wishing to get into that huge discussion between leccy and gas but that does seem to be quite a specific point, if we are talking about a large cold combi just to washing hands then yes, that may certainly be correct but that's only a small narrow area of use where gas may not be cheaper but in that specific case would it actually be any more expensive? Electric instantaneous heaters also have to warm up so they can all waste energy etc. Many combi's also now have a pre-heat function so HW is delivered much faster but when it comes to all other requirements for HW/stored and space heating then I'm sure the figures may still bear out that gas is still cheaper overall. When combined with renewables then that average cost would drop again.
I think as the energy sector balances out with other sources coming on stream, higher efficiencies/insulation/designs etc then it may be time the jury is recalled and it's time to have a new survey where 2 identical houses are put side by side, one all electric and one standard mixed setup, just to see current indicative costs over a year.

 

[Agile]

And to add to the advantages of retaining your existing boiler to heat a new cylinder!

A standard cylinder is about 90 litres or 26 gallons.

But they are available in larger sizes and you would need to be pretty careless to empty a 300 litre cylinder while the boiler was reheating it.

And if you must have filmstar baths, filling a standard bath with a combi could take 15 minutes.

Whereas a cylinder would fill it in less than half that.

 

[ericmark]

I agree with @Madrab my point was for years I was told to use the boiler to heat the DHW was cheapest option, in some cases this may be true, but I can't believe how much in my case oil has been saved by using the immersion heater, my boiler would fire up 4 times a week for around 20 minutes, set to half hour, but always stopped before that time had elapsed. So around 24 kWh per week with oil, 2.44 kWh with electric so electric would need to be in my case to be ten times cost of oil per kWh to break even. OK maybe my house an extreme case, however once a combi boiler is fitted, no option but use gas/oil for all DHW.

I have four hot water taps, three in regular use, I have considered under sink heaters, but even without solar, 2.44 kWh is not going to break the bank, so not worth doing anything in my case.

My dad when getting a combi boiler had a radiator fitted in the airing cupboard so it would still work as an airing cupboard, I would not have done that, he also had to get the power shower removed, and a mixer shower installed instead. I will admit I miss the shower direct from the combi boiler, or last house DHW boiler, there were not as good as the power shower, but better than the electric shower, however the electric I run maybe a couple of cups of water before stepping under, the combi boiler had to wait for shower to go hot, then cold, then hot again as it used up the small reservoir of hot water before the boiler could fire up to replace it, so likely near to a bucket of water wasted waiting for the hot water. This did not happen if Eco selected on the boiler, once hot it stayed hot, but then the taps in kitchen would run cold.

Parents when with combi for same reason as you, the tanks started to leak, and he could get a grant for combi, but not to fit new tanks, so for him combi was cheaper, he had to my mind a load of cowboys fit it, large firm specialised in fitting central heating systems with grants, they connected the power shower to the combi boiler which was against the law, they had to return to fit new shower, and non of the pipes were lagged, and the wall thermostat was not suitable for the boiler, and the lack of a TRV in hall resulted in heating not working very well, and they did not set even on lock shield valve.

So much was down to poor installers.

It may be a combi is the way to go, but do it with eyes wide open, not pushed into it by miss information.

 

[RandomGrinch]

And to add to the advantages of retaining your existing boiler to heat a new cylinder!

Can't we all leave it now.
The OP wants a combi - there is nothing wrong with a combi; especially in these circumstances - so you wait seven minutes longer for a bath, big deal!

As has been mentioned, apart from checks on gas pressure etc; to replace the existing boiler in situ should be possible. Hopefully there would be a cold feed nearby - does the existing boiler have a filling loop?
That would leave a connection to the existing hot water pipework - which possibly would have to make it's way back to the cylinder, unless there is existing hot water pipework near the boiler.
After that, the tank could be removed to create some (probably) much needed suitcase storage

 

[Fionageko]

Well, you’ll need to get your water pressure and flow rate tested before you even consider replacing your heat only boiler with a combi boiler. If okay, you’ll need to get a heating engineer round to carry out a survey to see what’s needed. What boiler do you currently have?

It’s a Potterton I believe.

 

[Pete the Feet]

Heating hot water for a cylinder is less efficient than using a combi for hot water. I worked out the efficiency was about 55% as opposed to a combi which is much higher. However there are many things to take into consideration. If your water pressure fluctuates all over the place or is low then a combi can be a real pain. We have a 300L unvented cylinder and this gives a really good shower [4-5 a day. An hour charging time, once a day]. It also fills a large bath up fairly quickly. It is however a luxury and expensive to run. Combis can now provide a really decent shower unlike the earlier models.
The other advantage of a cylinder is when the gas boiler packs in you have an electric immersion back up for your hot water.

 

[Madrab]

Heating hot water for a cylinder is less efficient than using a combi for hot water. I worked out the efficiency was about 55% as opposed to a combi which is much higher.

Under what conditions and scenario was that figure obtained???

Sorry but I have to disagree, unless there were duplicate systems setup for each in carefully controlled conditions then I'm afraid there are far too many variables involved to obtain accurate and comparable figures to make that kind of sweeping statement.

In your scenario that may possibly be the efficiency rating but that couldn't ever be compared to any others without knowing how every permutation of available system would/could perform.

 

[Pete the Feet]

Under what conditions and scenario was that figure obtained???

Sorry but I have to disagree, unless there were duplicate systems setup for each in carefully controlled conditions then I'm afraid there are far too many variables involved to obtain accurate and comparable figures to make that kind of sweeping statement.

In your scenario that may possibly be the efficiency rating but that couldn't ever be compared to any others without knowing how every permutation of available system would/could perform.

I read it somewhere and then measured the gas usage on several occasions to verify. A bit disappointing I must admit. I have two temperature monitors one at the top of the cylinder and one near the bottom, and a shunt pump for the cylinder. The lower sensor is the one that controls the boiler. The pump runs when charging the cylinder to maintain an even distribution of heat. Before I fitted the pump the water was unevenly heated in the cylinder and didn't really heat up all the cylinder and consequently seemed to run out of hot water sooner.
Initially I had a Ravenheat system boiler which had two stats. One for heating generation and one for DHW. The least said about that boiler the better. The current boiler is a Vaillant ecotec and controlled by Vaillant Vsmart which allows for weather compensation and low flow temperatures for heating and higher flow temperature when charging the cylinder. As you know you can't really have weather compensation if you can't crank the flow temperature up for generating DHW in a cylinder. The gas usage is still surprisingly high in Summer or non heating months, but as I said it is a luxury. The cylinder is set to charge at 7:30 in the morning for 1 1/2 hours but an hour is usually sufficient.

 

[flameport]

and a shunt pump for the cylinder.

That is throwing vast amounts of heat away.
Most hot water losses are in the pipework.

 

[Pete the Feet]

That is throwing vast amounts of heat away.
Most hot water losses are in the pipework.

How so? The main losses are every time a tap is run and the cold static water is first run off. It pumps between the top and bottom of the cylinder to prevent stratification in the cylinder. It runs only when the cylinder is charging and is not pumping water around the house as a secondary circulation pump. A destratification pump is recommended practice on larger cylinders. Maybe not on a 300L cylinder but it works well.

 

[martygturner]

Another pointless unhelpful series of posts for the OP to digest and have to argue against... She wants an combi, is it possible. Yes. end of.

Like the seminal I want a to build a railway sleeper retaining wall, will it retain 2ft of earth...Oh you don't want to do that you want brick.

 

[ChrisJP]

Valid points. But with respect I’ve repeatedly said I prefer a combi boiler. I am cabin crew so away from home for consecutive periods and live alone. When I return home I like a soak in the bath and need to hand wash a fair amount of clothes, I find I run out of hot water and have to wait for the cylinder to refill/heat up. I find this really inconvenient hence my preference for a combi.

I would still advise that you think carefully about the downside of relying totally on a Combi boiler for your hot water. If you return home from a spell away and the boiler won't start for any reason, you'll have no hot water until you can get a plumber out to fix it - which may take days.
If you also have a cylinder and immersion heater, you just switch it on and you at least have a hot water supply. The chances of an immersion heater not working when you switch it on vary from minimal to non-existent.

 

[Harry Bloomfield]

When I return home I like a soak in the bath and need to hand wash a fair amount of clothes, I find I run out of hot water and have to wait for the cylinder to refill/heat up. I find this really inconvenient hence my preference for a combi.

I have a cylinder, open vented, with a heat only boiler. I find a can quickly run a bath, much quicker than a combi can supply, and whilst in the bath, the cylinder is recharging. When back out of the bath, there is a fresh tank full of hot water.