combi gas boiler 2 showers simultaneously

Piriz

getting mixed messages on this one,

I would like a combi for my house as it sounds more efficient to just heat the water we use instead of hot water cylinder, we have two showers and often they are running at the same time when the kids take their showers each night..

We have only 7 rads and often just have 4 or 3 rads on depending on the weather.

We have a pump which provides high water pressure / flow to the house.

I had a plumber / heating specialist visit on Sunday and he feels a Ideal Vogue combi will supply two showers simultaneously.

I had phonecall with another plumber / heating company today and they advised it's not possible to supply two showers simultaneously with a combi, unless i spend big on a combi with inbuilt hot water cylinder.

I'm gonna get quotes / advice from two other companies.. but i'd like to hear the opinion from Boards experts if possible..

thanks

Sleeper12

I would seriously question if you could ever save enough on heating hot water to justify the upfront cost. It's worth remembering that the sale of gas boilers will most likely come into force in a little over 10 years. They are forcing heat pumps on us. You will need to install a hot water cylinder again for the heat pump

Anyway to answer your question, two showers could work at the same time. It depends on what the flow rate of the showers is going to be. Installer might have to reduce the flow of the showers to allow 2 at the same time. Also this is assuming you have a cold water tank.

Piriz

the boiler I have is Ideal mini, it's about 24 years old, doesn't give any bother and hasn't been serviced in a few years, yea have cold water tank.

I do have a problem with the safety valve on the hot water cylinder and the cylinder will need to be removed to access the valve to replace it, I assume this will cost a lot in labour..

because the boiler is old and probably inefficient and our heating controlls are a nightmare and dont seem to work (so we have constant hot water) I thought I'd get a A rated combi, remove the cylinder and get better control over the heating and subsequent costs and it'd pay for itself in a few years in cost savings..(quote 3800)

however, now I wonder if I just pay for the work to replace the (inaccessible) safety valve, service the existing boiler and get new heating control like Hive.. probably be cheaper..

welcome any advice thanks

antoinolachtnai

You have a pressurized (unvented) hot water cylinder? And four people living in the house?

Sleeper12

There is a grant for the heating zones, if you don't have it zoned. Grant is around €700 I think.

Personally I'd question if you'd ever save enough money to recover the original outlay. Average lifespan of a modern gas boiler is only 10 years. A condensing boiler is only fully efficient when in condensing mode. If you tend to turn it on for an hour at a time then it will never be efficient. If you have it on for hours at a time then it will be efficient

Piriz

note sure if it's pressurised / invented, there's 3 adults 3 kids in the house

Piriz

I have two heating zones, but don't have good heating controls, think hive could be an option..

what I take from what your saying is to keep old boiler instead of going for heating upgrade.. as the return on investment is unlikely to be as good as I expect.

antoinolachtnai

https://www.boards.ie/discussion/comment/121733557#Comment_121733557

With this many people in the house you won’t save much by not having a cylinder. You have a lot of demand for hot water. It would be different if you had low demand for hot water.

Any heat loss in winter gives a benefit to the house because the cylinder acts as a small radiator. In summer loss is more of an issue but I wouldn’t necessarily think it would be that big.

what does the cylinder look like? Can you see insulation on it? Or covered in white material? Or is copper exposed?

Your boiler is old and will need replacing no matter what. You are wise to consider the best replacement now.

Piriz

17073287864548994183619241053662.jpg

see cylinder, this will need to be taken out to replace the safety valve that's causing the leak to the overflow pipe going to the garage, with this in mind would it be wise to replace the cylinder? are new ones better? I know the thermostat has broken from this cylinder (bottom left) although this is probably easy to fix.

so the plan could be to not get a combi but replace the old boiler with a condenser boiler?

and get heating controls like hive.

antoinolachtnai

It’s certainly one very realistic option. I don’t mean to dictate.

The safety valves and thermostats on this type of cylinder setup are very critical. A faulty set up is highly dangerous because water is heating under pressure. The setup should be regularly tested and serviced and the safety valve should be positioned to allow this I think. (This type of cylinder is not something I have a lot of experience of.) Kingspan recommend they this type of cylinder be inspected every year for instance. (https://www.kingspan.com/gb/en/services/hot-water-cylinder-servicing/)

A suitably experienced plumber will have to decide whether this tank and setup is ok. If it has to be replaced that will have a big bearing on the overall price.

it is worth noting that none of the pipes are lagged.

I would guess that the cylinder is cool when you touch it even when the water inside is hot? If so that would indicate that you are not losing much heat from the cylinder itself.

Piriz

yea the cylinder is cool at all times.

I did have an experienced plumber have a look at this set up on Sunday but I was expressing interest in combi boiler and removal of cylinder and he gave quote for this e3800 with hive control and Ideal Vogue combi boiler..

I need to reevaluate, it's actually quite tricky to know what to do..

Lenar3556

https://www.boards.ie/discussion/comment/121736310#Comment_121736310

You would need a big combi to serve that type of hot water demand. Probably in the region of 50kw. The quote sounds too cheap.

I would second the other suggestions that there will be rather limited efficiency savings, and the thing won’t pay for itself.

Suggestion would be;

Review existing boiler and see does it need to be replaced. The efficiency savings on a new condensing boiler won’t be as big as you might think.

Certainly get cylinder inspected and make sure safety controls are working.

Hive or Heatmiser Neo heating controls will make a big difference.

Piriz

had another think about the combi today

basically I've two thermostat showers, both can run max 10ltrs per min (I tested this)

with a 26kw combi boiler this can run 11 ltrs per min of hot water, this would feed 5.5 ltrs hot water per shower when running simultaneously

the combi boiler has max water temperature setting of 65 degrees C, the average temperature of a hot shower is about 40 degrees C

5.5 ltrs of 65 degrees water would need to be supplemented by another 5.5 liters of cold water (assuming the cold water is about 15 degree which would suffice to produce +/- 40 degrees C and probably 8-10 ltrs per min water flow / pressure

is there anything significantly wrong with these calculations / assumptions ?

an 8ltrs per min flow rate is fine if two showers are on simultaneously

antoinolachtnai

I don’t mean to discourage but there is more to consider.

1.The combi wont necessarily achieve 65C just because you set it at 65. Whether it achieves it or not will basically depend on the temperature of the incoming water as well as the volume.

Look at the formula here https://heatingcalculators.com/2021/08/28/how-to-calculate-the-kw-required-to-heat-a-volume-of-water-in-a-particular-time/

note that this is per hour so you need to multiply your 8 l/min by 60 to work it out.

The temperature change required to get to 65 will be at least 50 and might be up to 65.

2.even if you have capacity, turning up temperature on the thermostatic valve in one shower will result in the other shower running cold.

3.At 26 kW you need to be sure the gas line at the boiler can deliver an adequate volume of gas.

4.Unless all your taps in the house have thermostatic valves there may be safety issues with having such hot water.

Piriz

thanks for this reply, it's a nightmare task, two companies have given quote for combi following on site inspection,

John.G

https://www.boards.ie/discussion/comment/121746004#Comment_121746004

A 26kw combi will give a flow rate of 10.96LPM at 40C from mains at 6C in winter and 14.91LPM at 40C from mains at 15C in summer, a 30kw will give 12.65/17.2 LPM, winter/summer

Piriz

i have to say the comment / opinion and knowledge shared on this thread has been really helpful.. thanks everyone !!

as a result ill not proceed with either of the two quotes ive received for combi boilers, i will get my cylinder inspected and ensure that its made safe (this will fix the leak from the safety valve) and I'll get Hive controls and I'll get my old boiler serviced..

antoinolachtnai

Sounds like a good plan. The other thing to remember is that you will need a new boiler in the next few years. 25 years is as long as I’d leave it. The new boiler will give you an efficiency improvement of course.