Combi boiler and pump

digitaldr

Doing a major house renovation (bought the house in Feb - never actually lived in it). We have decided to fit three unpumped Mira Digital showers (2 dual head and one single) on the basis that in the old house the kids always used the power shower in our ensuite and never bothered with the electric shower in the main bathroom leading to queues in the morning! Got a pretty good price on them on Amazon. One of the showers will be in the attic.

Both the builder and architect recommend a combi boiler and space hasn't been allocated for a hot water tank. Plumber won't be onsite for a few weeks yet. House is in Dublin so natural gas supply, not sure what the mains pressure is like but being Ireland probably not great.

Is it possible to fit a whole house pump with a combi boiler or would it better to pressurize the mains feed (I think some kind of sump tank would be needed for this to avoid robbing pressure from the neighbours?).

It's probably unlikely that all 3 showers would be running at once but certainly 2 might be so what Kw combi would I need?

has anyone come across powertanks? Would fitting one help?

Thanks

antoinolachtnai

Running two showers off one combi boiler is a very optimistic thing to want to do. It is not impossible and you might be ok, but you risk being disappointed on a cold morning when the incoming water is cold.

The answer to the question of which model of combi boiler is really for the architect to consult with the distributors of the various brands, providing information about the water flow required, and asking them which models would be most suitable for your needs. You would want to understand from the manufacturer too whether the gas supply will be adequate to reach the required performance (i.e., that the gas pipe between the meter and the boiler is sufficiently large).

Separately there is the issue of water supply. To comply with regulations, there has to be a storage tank, and to use a combi, you need a pump to provide pressure. I would say that for a house with three showers you would need a tank a good bit bigger than 125 litres (the one you have linked to). If the architect or the plumber are in doubt as to what is required they will no doubt consult the SIs, the guidelines, the bylaws and the Irish Water requirements.

What you are trying to do is sophisticated. Not terribly difficult necessarily, but it does need some thought. Really, you need for your architect, who is the expert you have engaged, to design this whole thing and then take responsibility for it. There has to be a clear division of responsibility between the plumber and the architect as to whether the plumber or the architect is taking responsibility for the design. The designer will need to work out whether you'd be better off with another approach, i.e., hot water cylinder and booster pump for showers.

The one thing to avoid is that the designer ends up being you!

digitaldr

Thanks for your sound advice. Agree that I shouldn't be the designer but I just wanted to have some idea when I'm talking to the plumber. First fix is a few weeks off yet but will be asking to meet the plumber soon. We got some work done on our last house by a clueless plumber and are anxious to get it right this time.

If it came to it there would actually be space under the attic stairs for a hot water cylinder. Will wait to hear what the plumber has to say but I'm thinking this may be the way to go.

antoinolachtnai

You are paying big money for a plumber and architect to solve all these problems.

If there is an issue about space, the architect really needs to start sorting out locations for tanks right away.

sami2015

Hi Digitaldr,
you will really love having a combi boiler. The idea of having a supply of hot water that is unlimited by a hot water cylinder is really pleasant. Some other posters have stated that you will not be the designer of the system and that is certainly true; however it is important to note that some plumbers installing such systems don't have them in their own houses and may work to a design/layout that suits them as opposed to you. I would have a few comments/advice which I would like to have known all of them before I installed mine. If nothing else it might help you review the design that you are presented with:

1) You will certainly need to have a tank or tanks. I put in the largest that I could find initially and then went back and added a second. It could be placed in the attic... but it would be better to put it outside (eg do you have an attic in your garage or do you have a garage?).

2) You will also need to have a pump. The pump will depend on the number of water outlets planned. If you have 3 showers (plus a kitchen/utility??) this suggests lots.... you will need to try and find a good constant pressure pump. Try to keep the pump outside to reduce noise inside. Even the water cooled ones are quite noisy. The air cooled ones cannot be boxed in/covered with noise reduction material and so can be even noisier. Try to keep the pump as close as possible to the water tank(s) and try to keep all the pipework tidy and as far as possible away from bedrooms (it can be noisy to have water flowing behind the wall of a bedroom).

3) With several outlets working off the combi please keep your toilets on a gravity fed line. This will mean some extra piping and extra effort during installation but will also reduce the likelihood of pressure variation in the feed when the toilets are flushed. (Remember the pump will pump from the tank and this high pressure line will then be split; half into the boiler to become heated and the other half to be the cold feed to the showers). If this cold feed also feeds the toilets it could be annoying if you have pressure variations on the cold feed (and could result in scalding depending on the pipe layout and whether or not you have a thermostatic mixer or not).

4) Get a decent sized combi (probably 33/35kw if you have several feeds).

5) Please ensure that you get your showers tanked really well (eg with fibreglass....the rubber paint that some builders use can have a problem adhering to the acrylic shower trays and can cause leaks, particularly on suspended wooden floors).

6) Please make sure that you bring the sink and shower/bath waste pipes out of the house before joining into the sewer pipe (otherwise you can have other kinds of problems)

7) You could also consider future-proofing the system so that it can incorporate a solar panel in the future to pre-heat incoming water (I have not done this due to space/costs constraints but it is something that I have seen done on other systems)

I think that is enough of a rant for now. Feel free to ask any other questions since I really think that combis are great but you need a good design and a good plumber to install them!


Good luck,
sami