View Full Version : Adding booster pump to combi boiler?

03-09-2009, 06:55 AM
Hi, My boiler is in a house with a well so the water pressure is just less than 1 bar, and my combi boiler needs between 1 and 1.5 bar. The boiler has a small tap which you have to open when you want to increase the pressure, so if I add a booster pump I assume I would have to leave that tap open - but then the water for the radiators and the tap hot water would be mixed?
Basically my questions are:

1. Can I add the booster pump and leave the refill tap open (because closing the tap would prevent automatic pressure adjustment from the booster)

2. Is it also necessary to add (apart from the booster) a water pressure regulator (with bar clock) to maintain the pressure at a constant 1.5 bar?

Thank you.

03-09-2009, 08:00 AM
Lots of things doen't like to work with that low pressure! I'd think I'd rather raise the house pressure, then things would work better, rather than just trying to do this for the boiler.

A boiler makeup water needs a backflow preventer to protect the potable water from the stagnent stuff in the boiler circuit, so you can't just leave that path open.

This is a creative guess...if you isolated the expansion tank, lowered the pressure to say around your supply pressure, then filled the system with what you have. If you then closed off the supply and pressurized the expansion tank by pumping it up slightly to the needed system pressure, you'd probably be okay. If there are leaks, you'd have to do this periodically, but since the boiler circuit is closed, once you fill it, it should stay.

03-09-2009, 03:58 PM
But how do I pump up the expansion tank in a combi? I suppose another possible workaround might be to install the booster pump in such a way that it is connected by a flexible copper pipe from a tee connector on the water supply pipe directly to the boiler side of the fill up valve, which can then be left closed. The other side of the tee connector would be for the hot tap water suply. With this solution of course the pressure in the hot water taps would remain as before.

03-09-2009, 05:05 PM
Maybe some confusion here. The boiler heating circuits require that closed circuit to be pressurized to (usually) something around 14#, or about 1Bar. If you input is less than that, since it doesn't take much to increase the pressure in a closed system with liquids (which don't compress), you could do that with the expansion tank. The expansion tank has both air and water in it, so if you fill the air tank with say a bicycle pump after you install it, you'll pressurize that closed part of the heating system. To not overstress the bladder in the expansion tank, you'd want to start with a lower pressure than what water pressure has so some water will partially fill the tank. Then, you can add air to bring the whole system up to the minimum supply pressure. Keep in mind that it will raise some when it expands while the water is heated, but most of that will be absorbed by the expansion tank.

Now, if you are talking about raising the whole potable water system's pressure...why not just use a larger well pump? That would resolve the whole problem.

03-10-2009, 02:43 AM
But how would I connect a bicycle pump to the expansion tank? Via a bleed nipple?
The problem is that at the moment, when I increase the water pressure to 2 bar by connecting the well pump directly to the boiler, the pressure drops to about 1 bar in a few hours (the recommended pressure is between 1 and 1.5 bar).
But since I have had to top up the pressure so many times, while bleeding the rads, sometimes the pressure went up temporarily to 3 or 4 bars.
I have checked all the rads and fixed the few really tiny leaks I found. And the pipework seems fine. I will try to have a more detailed look at the boiler for possible leaks, by removing the side cover.

I don't quite understand how more water pressure in the house would solve the problem, if you have to manually open the fill valve every time you want to top up? If I had been running the boiler at around 2 bar, would that damage anything inside it? I was hoping it would be possible to find an automatic way to keep the pressure at a constant 1.5 bar??

03-10-2009, 07:39 AM
There are auto-fill valves designed for this, but that requires the supply pressure to be above the needed pressure in the boiler. How do you do much of anything with that low pressure? I lived in the Middle East, and we had a storage tank on the roof, and we probably had that...made it very unsatisfying to take a shower, and it took forever to fill the sink or washing machine to clean anything. You need to find the leaks...constantly adding water will rust out things. That water needs to be fairly inert, but new water adds disolved oxygen and other gasses that will just rust and corrode the insides of the system. If all it can do is circulate, those gasses eventually react or are purged.

The expansion tank has an air valve on it just like your car tire, but this would not work easily for a frequent thing. Essentially, the procedure would be to lower the air pressure so you get some water in the tank. Then, once the fill valve is closed, pump air into the bladder of the expansion tank to get it to the needed pressure. Much safer and easier to get rid of the leaks.

If the expansion tank is bad, it will fill up with water, and then each time the boiler runs, the water expands, and increases the pressure and some leaks out. A working expansion tank gives that expanding water a place to go.

The fill valve may leak, and if you don't have an anti-siphon valve, the higher pressure, stagnent water from the boiler is pushing back into your drinking water. This is NOT GOOD!

03-11-2009, 12:41 PM
So it's not really normal to top up the pressure automatically in sealed room combis? I know the automatic topping up could also disguise leaks because the pressure would remain at 1,5 bar even if there was a big leak somewhere in the system. Then maybe the manual topping up is preferable. So I will try to get a close up look at the boiler with a side-panel removed and see if there's a leak....somewhere.

03-11-2009, 04:12 PM
The water in a typical boiler installation is closed...fill it, and the only time you should need to add more is if there is a leak. Having to add water constantly means problems. Now, some steam systems might vent off if the pressure got high, and if your expansion tank is shot, you could lose some during a heating cycle...the water expands, and there's no place for it to go except out the T&P valve. Do you get any drips out of that valve? Is your expansion tank shot?

03-12-2009, 11:23 AM
T&P valve?? Do you mean the valve I have to open when I top up? This valve does have a small leak when it is open, but when I close it, it seems quite dry.

I will also try to take a close look at the Pressure release valve, to see if there's any sign of water escaping there.
The expansion tank? I'm not familiar with the procedure for pumping air into it, but I'm sure I can follow the detailed steps I have seen in forums... after reducing the pressure to nearly 0 bar? Is it normally possibly to "fix" the expansion tank with that procedure? Obviously I won't attempt any repairs unless I consider them to be completely safe.

03-12-2009, 05:44 PM
The T&P valve is the safety valve on the boiler...it opens if the pressure gets too high or if the temperature gets too high.

An expansion tank is required in a closed boiler system. Depending on the brand, they usually have a rubber bag (bladder) inside them. There's an air valve on one side - same as on a tire; the other side is connected to the boiler piping and will have water in it. The expansion tank is normally filled with about 1Bar for a boiler, but it would depend on the normal working pressure. Once you connect it to the boiler piping, the water pressure will equal the air pressure, so checking the pressure while installed won't tell you how much air there really is in there, only what the water pressure is. When the water is heated, it expands and pushes on the rubber bladder in the tank, compressing the air, and if sized properly, will limit the pressure increase that would normally occur with the water trying to expand in a rigid structure.

03-15-2009, 05:21 AM
It looks like it may be the pressure release valve that is dripping, even when the boiler is turned off, and with a pressure reading of 1.2 bars, so I'm going to change it. But one question:

Would it be ok to drain the boiler, take off the prv so I can show the shop the type I need, and fit the new valve a week later? So the boiler (turned off and empty of course) would be minus a valve for a week. Would leaving the boiler empty for a week encourage oxidation inside?

Or would it be better to get the new valve (quoting name and model of boiler) and remove the old one just before I fit the new one?

Is it essential to get the exact same valve for this specific boiler (a Domusa)?
I thought maybe I might be able to find a better valve, as long as it has the same charateristics??