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Author Topic: Combi/condenser Boiler upgrade noise problem.  (Read 38631 times)
desperate
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« Reply #15 on: February 08, 2013, 08:39:29 AM »

Hallo Mart,
I would hang on to your CDi, they are pretty well made, apart from some dodgy plastic bits, spares are readily available at reasonable prices, and although they bare not as efficient as a modern condenser the difference is not great enough to justify on its own merit.
Your problem is is the worst aspect of combi boilers, they have to be rated at 24kW and upwards to get a halfway decent flowrate for domestic hot water, but this as you say is way too big for the heating demand. With a modulating burner the efficiency does not suffer too much, but normal pump does not modulate so in effect it is way over size, gotta go now , the BT man has turned up to do our infinity broadband.

Desp
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desperate
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« Reply #16 on: February 08, 2013, 04:11:23 PM »

Hallo again Mart, you'll have to read this really really fast now I am on infiniteeeeeeeethinkpatrickmooreand dontstoptoblinkorbreathasthereisnotimeforanyofthatanymoreormaybeihaveinterpretedthisallwrong wackoold wackoold wackoold wackoold

anywayasIwassayin fight fight thanks Dear, the Mrs just bashed me on the head and service is back too normal whistlie..............mmmm well normal ish

the pump in your boiler will be sized to keep the heat exchangers happy, but that flowrate will cause all sorts of turbulence in particularly the rad valves especially if they are closed down to half or three quarters or a turn open. It is worth opening the lockshield valves fully on the rads that have TRVs on them, and any rads that don't have TRVs you could open both valves until the noise just stops. This may mess up any balancing that has been carried out on your system, but usually it is possible to balance a system while still having the valves open enough to reduce the noise to an acceptable level.

Time to fiddle with the plummin Grin

sprat Deee
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Philip R
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« Reply #17 on: February 08, 2013, 09:00:30 PM »

If I can add my 2 pennies worth.As has been well said already, combi boilers are way to big for central heating duty for the average UK dwelling, they need to by powerful to generate more than a piddle flow of instantaneous hot water. Modern condensing models now are capable of modulating down to 20%  and some lower so better match the heat load of the house. Those boilers with downfiring burners are a little more limited in their down power modulation capabilities, ie WBoschs and Ideals. For real modulation, you need the Drum geometry heat exchanger types with the latest SIT two stage zero governor gas valve. However, this limits your boiler choice to those of Dutch connection, Baxis, Atags and Remehas. I have not installed any of these (yet).

Getting back to big boiler, fitted with big pump running on high speed setting, hence radiator valve noise. Just retrofitted the mother in laws bungalow with Glowworm Ultracom CXi 24kW (DHW) CH max 18 KW but can modulate down to 5 KW even that to much as it heats the rads up very quickly. Again had issues with whistlie from t/static valves and lockshields. Out went the balancing to get 10 deg diff across radiator as per Benchmark and good heating guide. Did what Desp suggested and opened lockshields up a bit more to lower rad diff to 4 /5 deg C. and balanced with diff thermometers. This boiler has two speed pump, speed dependant on firing rate, but could do to go to even lower pump speed, at low firing rates, again heat exchanger cooling is 1st priority here.

Issue still remains that combi boiler manufactures need to get to a minimum firing rate of 2/3 KW with commensurate pump flow rate before I can be satisfied that a combi is still too much of a compromise. I still like heat only combis with seperate pump ( now good choice of variable speed jobs on market).

Many modern Thermostaticc valves on market are now two way capable, i.e. not water flow direction dependant, or do you know different.
Philip R
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desperate
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« Reply #18 on: February 08, 2013, 09:27:48 PM »

Philip,

I think the manufactureres are going to struggle to get the outputs down to that level from a burner that is probably optimised for 20kW, I remember reading in the gasrag (I think) that burners running at much less than 25% of the maximum capacity would have a gas velocity approaching the flame velocity, and therefore getting close to the point of flame blowback into the mixing tube/venturi. It is easy enough for the makers to control gas rates, fan speeds, pump speeds and etc, but with a fixed aperture burner you are stuck between flame lift off and flame blowback by the burning rate of the mixture.

Maybe we should invent the variable aperture burner and make our fortune.

You heard it here first  folks Grin

Desp
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Philip R
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« Reply #19 on: February 08, 2013, 10:06:52 PM »

Good point Desp.

You would be looking at a two stage burner or variable aperture device as you have suggested ( I have worked on heavy oil burners using a variable flow valve that could modulate right down to 10%). Then you are looking at a small cascade like on small commercial stuff, gets expensive. Point taken.

Those new Netatecs/ Duotecs look good with respect to modulation, about 1/7 approx 14%, most of the others are at 20%  in that repect, but are a bit lacking in the hot water flow compared to some of the other similarly sized gas boilers from their competion.

PhilipR
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« Reply #20 on: February 08, 2013, 10:10:36 PM »

Thanks kindly guys for the advice. So, unless anyone thinks this will lead to big problems, I'll suck it and see for now with the pump speed changed from III to II when the heating is on, coinciding with colder incoming water. But when the heating is switched off, roughly early April through to late September, I'll pop the switch back to III to protect the heat exchanger (Huh).

The whistling has completely stopped for now, so, so, so pleased. The boiler also sounds much quieter. It was never that loud, but it's own noise seems to have reduced more than proportionately from 95W to 65W (if you see what I mean). I might be looking for problems, but I suspect warm up times are a little slower, but still perfectly acceptable.

Just out of curiosity, would the fact that I was recommended a 24CDi but went for a 28CDi (for expansion purposes as described) be relevant. What I mean is, do I have a pump designed for a bigger job, than it is being asked to fulfill?

One last question, am I right in thinking that I should put the pressure back up to 1.5bar, rather than the 0.7bar that helped reduce the whistlie a bit?

Mart.
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Just call me Mart.     Cardiff: 5.58kWp PV - (3.58kWp SE3500 + 2kWp SE2200 WNW)
Philip R
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« Reply #21 on: February 08, 2013, 10:47:31 PM »

Mart,

The heating water charge pressure, "static pressure", should not affect whistling unless you have entrained microbubbles circulating. this will detrain in time. Excessive charge pressure reduces the expansion vessel capability and may result in loss of water through the pressure relief valve on boiler warm up and then followed by oxygenated makep, corrosion and microbubbles!!! should you top up the system. ( I have seen a lot of new customers lately having this problem with their boilers and their former heating techs not understanding principles of setting up the EV system, good business for me).

24 KW is big for heating duty, and 28 kW in most cases excessive for heating, just warms up a bit quicker and better hot water flow. As Desp has stated earlier with modulating boiler not so much an issue. Going up in boiler size may have resulted in the fitment of a 7meter head pump instead of a smaller 5 or probably 6 meter pump, is is variable speed, controlled by the boiler? This bigger pump, infers a higher flow rate and hence more liklihood of flow induced noise.
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