Replacement boiler considerations

[AnthonyL]

As per subject I would appreciate any comments on my notes for a replacement boiler.

Property is an extended bungalow, late 60's build, fairly well insulated, 11 radiators, shower room and bathroom with bath. Just the two of us, retired.

Existing Baxi 105HE Combi boiler is ~2007/8 install and is "limping" along. The boiler is in an end extension/utility room in a cupboard against an outside wall.

1. Boiler Size ==============

Obviously will take guidance from the installer but would like to check my thinking is right.

Existing dials go from 0-9 (as per a clock). CH never set above 4 even in cold weather. DHW is at 1 which is ok for the shower (6L/min) but I need to wind up to 4 if running a bath (12L/min). I need to manually change as the modulation is not working very well.

This is a 30kW boiler and from the above I reckon a smaller boiler will be quite adequate. Baths are rarely run, but nice when they are.

This 25kW unit for instance seems sufficient:

I understand that a larger boiler doesn't cost much more but I don't want to pay for something I won't need.

1. Features ============

As I understand it good modulation allows the boiler to run at optimal efficiency and I believe this boiler has 1:10. It also has Weather compensation which if I have understood correctly means it automatically puts it on a higher setting on colder days, equivalent to me turning the dial up to 4 or down to <3.

We have a hot water delivery problem to the kitchen which is at the front of the house (boiler right at the back). My wife likes to wash with running water (makes some sense) but with a combination of poor modulation (burners turn off when water too hot) and the warm up time, she ends up using the cold water and screaming frost-bite at me. Again AIUI this boiler has a Quick Tap feature: turn tap on/off to let the boiler know you are about to use it, 30s later it's ready to deliver hot water. Just realised at the moment we have separate hot and cold water taps - might the water come through too hot, though it'll be set for reasonable temperature at the shower near to the boiler.

The Wiki notes suggest upward firing burners are not preferred. The existing ones are. Is that because downward firing burners will burn more gasses? Where is the heat exchanger, still above? And how do I find out what the Worcester Bosch have?

1. Installation ===============

I had some Fernox F3 Cleaner Express added to my boiler a month or so back and the engineer said that this would save the need for draining/flushing the boiler when a new one is installed. I need convincing though I can see the logic of not introducing a whole load of fresh water to a system that has had 1L Calmag Chem Protector Inhibitor added a few years ago plus the Fernox.

One engineer suggested that flue should have an upward gradient and he plume of condensation when the boiler fires up is due to this not being the case.

Should I insist on a full rebalancing of radiators? My study doesn't get up to a nice temperature but it only has an 80cm radiator, all the others are larger or double. Might be a good time to change it?

Is a Greenstar System Filter (magnetic filter) recommended, bearing in mind steel radiators, 20yr old install and water on the hard side?

Proposed installer if Worcester Bosch accredited and Gas Safe etc. But a one-man band.

1. Radiators ============

These are the round top metal ones with the bleed screw at the back. A couple of bleed screws can't be turned any more. All the radiators are of the same basic design so I'm minded to close one of the doubles off, take the screw out as that is easy to get at, go to the local merchant and get a handful of the right size and then remove the dodgy radiators and replace the screws. Is that a good plan and any suggestions on ensuring they remain free in future. Just a thought, how do I ascertain whether the radiators are metric or imperial?

1. Add-ons ==========

I've only added a Drayton Wiser room thermostat system in the Autumn of 2022. The hub is hard wired to the boiler. On the whole it does it's job and I'm not sure what benefits a different system could offer. It offers any number of schedules for each day of the week, a Comfort mode setting (anticipate the need to get the temperature up) and an Eco Mode (anticipate when it can cut out the boiler based on warmer temperature imminent). Any views?

Anything I've missed?

[Animal]

if 1 is 3kW, 9 30kW, probably 18kW is enough. Some people choose a higher flow shower later, requiring more power.

yup, alters the primary temp set point, improves efficiency a bit

WB & Vaillant are the most reliable, the rest a good bit behind. That's what matters.

so would I :)

Flues are normally slightly sloped. No big deal if not

Better for you to do that job. Estimate, then come back later & tweak until all rooms run at the temps you want. A plumber can only do the estimating bit of that process.

yup. Primary temps are on the way down over time to improve efficiency, meaning an underperforming rad will output less if that's done in future.

A filter is essential with a new boiler. I don't know of any brands that don't work. FWIW I've found the screen catches most debris, the magnet less, so am a preferrer of screen type filters.

sure

plumber's grease on the thread

measure em

dl the manual

Someone else mentioned a combi leaves you high & dry when it breaks down. A tank therefore gives you time to get good advice & a fair price. So it depends what your space is worth - I would not automatically go combi.

Tim mentioned heat pumps. They cost more per output to run, and can be noisy. I wouldn't. Don't overlook insulation. Doing that at the same time can reduce the max boiler power you need.

[Harry Bloomfield Esq]

The combi boiler will probably need to be sized, to meet your hot water requirements, rather than that of your central heating. The boiler has to have enough Kw to heat your water, on the fly, unlike a stored system. In other words, the lower the Kw, the longer it might take to fill a bath, whilst you wait.

[Fredxx]

That sounds a lot of radiators.

Best way is to do the calculations yourself, assuming wall conductance and insulation. It's a good time to review insulation too.

I might also look at modulating ratio and the lower figure will give the point at which the boiler with start cycling through a lack of load.

As above.

Most combis allow independent control of DHW and CH water temperatures.

Perhaps an under-sink heater is called for?

That doesn't sound quick. My combi stores hot water in the heat exchanger. I have immediate hot water before it's fired up.

Perhaps consider thermostatic taps, or a mixer where you want lower hot tap temperatures?

Not heard that one.

Many warranties rely on a flush. I doubt they would agree to Calmag Chem Protector Inhibitor in lieu of one. I think you've been had.

Yes. Though some flues have the exhaust angled, rising within the inlet pipe, such the inlet is installed horizontally.

Invest in a non-contact infrared thermometer and balance them yourself. You'll do a better job than a plumber. I suppose he could set them going by experience and you tweak the result.

I would look at the warranty terms. Most manufacturers stipulate a flush and a magnetic filter. Ones incorporating screens aren't the norm. BICBW

As TNP has said I would avoid Combis if you can. However, on the advice of some here I went for an Intergas combi, on the basis of it's simplicity. They are unique in having a heat exchanger where both DHW and CH water flow in parallel. So no motorised valve and no secondary heat exchanger. Plus DHW still works in the CH side isn't working or empty of water.

If they're the ones I think you've got then that can seize and all I can say is good luck.

I thought they use imperial bleed screws and imperial fittings?

This is also a time to consider replacing radiators in cold rooms with oversized rads that can cope with a lower CH water temperature to improve boiler efficiency.

Isn't that bespoke for your current boiler? Intergas boilers are compatible with OpenTherm controls. Worcester Bosch may have their own controls if you want more than a bang on-off thermostat.

[John Rumm]

For CH use even 24 might be overkill - however with a combi it is usually the DHW that drives the power requirement.

Go too large and it might cost more because it can't modulate low enough and hence cycles more.

Yup wide modulation range especially important on a combi sized for DHW performance. Weather compensation also very much worth having from both a cost saving and comfort PoV

Perhaps a small under sink unvented cylinder heated from the CH side of the boiler?

The temp should not exceed the set point DHW temp you set on the boiler. The pre-heat / tempering capabilities of some combis will deliver hot water faster, but it still will not make DHW "instant" if there is a long pipe run.

I doubt there are many upward firing ones about these days. (condensate running into the burner all the time is not ideal)

Many have drum style HX with the burner assembly as the "hub".

You may find your warranty depends on it being properly flushed. Generally it is a good thing with high efficiency HX since their small water paths are easily blocked.

It should - although some may have copied the Vaillant way by now - you drill the flue horizontal, but the outlet terminal positions the inner tube near the top edge of the flue pipe - so the exhaust carrying bit has an upward slope even if the main outer tube does not.

I don't follow that logic... liquid condensation dripping out of the flue would be a more likely manifestation of insufficient flue angle. (the idea being that any water that condenses out of the gases in the flue will run back into the boiler for collection.

Probably a good time to change it. If you are having weather compensation, then oversized rads are good since it will allow a shallower compensation curve that runs the boiler flow at lower temps for more of the year.

Generally a good idea on an older system... again many makers now require them for extended warranty cover.

Are you sure it is the screw that is the problem and not the bleed valve itself?

Chances are they are imperial - just measure the width of one and see if it is a nice round number in mm

Weather compensation may require a more sophisticated stat. It works best when the boiler knows what the internal temperature actually is, rather than just whether it is above or below a set point (even if that point is variable over times slots)

Probably :-)

[alan_m]

+1

The warranty will probably require a power flush and the fitting of a boiler manufacturer _approved_ magnetic filter.

In my experience a chemical flush followed by a drain down doesn't remove the sludge in the bottom of the radiators that well. When you refill the system that sludge may/will be disturbed and some will start circulating in the system. A few years back I fitted a magnetic filter to my system that had been installed perhaps 20/25 years previously. Prior to this I had had dosed the system with a chemical cleaner and let the system run for a week. The system was fully drained down, filter fitted and then refilled. On checking the filter after a few days a massive amount of magnet debris had been caught. Similarly after week the amount being caught was quite high although subsequent checks on a 6 month basis revealed moderately small amounts. The system had always been run with an inhibitor and over the years had a number of drain downs, some with a chemical cleaner beforehand.

My boiler has since been replaced but with a system power flush beforehand.

If one radiator is being replaced I suggest the OP take it outside and turn it upside down and run a water through it using a garden hose. The amount of black sludge coming out of it may be a surprise :)

Possibly what the OP is referring to is the boiler automatically setting the flow temperature based on the outside temperature. I agree that a more sophisticated stat or controller may be needed to override the boiler modulation as the room temperature is reaching its set value.

It's worth investigating how things like weather compensation work with the proposed boiler. Sometimes an external temperature sensor requires a more sophisticated (more expensive) interface than is supplied with the bare boiler. Once you have to go down that route then you may find that it may be better to invest in room stats etc. that may wirelessly connect with replacement interface/controller.

[The Natural Philosopher]

My electricity is 30p/unit. My oil cost essentially 7p/unit.

Go figure. .

[Jeff Layman]

Not really. For our unextended 3-bed bungalow there are 7 rads and 2 rads/towel rails.

It would also depend on what size the rads are, and where it would be most convenient to put them. Do you go for one 1500mm rad or two 750mm rads? If the OP's bungalow has been extended from 3 to 4 bedrooms, that could account for a couple of extra rads.

Anyway, as it's a bungalow, there'll be room for a decent cylinder in the loft to get rid of the combi. That also gives a small advantage of having an immersion heater for hot water if there's any problem with the gas or boiler.

[Animal]

??? A gas boiler in a house is just the job.

?

he isn't

then your control system is faulty.

[Animal]

some houses you can calculate, some are full of all sorts of hidden surprises that will give you wrong numbers, especially old houses.

important if you use a low flow shower, showers as low as 3kW are perfectly practical.

another option is a circulation pump & pir. When you approach the sink, the hot water runs to the tap briefly (& back). Result is instant hot most of the time.

Setting the rad temps the same is a good start, but will not get you a fully balanced system, as heat requirement is not consistently proportional to rad size, various factors make it only rough. Final tweak is to balance room temps - so you may as well just set the rad temps initially by feel.

Yup, you can get filters that have both.

[Fredxx]

It's a starting point.

I dread to think of the flow rate at 3kW. It is the norm to have a significant flow with a combi, otherwise you might as well go electric.

And a significant loss of heat every time it runs.

My idea of a balanced system is where the return temperatures match. Any control of temperature is by way of a thermostatic valve.

[Animal]

Gas heat is about a quarter the cost of electric. And it becomes space heat, so 2/3 of the year no real loss.

The goal is for room temps to match. The less trvs need to do the less temp inaccuracy they add.

[alan_m]

and it doesn't have to be the heat loss to 3 decimal places!

Since 2022 a heat loss calculation must be undertaken on the property when a new boiler is fitted to ensure the boiler is not oversized. I'll bet that in many cases this will not happen and the installer of a replacement boiler will just look at the rating of the old boiler or do a 2 minute finger in the air calculation.

[alan_m]

Not necessarily, do you need bedrooms etc. to be maintained at the same temperature as the main living rooms.

[The Natural Philosopher]

In my case absolutely not, is the answer,

I have between 4 and six rooms I only use fore guests. And two more I only use at weekends.

My homebuilt controller plus room stats in the rooms is doing a sterling job of keeping only the four rooms I actually use warm during the week.

In an ideal world every room would have a stat and a motorised valve, but in the absence of that a mixture of TRVs and controlled wet fan heaters do the trick.

[AnthonyL]

All of our radiators have TRVs.

I can't stand warm bedrooms so the main bedroom is turned down anyway.

The two spare bedrooms are on minimum and will get turned up the day before any guests arrive.

The hall, rather large and arguably a bit of a waste of space except it well separates the living and bedroom areas, has a large radiator which is also turned down to a minimum.

The main bathroom is the "always on" radiator. The room thermostat (temperature driven) keeps the lounge/dining area pleasant, the kitchen is generally low, and the rear utility (next to the boiler in the end extension) is generally kept warm as there is a shower in the area and also handy for hanging the washing.

That just leaves me in my rather cool study. I've misplaced the thermometer but I'm fairly sure that whilst the lounge is a pleasant

19.5degC I'm sitting in 18degC which for an old man sitting on the computer is a tad cool.

[Animal]

But the result can be out by a factor of 2 when the construction is misunderstood, which is common with 1800s houses. The OP already has a boiler, so knows what's required already.

[Animal]

I didn't think I needed to complicate things by explaining the obvious

[alan_m]

You don't just double the size of the boiler "just in case". That has been the problem for many decades.

It is well recognised that the skill level in the industry has been quite low over the past 30 years and many properties have over size boilers. With modern condensing boilers being oversize is likely to result in inefficient fuel economy.

Furthermore for a system installed 20+ years ago the heat loss of the house may have reduced with better windows and/or insulation.

[The Natural Philosopher]

You see what happens when a combi boiler is fitted to a family house: My sisters house in Germany had a simply MASSIVE one to cope with three simultaneous showers. Must have been about 60kW. The house itself was insulated to the hilt. Most of the time was spent switched off.