DIY Doctor

Replacing an old gravity fed system

Postby 1606mel » Fri Jan 01, 2021 7:20 pm

I have an old (25yrs+) gravity fed system in our 1933 3 bed house, there is a header tank in loft, water tank in back bedroom & Potterton Flamingo II boiler in the kitchen, there are 6 rads. It all works perfectly fine but we are looking at a kitchen refurbishment & I have been told I am paying far more on bills with this old system. My query is a couple of yrs ago I got quotes to replace with a combi boiler, quotes ranged from £2500 to £6000, some said I needed to replace all the pipe work others not (I have 2 pipes that run up either side of the downstairs back window) which apparently is the wrong type of pipe work. It confused me so much I knocked it on the head. So questions are 1. Should I replace the whole system at all as it’s never once failed 2. Would I need all new pipe work 3. What would be a ball park figure?. I have 1 bathroom with a family of 4 & live in the West Midlands, many thanks
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Postby stoneyboy » Sat Jan 02, 2021 11:28 pm

Hi 1606mel
Suggest you leave well alone, yes you are paying for a less efficient boiler but you will not recover the cost of a new boiler and system.
Either Wait until the boiler fails or replace it to fit in with your new kitchen, go for a conventional condensing boiler and see how well the system works.
Regards S
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Postby 1606mel » Sun Jan 03, 2021 2:22 am

Thank you, I’ve always thought “don’t try & fix it if it’s not broken” & if it’s like other things we’ve updated in the house it never seems to be as good as the original.
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Postby ericmark » Fri Jan 08, 2021 11:29 pm

I have asked the same question, in the end boiler started to leak so no option, however so much has changed can't really say how much cheaper to run.

The old system only monitors output water temperature, so you can set radiators with only 5°C drop across the radiator, but the new system needs the return water under a set temperature so around 20°C drop across the radiator, also output temperature may be lower, so the radiators can't put as much heat into the room.

In the main we have all had new windows, cavity wall insulation etc, so don't need as much heat from the radiators.

My on/off boiler has an output of around 18 kW but a combi boiler is more around 28 kW in the main so you can fill bath in reasonable time, but for the boiler to gain the latent heat, it needs to turn down, and the amount they can turn down varies a lot. Some can drop as low as 6 kW others can only drop to 12 kW, once the boiler reaches the minimum output it starts to cycle on/off just like the old boiler.

While the boiler output is between limits the control is handed over the the thermostatic radiator valve (TRV) and they gradually open and close, and once so many have closed the by-pass valve opens. So you get the radiators gradually heating and cooling rather than on/off, this in turn means very little hysteresis (over shoot) however the job of the wall thermostat completely changes.

Using a non electronic TRV head it can't send a signal to the boiler to tell it to start, so you need a wall thermostat in a room normally kept cool, on ground floor, with no alternative heating, and no outside door, and the lock shield valve is set so this is last room to reach temperature, if it ever reaches temperature until warm weather arrives, so it will switch off boiler. The room rarely exists, so some compromise is called for.

The problem is the boiler will still warm up the home even if not set correctly, so the user is often unaware when not set up to use the latent heat, even some proprietary controls can fail to use boiler to its full potential.

Mothers house I found a wall thermostat designed to stop hysteresis in an old boiler or oil boiler, it did this by switching off/on as it approached the target temperature to stop it over shooting, but each time it turned off boiler, it switches back on at maximum output, so all the cleaver stuff in the boiler designed to modulate the output was to no avail as wrong thermostat fitted.

Some boilers allow the use of third party controls direct to the ebus, normally OpenTherm so the boiler can have some very clever controls, others do not allow third party controls, so often whole home controlled by one wall thermostat.

In my old open plan house one thermostat would work, but most houses have doors on the rooms.

So what I am saying old boiler may be 80% efficient and new boiler 95% efficient but unless installed and controlled well, once installed both boiler could end up with same gas usage.

My annual oil bill as around £350, and the oil boiler would cost around £4000 and will last around 20 years (oil lasts longer) so to pay for a change the new boiler will need to used £200 worth less fuel, it is quite clear it can never save that much.

So gas boiler unlikely to last more than 15 years, so work on 15 years, although seen some only last 7 years, so cost say £2000, so it needs to save £130 per year to break even. With interest on the money spend it will be more. So really your looking at a £200 saving to be worth while.

My son proudly tells me my old gas fired central heated house fuel bill has dropped more than £200, however new front and rear doors fitted which seal better, and house heated to 18°C where we heated it to 20°C and now not heated mid day and it was heated mid day when we lived there, so not a true comparison.
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Postby 1606mel » Sat Jan 09, 2021 5:11 am

Thank you, there’s more to central heating than I could of ever imagined!
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Postby ericmark » Mon Jan 11, 2021 3:34 pm

I have looked at some old systems, the old hypocaust for example good example in Chester, and wondered why we don't have them today?

To my mind the problem is speed, even heart of winter, when we do see the sun, it can heat up our homes quickly, specially with bay windows and the like, so any heating system must be able to adjust just as quickly.

Under floor heating sounds good (the hypocaust) but it takes too long to cool, same with storage radiators, we want radiators which store as little heat as possible, however although the fan assisted radiator seems perfect, that too has problems.

In the main we want cool and warm spots in the room, allowing the air near widows to cool means less waste, and the same in reverse when sun comes out. So circulating the warm air has a limited plus, it also has some minus points.

But the biggest advance must be modulation. The ability to gradually increase and decrease heating rather than off/on, analogue rules KO.

It really seems odd in this so called digital age, that with heating analogue is better, but it is.

If the radiator is at 40°C when sun comes out, it does not take long to cool, so we can maintain room at 20°C, but if radiator at 70°C then likely room temperature will over shoot.

So we have words like hysteresis, or the sine wave caused as heating switches on/off, and the gas central heating system has the ability to remove this by using analogue control.

However it seems we are slow to learn, with even the government introducing rules which are counter productive asking for zones, which in the main stop analogue control being implemented. Although I think that is more down to how people have interpreted the law, rather than what the law says.

In every house I have owned heat raises, so upper rooms are always warm, so upper rooms we need to prevent over heating, there is no need for linked TRV heads, or zone valves, the main point is to stop over heating, so the TRV head either simple or programmable will control upper floors.

The lower floors it is the rate of cooling that matters, we can control rate of heating using the simple lock shield valve, but it is rate of cooling that matters.

So boiler control, once weather warms up, we want boiler to stop working, but if we still have cool rooms we still want it to run, so often we really need two wall thermostats, in parallel so either can make boiler run, but one monitors coolest room, and other monitors fastest room to cool, which are not always the same room.
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