Moved house - system boiler & water tank programmer settings

[wizbongre]

Morning. I’ve moved house and the new place has a different heating and hot water setup than I am used to (came from a combi controlled by Hive.)
We now have a Worcester Bosch Greenstar 40CDi system boiler and a Megaflow indirect water tank with 170l capacity. All appears to work as expected.
I need to set the boiler to come on regularly to heat the water in the tank but don’t know what schedule would work.
There are four of us in the house typically having a total of 2 showers a day and 3 full baths a week between us. Plus the usual washing up, washing hands, etc.
How often and for how long should I expect to run the system boiler to heat the water? I have a controller that allows for 7 day programming and 3 time programmes per day, as well as manual/boost modes. It’s currently set to run for 2 hours 3 times a day, which seems excessive.

Suggestions?

 

[ericmark]

I have an oil boiler and not DHW thermostat on my hot water tank, it is 20 kW and for us two hand washing only I would set it at 30 minutes 4 times a week, and it would actually only run for 20 minutes before the boiler thermostat cut out. But C Plan today is rare, most systems today are Y or S plan, and the hot water tank has a thermostat something like this strapped to the water tank, or even built into the water tank to auto turn the central heating on/off as required.

I now use an immersion heater with iboost+ which detects when solar panels are exporting, so costing me nothing to heat hot water, once I start getting payment for export it will cost the loss of money from the export, but it means only the tank is getting hot, we are not heating un-lagged pipes between the boiler and the tank.

Gas and oil is cheaper than peak rate electric, but by time you allow for losses from un-lagged pipes, plus they are heating your house which likely you want to keep cool, one has to question if worth running boiler? I have off peak electric at 8.95p per KWh so likely a timer on the immersion is the order of the day, set the immersion higher than central heating in summer, and lower in winter, but in the main no point having the DHW on a timed control with gas or oil as does not matter what time of the day it runs, only with electric do you need a timer. That is assuming there is a tank thermostat.

If like me you have not got a tank thermostat, since you have more use of water to me, I would say start at an hour per day, most of my 20 minutes was to heat the pipes, and most of the heat into the tank was to replace losses from the tank.

 

[fixitflav]

Morning. I’ve moved house and the new place has a different heating and hot water setup than I am used to (came from a combi controlled by Hive.)
We now have a Worcester Bosch Greenstar 40CDi system boiler and a Megaflow indirect water tank with 170l capacity. All appears to work as expected.
I need to set the boiler to come on regularly to heat the water in the tank but don’t know what schedule would work.
There are four of us in the house typically having a total of 2 showers a day and 3 full baths a week between us. Plus the usual washing up, washing hands, etc.
How often and for how long should I expect to run the system boiler to heat the water? I have a controller that allows for 7 day programming and 3 time programmes per day, as well as manual/boost modes. It’s currently set to run for 2 hours 3 times a day, which seems excessive.

Suggestions?

I doubt whether you'd notice any difference in cost if you leave HW on 24/7, over using timed periods.

There might be more of a case for CH on timer, but I run both HW and CH 24/7.

 

[fixitflav]

If like me you have not got a tank thermostat,

From the description I'd be surprised if he hasn't.

 

[Rodders53]

That boiler will likely reheat that tank from cold in about 31 minutes and recovery from 70% HW capacity drawn off in 22 minutes... It'll lose 1.7 kWh per day whilst warming the airing cupboard. Give or take a bit.

https://www.heatraesadia.com/produc...water/unvented-cylinders/megaflo-eco-slimline find the exact model you have for the install/maintenance manual; and that'll have all the detailed ratings and timings.
It'll need the expansion air bubble replacing routinely.
The 'floating baffle' had a tendency to disintegrate at one time, blocking filters on mixer taps. Hopefully yours is not one of the affected units?
It is recommended to have an annual 'service' by a G3 unvented qualified person (to check over the safety valves etc.,.).

Timers: Just 'suck it and see' (in other words do it by trial and error).
I'd suggest if you all shower between 6 and 7 am to have it on for 90 minutes between 5:45 and 7:15... If baths are 6-7 pm then another similar PM period?

With just the two of us here, we have it on once a day for an hour around the time we usually shower. IF we ever need extra hot water (e.g. for guests) I just kick my Wiser smart controller on for a 30 minute, or slightly longer, boost. The tank stat will switch off the boiler when the water temperature in the tank is sufficient.

 

[Harry Bloomfield]

How often and for how long should I expect to run the system boiler to heat the water?

Depending on if your system is designed to serve heating both radiators, and water simultaneously- you might find it beneficial to heat water, when CH is not needed. You don't want the house going cold, whilst the boiler reheats the water, if it serves one at a time...

30 minutes to an hour, will see the tank of hot water reheated, depending on the starting temperature. Once up to temperature, if none is used, it should keep hot, easily, for at least 24 hours.

 

[Harry Bloomfield]

With just the two of us here, we have it on once a day for an hour around the time we usually shower. IF we ever need extra hot water (e.g. for guests) I just kick my Wiser smart controller on for a 30 minute, or slightly longer, boost. The tank stat will switch off the boiler when the water temperature in the tank is sufficient.

Mine comes on for HW, around 15:30, then off around 20:00. That makes up for and covers the washing up, then baths, if wanted, with tie remaining to top the cylinder up after. We effectively, almost always have a full cylinder of HW - failing which, the control includes a HW boost function.

 

[wizbongre]

Thanks so much for all of the replies. Not had a chance to come back to reply to them before now as still unpacking boxes everywhere!

So, some more info. and responses to comments so far:

The system boiler and cylinder are both down in a very large, slightly damp cellar. There is almost zero lagging/insulation on any of the pipework, which also seems to do at least 2 laps of the whole house (and possibly next door) before it makes it to the bathrooms. Its a 4 storey house and the pipework literally goes from cellar, up to attic, and back down to hot water taps. It takes AGES to draw hot water first thing in the morning. I'm no plumber but even I can see it's been very badly designed.

A few observations/thoughts/questions now we've been in for 2 weeks:

As a family of 4, a 170l tank seems too small. It can just about do 2 baths or 3 showers before it needs a re-heat. First up best dressed and all that!

From the description I'd be surprised if he hasn't.

There is absolutely no thermostat on the cylinder that I can see (no 'stat for the heating anywhere either but that's another challenge for another day.)

Given there is no 'stat on the tank, I'm assuming leaving the DHW on 24/7 would result in massive bills as the boiler is running constantly, even if its modulating right down? It's set to hit ~63degrees temp.

That boiler will likely reheat that tank from cold in about 31 minutes and recovery from 70% HW capacity drawn off in 22 minutes... It'll lose 1.7 kWh per day whilst warming the airing cupboard.

This has really helped as a starter for 10. I've gone for 30mins early morning before anyone gets up, then another 20 mid afternoon, with a final 30min in the evening. Seems to be working ok so far. Honeywell 2-channel controller has a boost which has got us out of a spot or two.

Depending on if your system is designed to serve heating both radiators, and water simultaneously- you might find it beneficial to heat water, when CH is not needed. You don't want the house going cold, whilst the boiler reheats the water, if it serves one at a time...

I have no idea how its set up yet. Hoping to get a local trusted heating engineer around in the coming weeks to give it the once over and go from there. For now, and as its the warmer months (aledgedly!) I've only got the heating doing a quick blast early morning to dry bathroom towels after the DHW has had its first run.

Mine comes on for HW, around 15:30, then off around 20:00. That makes up for and covers the washing up, then baths, if wanted, with tie remaining to top the cylinder up after. We effectively, almost always have a full cylinder of HW - failing which, the control includes a HW boost function.

The tight-ar5e in me feels like that's just burning gas for the sake of it - maybe I need to adjust my thinking to how efficient a system boiler and tank are, after about 30 years of using combi's?! Maybe I should review after a month or three of usage recorded on my smart meter?!

 

[Harry Bloomfield]

There is absolutely no thermostat on the cylinder that I can see (no 'stat for the heating anywhere either but that's another challenge for another day.)

Given there is no 'stat on the tank, I'm assuming leaving the DHW on 24/7 would result in massive bills as the boiler is running constantly, even if its modulating right down? It's set to hit ~63degrees temp.

Those issues need addressing urgently.

 

[fixitflav]

There is absolutely no thermostat on the cylinder that I can see

That's odd if it's a genuine Megaflo Eco

Given there is no 'stat on the tank, I'm assuming leaving the DHW on 24/7 would result in massive bills as the boiler is running constantly, even if its modulating right down? It's set to hit ~63degrees temp.

Probably right, and you don't want the HW temperature to be determined by the boiler control-stat. As Harry said, by the sound of it the whole system needs updating.

 

[Rodders53]

Find the exact Megaflo model and download the manual for it.

The thermostats (one for boiler, one for the immersion) will be "hidden" under the grey plastic cover near the bottom --- there should / will be cables exiting from there to the system wiring station terminals and an immersion switch. Some have another heater element / cover higher up.

Some CH systems rely on TRVs on all rads and then just run on timers... Somewhat deprecated nowadays, I guess?

EDIT: You can check how the boiler runs if you leave the HW ON for a couple of hours... it'll either keep pumping for the whole two hours or stop once the tank gets to the tank stat's temperature setting (assuming it's lower than the boiler's 63C).

EDIT2: 170 litre doing three showers or two baths seems good to me. Your boiler's HW just need to be heating while taking baths or showers and a bit after to keep a full tank!

 

[ericmark]

I live in a three story house, and like you found the boiler system a complete mess, to get a thermostat on the tank to control the boiler using a hard wired methods was simply not going to happen, it would be a major job to remove ceiling to route wires, and even worse trying to raise floor.

I remembered my dad having same problem and using a motorised valve to control his DHW in turn controlled by a wireless thermostat, but they no longer seem to be marketed. What there was seemed very expensive, so I was left looking for a cheap alternative method, also no thermostat for central heating, and I found the triple and earth to the boiler from the programmer changed colour enroute and had one core open circuit, so seemed prudent to convert to extra low voltage, and I went for Nest Gen 3, in hind sight not a good selection, but it does with two ELV wires allow me to control central heating and domestic hot water, and power the wall thermostat.

I found 4 slots a week at 30 minutes each was enough to give us hot water at taps, winter no problem no way to turn DHW off, so just summer where I needed to find a way to get DHW without boiler cycling on/off all day and keeping all the pipes hot as well as tank.

The immersion I did not use, as knew it was expensive, one time I tried it, the cut out popped. That was until I got solar, and I (it seems in error) thought it would be a good idea to heat the DHW with solar, so had an iboost+ fitted, which uses a battery powered CT coil and wireless link, but the cut out popped again, I turned the temp down on the immersion heater thermostat and it has worked fine since.

However I found the immersion is rather short, so any day without enough sun to be exporting, and the water gets cold, but that is rare. However one full cylinder of water is required to fill a bath, so must use central heating to get a bath, the immersion does not heat enough water, but not had a bath in years, use electric showers. Yes expensive, uses around 6 kW from the grid on a sunny day, but they are short, and not worth the expense getting hot water to the showers.

 

[Johntheo5]

As a family of 4, a 170l tank seems too small. It can just about do 2 baths or 3 showers before it needs a re-heat. First up best dressed and all that!

I just cannot see how any HW cylinder with a internal expansion bubble can deliver what it states is its nominal capacity, as HW.
From the megaflow brochure, it says the "170SB" has empty weight of 48kgs and a full weigt of 230 which = 182L, it also states that its nominal capacity is 170L which, because of where the T&P RV is located and presumably the bottom of the dip tube (dimensionally, 26.3% from the top) is also correct by my calcs once the cylinder is pressurized to 3.0 bar (cold). At the end of the heating cycle there will be 173L of HW in the cylinder but only 134L of this, due to the position of the dip tube is available, the 39L of HW above the bottom of the Dip Tube is IMO unavailable. If so, then you effectively have "only" a 134L HW cylinder.
For anyone interested, a snapshot of my calcs.

 

[Rodders53]

I just cannot see how any HW cylinder with a internal expansion bubble can deliver what it states is its nominal capacity, as HW.

My OSO air bubble unvented cylinder range states that one can remove the draw-off dipstick and gain circa 20 % capacity of HW... 170litre -> 200 litre from the installation manual. It requires an external EV to do that of course!

Recheck your calcs? as without a cutaway dimensioned drawing of the cylinder I think you have likely made an error? Remembering that an air bubble weighs next to nothing, no matter how large.

 

[Johntheo5]

The air bubble, above, only occupies 9.2L, less than 5% of the total volume, the reason for the huge expansion vol required before refilling (26%) is that, for obvious reasons, one cannot pre charge a Megaflo to 3.0bar, in the example below, of a conventional HW cylinder with a EV but no precharge then, that EV would have to be practically the same % vol (26%) of the cylinder to keep the final pressure to 4.2bar. With a precharge of 3.0bar, 10% is all thats required. I've been doing these calcs for years on EV sizing so would be pretty confident that the air bubble calcs are also correct.