Advice for replacing decommissioned boiler

[LeserattePD]

Dear all,

 

Our boiler had to decommissioned yesterday for a CO leak - it was a Baxi WM70 that we hoped to nurse along until we can do an extension/refurb and replace it. Unfortunately, after more than 2 years, we’re still having planning issues with the council. 

So we need to replace the boiler but ideally with one that will accommodate what we want to do with the extension/refurb.

The house is a 1990 end terrace - the current hot water system is gravity-fed - the water pressure we have means that the bathtub can be filled in a reasonable time, but showering upstairs is only possible with the power shower (on it’s own pump) we currently have in the single (upstairs) bathroom.

For the extension we plan to have a pressurised system to address these issues, especially as we’re adding a second bathroom on the first floor. We are also planning to install solar PvP. 

 

My understanding is that we need an indirect thermal store for the future solar power to be used and this would be via a new storage cylinder when that is installed. This I understand excludes combi boilers. We do not want to change radiators or piping until we do the refurbishment if at all possible. 

 

So we are looking for a boiler that can be installed now and use the current setup but also accommodate our intended pressurised system and solar PvP, and maybe a heat pump down the line (not currently planned but would like to leave the option open). 

 

What boiler can you recommend? Also, any recommendations or comments on the future heating system plans would be appreciated. 

 

[JohnMo]

Nearly every boiler ever installed is massive compared to actual heating needs. So doubt you will have any issues after refurb and extension. Heating load calculation is needed any way - not a plumber finger in the air and guess

 

Aspects that are important

Boiler modulation - boilers spend their whole life modulated to a lower output than design output. So look for a boiler than can modulate to 6kW or below (some will go down to around 1.5kW)

S or Y plans are just shockingly poor for boiler efficiency and should be avoided at all cost.

Design new system to be Priory Hot Water Demand (PHWD), this will boiler to run at a high temp for DHW production and a low temperature for central heating - this change alone could drop gas consumption 20 to 30%.

Always install a  heat pump cylinder (even with a boiler) this will give very rapid water reheating.

Run heating on weather compensation or load compensation.

Do not zone house to death - a couple of big zone is needed. 

 

Never heard of PvP solar - but all you need is Solar PV. Don't bother with solar thermal expensive and a one trick pony

 

Cylinders - Unvented cylinder only - PV will divert to immersion if you want. No need for thermal store.

 

Combi boilers and solar DHW. Yes it is possible and quite easy - you need a combi boiler that can take preheated water. Atag, Intergas are examples that will and the manufacturers freely state this. 

Instead of cold water feeding from the main to the boiler you need to add a cylinder (small 50 to 100L UVC) upstream of boiler, cold feed into bottom and top feed to combi via a solar diverter valve.  Use immersion to heat so no coils needed in cylinder.  Solar diverter valve, will divert water above 45 degs direct to taps and miss out the boiler - this stops the boiler firing up when there is no need saving gas. But over all you better taking an Unvented cylinder.

 

I would choose a good system boiler instead of a good heat only boiler. Then the circulation pump is included and flow modulation is done automatically.

 

I have an Atag boiler and think its great. Wouldn't look much further than Atag or Intergas boiler, they all PDHW out the box and weather compenstaion is built in, they are both very simple and efficient boiler makes. If your system starts to look complex - there is something not right.

 

Best systems generally look like

Boiler controlled only by boiler manufacturer controller - no third part stuff.

3 port diverter valve or 2x 2 port valves (on normally open the other normally closed)

Heating system one side of diverter, and DHW cylinder the other.

A system like that doesn't even need a wiring centre, every is taken directly back to the boiler

 

 

[Nickfromwales]

PDHW = priority domestic hot water

DHW = domestic hot water  

 

If you’re going to have a new cylinder then you just need to install a “system boiler” which replaces your current boiler and will just drop right in with minimal disruption.

 

Assuming a typo on PvP and should read PV, so yes, that’s compatible when the time comes but you’ll need to ask the PV installers to fit a hot water diversion controller (such as an Eddi from MyEnergy) for that to feed to the immersion in the new cylinder.

 

Make sure the boiler installer flushes the system and fits a magnetic filter so none of the cr4p in the existing pipework and rads gets into the new boiler.

 

Ask for new thermostatic radiator valves to be fitted throughout, but not on any rads you intend to replace / upgrade eventually.

 

Currently looking at the market offerings for gas boilers so I’ll check out Atag, Intergas, Veissmann first, then WBosch and Vaillant I guess. First one will be a combi, so time to go back to the school of gas and see what’s what! 🙃

[LeserattePD]

1 hour ago, Nickfromwales said:

PDHW = priority domestic hot water

DHW = domestic hot water  

 

If you’re going to have a new cylinder then you just need to install a “system boiler” which replaces your current boiler and will just drop right in with minimal disruption.

 

Assuming a typo on PvP and should read PV, so yes, that’s compatible when the time comes but you’ll need to ask the PV installers to fit a hot water diversion controller (such as an Eddi from MyEnergy) for that to feed to the immersion in the new cylinder.

 

Make sure the boiler installer flushes the system and fits a magnetic filter so none of the cr4p in the existing pipework and rads gets into the new boiler.

 

Ask for new thermostatic radiator valves to be fitted throughout, but not on any rads you intend to replace / upgrade eventually.

 

Currently looking at the market offerings for gas boilers so I’ll check out Atag, Intergas, Veissmann first, then WBosch and Vaillant I guess. First one will be a combi, so time to go back to the school of gas and see what’s what! 🙃

Thank you for your response! Yes, I meant solar PV - planning to have a fairly big system for a future electric car as we live in NW London and we also both currently work from home on computers. 

 

I will definitely look at the Intergas system boiler - interesting that W/Bosch and Vaillant are your last choices - any particular reason? We’re not keen on a combi boiler as we definitely would have shower and kitchen use at same time and I’ve heard from friends who have one that you end up with a cold rush in the shower when the kitchen tap runs, but maybe their boiler wasn’t specced properly 🤷🏻‍♀️ So I kinda had arrived at the conclusion that we needed a system boiler but really wasn’t too sure. Do you by any chance install boilers in NW London?

 

Thank you for the tip about magnetic filter - all the radiators will be changed (they’re currently abysmal old style ones!) and some will be repositioned during the refurb but not right now. The refurb will be a big job anyway as we want to get rid of asbestos artex ceiling and the floor boards need to be replaced since they are too thin. We don’t even have stud walls, just 2 layers of plasterboard stuck together. So basically the refurb will be a tear down to wall and support beams 😂 I expect I’ll have loads of questions on these forums…

[Nickfromwales]

Yes, ask lots of questions as there’s a bunch of very knowledgeable people on here in fairness.

 

WB and Vaillant I’m assuming won’t have the ultra low modulation rates that I require for my clients projects, so please don’t compare my investigations with your thread, no relevance there. 

[LeserattePD]

1 hour ago, JohnMo said:

Nearly every boiler ever installed is massive compared to actual heating needs. So doubt you will have any issues after refurb and extension. Heating load calculation is needed any way - not a plumber finger in the air and guess

 

Aspects that are important

Boiler modulation - boilers spend their whole life modulated to a lower output than design output. So look for a boiler than can modulate to 6kW or below (some will go down to around 1.5kW)

S or Y plans are just shockingly poor for boiler efficiency and should be avoided at all cost.

Design new system to be Priory Hot Water Demand (PHWD), this will boiler to run at a high temp for DHW production and a low temperature for central heating - this change alone could drop gas consumption 20 to 30%.

Always install a  heat pump cylinder (even with a boiler) this will give very rapid water reheating.

Run heating on weather compensation or load compensation.

Do not zone house to death - a couple of big zone is needed. 

 

Never heard of PvP solar - but all you need is Solar PV. Don't bother with solar thermal expensive and a one trick pony

 

Cylinders - Unvented cylinder only - PV will divert to immersion if you want. No need for thermal store.

 

Combi boilers and solar DHW. Yes it is possible and quite easy - you need a combi boiler that can take preheated water. Atag, Intergas are examples that will and the manufacturers freely state this. 

Instead of cold water feeding from the main to the boiler you need to add a cylinder (small 50 to 100L UVC) upstream of boiler, cold feed into bottom and top feed to combi via a solar diverter valve.  Use immersion to heat so no coils needed in cylinder.  Solar diverter valve, will divert water above 45 degs direct to taps and miss out the boiler - this stops the boiler firing up when there is no need saving gas. But over all you better taking an Unvented cylinder.

 

I would choose a good system boiler instead of a good heat only boiler. Then the circulation pump is included and flow modulation is done automatically.

 

I have an Atag boiler and think its great. Wouldn't look much further than Atag or Intergas boiler, they all PDHW out the box and weather compenstaion is built in, they are both very simple and efficient boiler makes. If your system starts to look complex - there is something not right.

 

Best systems generally look like

Boiler controlled only by boiler manufacturer controller - no third part stuff.

3 port diverter valve or 2x 2 port valves (on normally open the other normally closed)

Heating system one side of diverter, and DHW cylinder the other.

A system like that doesn't even need a wiring centre, every is taken directly back to the boiler

 

 

Thank you so much for your response!

 

I had read about boiler modulation, but what are S and Y plans?

 

Yes, we weren’t considering solar thermal panels - I meant that we wanted to feed in electricity from PV into the cylinder when it is in excess of the normal house and proposed future electric car charging needs. We are thinking of having a battery to store electricity as well.

 

Thank you for confirming that an unvented cylinder appears to be best for us - I’m an amateur trying to decide these things and feel rather unsure about my conclusions from what I have read.

 

Interesting that both you and Nickfromwales prefer these over the Vaillant and WBosch the boiler service guy who decommissioned the boiler immediately mentioned - what are your reasons here if I may ask?

[JohnMo]

1 hour ago, LeserattePD said:

but what are S and Y plans

S and Y plans basically set the boiler to a fixed flow temp - normally at or close to 70 degs or even well above that. The wiring plan just moves either 2x 2 port valves or a mid point valve to different positions - so you can heat the house, heat the cylinder or heat both at the same time.  PDHW only allows you heat one or the other and also makes the boiler flow temperature different for each duty.

 

So what happens you have a thermostat in each room and the room temp swings as the thermostat switches on/off. A bit driving your car with foot either fully down or up on the accelerator.  WC or load compensation is more like cruise control, just enough throttle to maintain a steady speed, or steady house temperature. You run it all the time at a steady or may have a 1 or 2 deg setback at night. The boiler just ticks away and uses way less gas than than on/off thermostat demands and on and off timers.

 

Flow and return temp to boiler make a big difference in the the efficiency of the boiler the lower the temp the more efficient it operates. And because the boiler isn't for ever switching on and off the boiler life span is way better

 

 

1 hour ago, LeserattePD said:

Vaillant and WBosch

I have had both the above and an Atag, just like the Atag its very efficient, very little noise.

 

1 hour ago, LeserattePD said:

Thank you for confirming that an unvented cylinder

But I only recommend a heat pump cylinder - they are different from a gas boiler cylinder and unless pushed the plumber will install just a normal gas boiler cylinder. 

[LeserattePD]

14 hours ago, JohnMo said:

 

But I only recommend a heat pump cylinder - they are different from a gas boiler cylinder and unless pushed the plumber will install just a normal gas boiler cylinder. 

Oh I understand now - the heat pump probably won’t happen during refurb as we’re prioritising the solar PV and also aren’t currently eligible for the government grant - EPC out of date plus the recommendation on that is more insulation as far as I remember, again something that’s going to be fixed during the refurb. For example, there is NO insulation on our floor plate 😢 or in between floors - so downstairs is freezing and upstairs toasty all the time - we don’t even run the upstairs radiators except in the bathroom. Honestly we bought the location, NOT the house 😂 it’s at the end of a cul-de-sac in a little estate around a village green with 5 min walk to the tube station/local High Street but incredibly quiet. The double width garden was the selling point because we always intended to do a side extension.

 

We’re interested in a heat pump long term partly because of the cooling option in summer. As the SW facing end of the terrace, the SW bedroom gets unbearably hot in summer, we’re hoping that increased insulation will mitigate that to some degree and planning to have climbing plants on both the south and west facing sides to help. That and it’ll look nice too and help the local wildlife.

 

We’re keeping the current cylinder for the time being and then will change it during the refurb. I don’t mind the tanks in the roof space either as that is unusable for anything due to the hip roof with only about 50cm straight line. The two story extension as currently planned will have a slightly lower hip (a hip-to-gable roof conversion on the main house was denied by the planners) - but the advantage of the hip is that will give us pretty optimal positioning of the solar panels to SSW and the slope is going to be pretty optimal too.

 

I had read about the lower flow rate heating and that is definitely something we want, though we had pretty much decided against underfloor heating. 

[JohnMo]

1 hour ago, LeserattePD said:

lower flow rate heating and that is definitely something we want, though we had pretty much decided against underfloor heating. 

Your first step for this is a room by heat loss. Then define your max target flow temperature. That will help you define the radiators you need, I would do this during the refurb stage. Or keep what you have, put new boiler in and set on WC and then assess what room may benefit a larger radiator.

 

UFH is good on a new build but just hard work on an existing house.

 

1 hour ago, LeserattePD said:

heat pump long term partly because of the cooling option in summer

That is why we got ours, but cooling doesn't work with radiators. You need fan coils or UFH. But you can replace radiators with fan coils.

 

1 hour ago, LeserattePD said:

We’re keeping the current cylinder for the time being

Your existing cylinder can also be set to PDHW.

 

A write up on the subject 

https://theintergasshop.co.uk/content/189-why-hot-water-priority-pdhw-is-the-reason-s-and-y-plan-should-be-banned

 

[LeserattePD]

45 minutes ago, JohnMo said:

UFH is good on a new build but just hard work on an existing house.

 

That is why we got ours, but cooling doesn't work with radiators. You need fan coils or UFH. But you can replace radiators with fan coils.

 

Your existing cylinder can also be set to PDHW.

 

🤔 that may make us reconsider the underfloor heating but we mainly need the cooling upstairs (at least with current setup).

 

UFH heating in the extension would be easy to do, and the ground floor in existing house would be doable since currently there is no insulation on the floor plate and the existing underlay&flooring will be replaced anyway in refurb to add such insulation.

The ground floor temperature in summer is ok - though that may change once we actually have insulation between floors. Currently the heat just accumulates upstairs.

 

On the other hand, the extension includes a garage and downstairs bathroom on the south side so should add further protection from heating up; though the new upstairs bedroom will be above the garage. All big windows will be either east or west side, with any on south side smaller >170 cm high horizontal windows to avoid overlooking neighbours.

We weren’t looking at UFH on first floor - I’m not sure our supporting beams would deal with the necessary load, plus head height is limited to 230cm. Everything in this house is late 1980s Wimpey’s cheapest… see the too thin floorboards that are bending all over the place and the 2xplasterboard “walls”.

 

Thank you for pointing out the need for fan coils if we do not use UFH.