Adding weather compensation and HW override to boiler?

[mkilpatr]

I've just started reading up on weather compensation systems that can be retrofitted with the aim of reducing the flow temperature to maximise the efficiency of a condensing gas boiler, as and when appropriate.

In 2009 with the extension I had a new boiler fitted, a Baxi Duo-tec combi boiler. It is very clear online that weather compensation can be retrofitted to a Duo-tech boiler. I also suspect Baxi use a common control PCB (Siemens made) for various models boilers.

The problem is a little more complicated here.

There is a hot water cylinder for the bathrooms /and/ there is a combi setup for hot water on demand for the kitchen and utility room which I thought practical because of the distance from one end of the bungalow to another.

The problem with weather compensation or system boilers specifically is that lowering the flow temperature risks legionella etc developing in the hot water cylinder.

The question is, when used as a combi-boiler /and/ with a hot water cylinder, does the Duo-tec have the capability to have an input from the timer/controller (or via the valve wiring) to set the flow temperatute back to 80/60 when the hot water cylinder is demanding heat (regardless of whether the heating zones 1 & 2 are) thus sastisfying all the safety requirements?

I wasn't easily able to discover if this is the case. I gave up waiting on the phone to Baxi after 40 mins. Anyone have experience of this?


Michael

P.S. Until this morning I wasn't aware that most of the time, condensing boilers aren' actually condensing at all and they don't run near max efficiency if the flow temp is set constantly to cater for a HW cylinder. Blimey.

 

[fixitflav]

I've just started reading up on weather compensation systems that can be retrofitted with the aim of reducing the flow temperature to maximise the efficiency of a condensing gas boiler, as and when appropriate.

Assuming you're still on DIYnot, did you get any joy on this? I came across it while looking up weather compensation retrofit on the web.
The ones I've found don't make it clear what the kit includes, usually just one item, or just what it does.
I think it needs
1. An outdoor temperature sensor
2. A control box taking input from the outdoor sensor, with temperature profile adjustment.
3. A pipestat for the boiler flow pipe, trip point varying in line with a signal from the control box.

I'd like to get a kit like that, if it's available.

P.S. Until this morning I wasn't aware that most of the time, condensing boilers aren' actually condensing at all and they don't run near max efficiency if the flow temp is set constantly to cater for a HW cylinder. Blimey.

The situation isn't as bad as it sounds. The boiler control-stat must be set high enough to cater for the HW cylinder, but that doesn't mean it will reach that temperature in CH mode. On CH the water reaches an equilibrium temperature, and usually the boiler stops on the roomstat before the water gets too hot, so some condensation takes place.
My boiler goes out on control-stat in HW mode, but never in CH.

 

[Harry Bloomfield]

The situation isn't as bad as it sounds. The boiler control-stat must be set high enough to cater for the HW cylinder, but that doesn't mean it will reach that temperature in CH mode. On CH the water reaches an equilibrium temperature, and usually the boiler stops on the roomstat before the water gets too hot, so some condensation takes place.
My boiler goes out on control-stat in HW mode, but never in CH.

The boiler, needs to have something similar to an ebus, so it can instruct the boiler as to what flow temperature is necessary to meet the actual heating demand. Aside from the ebus, it needs to know the actual room temperature, rather than on/off, demand/no demand. The outdoor sensor simply helps the boiler predict the correct level of demand for heat, and modulate appropriately.

The better the boiler is able to reduce the flow temperature, by modulating, the more it runs in the condensing temperature range, the more efficiently it makes use of the gas.

Heating stored water, the flow has to be hotter, 60 to 70 degrees, therefore it simply cannot condense.

 

[fixitflav]

The boiler, needs to have something similar to an ebus, so it can instruct the boiler as to what flow temperature is necessary to meet the actual heating demand. Aside from the ebus, it needs to know the actual room temperature, rather than on/off, demand/no demand. The outdoor sensor simply helps the boiler predict the correct level of demand for heat, and modulate appropriately.

The better the boiler is able to reduce the flow temperature, by modulating, the more it runs in the condensing temperature range, the more efficiently it makes use of the gas.

Heating stored water, the flow has to be hotter, 60 to 70 degrees, therefore it simply cannot condense.

I've never been into ebus, Opentherm, Nest etc and I'm not convinced they add much in the way of efficiency.
But I came up with a way to add adjustable flow temperature to W-plan, using a pipestat on the flow, (very) rough sketch attached. This shows manual adjustment of the pipestat switchpoint. For weather comp it would be automatically adjusted by a signal from a control box. The boiler could be on/off or modulating, but if modulating it would be on/off in WC mode.

 

[Harry Bloomfield]

I've never been into ebus, Opentherm, Nest etc and I'm not convinced they add much in the way of efficiency.

I would say they add quite a lot to efficiency, whilst ever the boiler is running in modulation, the difference between condensing and very little time spent in condensing mode. I kept an eye on my instantaneous gas consumption, for the past few years, out of curiosity, and saw during nearly all the time, serving CH, it was definitely modulating. There was none of - call for heat, boiler runs flat out to satisfy it, then off, just steady constant background heat, perfectly matching the heat lost.

Your diagram suggests your pipe-stat idea, will simply have the boiler running flat out, or off, plus you will need to be constantly tweaking that setting. All I need do, should I want things warmer, is turn my room control up a touch, and leave it to sort out what flow temperature, it needs from the boiler, and how long it needs it to run for, so it precisely hits the spot.

 

[fixitflav]

it was definitely modulating. There was none of - call for heat, boiler runs flat out to satisfy it, then off, just steady constant background heat, perfectly matching the heat lost.

Maybe modulation gives some improvement, I just don't think it's as much as is often supposed. As long as the heat goes into the water it doesn't matter if it's firing at max. Most kit is most efficicient at design load - electric motors, IC engines, hydro turbines etc.

Your diagram suggests your pipe-stat idea, will simply have the boiler running flat out, or off, plus you will need to be constantly tweaking that setting.

The manual adjustment is Mk 1. The Mk 2 version with weather comp has the pipestat setting automatically adjusted.

All I need do, should I want things warmer, is turn my room control up a touch, and leave it to sort out what flow temperature, it needs from the boiler, and how long it needs it to run for, so it precisely hits the spot.

So is that an additional input to the outside temp/boiler flow temp algorithm? Maybe if the room is well below the new temperature setting it could help, but for a small rise not so sure.

 

[Harry Bloomfield]

Maybe modulation gives some improvement, I just don't think it's as much as is often supposed. As long as the heat goes into the water it doesn't matter if it's firing at max. Most kit is most efficicient at design load - electric motors, IC engines, hydro turbines etc.

Condensing mode, gets you an extra 5-10% of efficiency, in all but the times when I make a sudden large change in the demanded room temperatures, or when heating my cylinder, my boiler runs well into condensing mode. Which suggests my system is well worth having, for the savings in energy/gas costs.

That compares well with simple on/off demand, plus the boiler having to self modulate. Which was what I had, prior to installing my bus controlled system. Back then, the boiler would come on, amid lots of creaking of pipes, overshoot, and undershoot of temperature - all very wasteful of gas. Now it just quietly maintains the set temperature.

The manual adjustment is Mk 1. The Mk 2 version with weather comp has the pipestat setting automatically adjusted.

I don't know how you will manage that, without a very complex system.

So is that an additional input to the outside temp/boiler flow temp algorithm? Maybe if the room is well below the new temperature setting it could help, but for a small rise not so sure.

The system knows the present room temperature, the preset room temperature, the flow temperature, return temperature, the outdoor temperature, and has a close idea of how much heat input it needs to generate, to meet the likely demand for heat, from past-experience - and so sets the boiler appropriately.

 

[ericmark]

it needs to know the actual room temperature, rather than on/off, demand/no demand.

This is the main point with any modulating boiler,

P.S. Until this morning I wasn't aware that most of the time, condensing boilers aren' actually condensing at all and they don't run near max efficiency if the flow temp is set constantly to cater for a HW cylinder. Blimey.

I was rather surprised in mother's house how well the boiler worked with simple return water control. The whole idea as @Harry Bloomfield says is for the control to be analogue, which is how TRV's work, which begs the question of why fit any on/off zone valve? Setting the TRV and lock shield valve, each room was within 1ºC of the target setting within around 2 hours of a temperature change. I found this too slow, so would cheat, and set living room to 22ºC at 7 am, then 20ºC at 8 am to speed up temperature change, more expensive TRV heads one can actually set time you want room to temperature rather than time to start heating.

However, I would place a hand on the radiator, and they were warm, very rare hot, just warm enough to maintain the room temperature. Spot on in the heart of winter, no real need for a wall thermostat, the TRV's worked A1.

The problem was Autumn and Spring. When the demand was dropping off, and the boiler was cycling as it could not reduce the output any more, so looking for a method to auto turn off boiler when all rooms satisfied, This was not so easy, to start with the wall thermostat was not suitable for a modulating boiler, this thermostat is designed to start turning boiler off/on as it approaches the set point, so the boiler does not over shoot, but every time the boiler is switched off, on being switched on again it does so at flat out setting until the water returns to boiler for the boiler to start modulating, great thermostat for non modulating boiler, but with a modulating type, want a far more simple wall thermostat. This one is great, but hard-wired, so not something I could use. I looked for a wireless type, this seemed to fit the bill, however I found it would lose connection and fail to turn the boiler off. So really want a wireless with some fail-safe, the original had that fail-safe, if no signal in ½ hour it would turn off, but as said not suitable due to the mark/space control system. So it seems hard-wired £35 would buy a thermostat to do the job, but for wireless one jumped to over £100 and most were so-called smart.

It was not just the wall thermostat, it was also the price of the TRV head, at the time, I could buy a eQ-3 programmable TRV head for £15, but a linked TRV head, was more like £60, multiply this by my 12 TRV heads, and it gets expensive.

The cheapest wall thermostat was Hive, but add the cost of TRV heads, and in total some of what seemed more expensive worked out cheaper. Plus, Hive will not accept a demand for heat if the wall thermostat is over 22ºC.

All the instructions say, put the wall thermostat in a room kept cool, on a lower floor, with no outside door, or alternative heating, which includes sun through the windows, And to put it simply, I have no such room.

So some sort of compromise is called for, either a linked wall thermostat, or the wall thermostat and TRV are carefully set to work in harmony, I selected the latter. Hall TRV set to 17ºC and wall thermostat to 18.5ºC because the TRV is lower, and closer to radiator it would on cold days never let the wall thermostat turn off, but as the weather improved, it did allow the wall thermostat to turn off, stopping the boiler cycling, but it was very much trial and error, this house failed, the hall cools too slowly. The only option is two wall thermostats, both in parallel.

I have owned four UK houses during my life, and the central heating control was different with every one. This is the problem, we look at our own home, and tend to forget, all homes are not the same. Sun into a bay window can really mess up the system, and the easy way out has to be linked TRV heads, this in spite of being old, is likely one of the best systems, why do we really need a wall thermostat? Answer to turn off heating automatic in summer, if the system can do this by reading the TRV's, then no need for a wall thermostat.

 

[Harry Bloomfield]

Harry Bloomfield said:

it needs to know the actual room temperature, rather than on/off, demand/no demand.

This is the main point with any modulating boiler,

Exactly, to maximise time spent in condensing mode, the boiler needs to know the actual difference between actual room temperature, the demanded room temperature, and an idea of how meet that requirement efficiently. Demand/no demand, modulation set solely by flow temperature, comes nowhere near good use of modulation, or gas use efficiency.

 

[ericmark]

Exactly, to maximise time spent in condensing mode, the boiler needs to know the actual difference between actual room temperature, the demanded room temperature, and an idea of how meet that requirement efficiently. Demand/no demand, modulation set solely by flow temperature, comes nowhere near good use of modulation, or gas use efficiency.

In mother house, the Worcester Bosch boiler, had no option for any e-bus control. The law changed to require new boilers to be able to use the latent heat from the flue gases, but only to be able, it did not have to do it in real terms.

I wanted to see how the boiler modulated, but can't get a clamp on meter to work with gas pipes. So in real terms, no way for the user to see if working or not. What was a real surprise, was moving here, with a non modulating oil boiler, and far larger house, my central heating fuel bill went down.

The only way I know the Worcester Bosch boiler did gain the latent heat, was it stopped working when the condensate pipe froze. But there is no point in fancy weather compensation if not using analogue controls.

 

[Harry Bloomfield]

In mother house, the Worcester Bosch boiler, had no option for any e-bus control. The law changed to require new boilers to be able to use the latent heat from the flue gases, but only to be able, it did not have to do it in real terms.

Thats where a smart meter really comes in, with it's indoor display, you can see exactly how much gas is being used from minute to minute.

The only way I know the Worcester Bosch boiler did gain the latent heat, was it stopped working when the condensate pipe froze. But there is no point in fancy weather compensation if not using analogue controls.

So you agree, Fixitflav, is mostly wasting his time with his efforts?

 

[fixitflav]

Condensing mode, gets you an extra 5-10% of efficiency, in all but the times when I make a sudden large change in the demanded room temperatures, or when heating my cylinder, my boiler runs well into condensing mode.

I get that, but have you any comparison with previous figuyres, to confirm the savings?

Which was what I had, prior to installing my bus controlled system. Back then, the boiler would come on, amid lots of creaking of pipes, overshoot, and undershoot of temperature - all very wasteful of gas. Now it just quietly maintains the set temperature.

Mine is On/Off, installed in 1999, before modulation came in (if I'm not mistaken). Room temperature is controlled pretty well, I just leave the roomstat at at 20°C all the year round, maybe drop it to 10° if I'm going to be away for a while in winter. I don't get any creaking, though admittedly I did in a previous house. Must be the way the pipes are installed.

I don't know how you will manage that, without a very complex system.

I was hoping a retrofit kit as described in #2 might be available, but I haven't found anything, and the OP seems to have left. Weather comp when provided by boiler suppliers must incorporate the software, OK if you want a new boiler, and the wiring varies between suppliers.

Exactly, to maximise time spent in condensing mode, the boiler needs to know the actual difference between actual room temperature, the demanded room temperature, and an idea of how meet that requirement efficiently. Demand/no demand, modulation set solely by flow temperature, comes nowhere near good use of modulation, or gas use efficiency.

That's quite sophisticated, and possibly improves efficiency, but as I understand it a standard W comp system doesn't have an input of room temperature, just outside temperature. It varies the flow temperature modulation setpoint (assuming all W comp boilers have modulation). A recent thread about a Viessmann system doesn't have room temperature input, just outside. https://www.diynot.com/diy/threads/indirect-water-cylinder.630918/
I don't know the current thinking on whether it's best to control flow or return temperature, some years ago a Danfoss system used return.

Thanks for the interest!

 

[fixitflav]

The only way I know the Worcester Bosch boiler did gain the latent heat, was it stopped working when the condensate pipe froze.

I haven't had time yet to read and digest your posts, but that hasn't much to do with W comp. Condensing boilers condense some of the time without W comp, just that it should do it more using W comp (with or without room temperature input). Also a lot to do with having plenty of rad area.

But there is no point in fancy weather compensation if not using analogue controls.

Not sure about that. Analogue control (of flow temperature?) might help, but On/Off will give some improvement.

So you agree, Fixitflav, is mostly wasting his time with his efforts?

I possibly am, but it interests me and I'm retired, so what the hell?

 

[Harry Bloomfield]

I get that, but have you any comparison with previous figuyres, to confirm the savings?

No, none, but they would be pointless anyway - with so many variables in the mix, like weather itself. I got a distinct gut feeling, that it was saving me some on the bill, but with a traceable improvement in indoor temperature stability. That was traceable, because my weather station records the indoor temperature, every minute, 24/7. You could see the undulations of the temperature graph, prior to my installing the system, as the boiler cycled - none of that after.

 

[fixitflav]

No, none, but they would be pointless anyway - with so many variables in the mix, like weather itself. I got a distinct gut feeling, that it was saving me some on the bill, but with a traceable improvement in indoor temperature stability. That was traceable, because my weather station records the indoor temperature, every minute, 24/7. You could see the undulations of the temperature graph, prior to my installing the system, as the boiler cycled - none of that after.

OK