Weather compensation and HW cylinder overrides - boilers?

[Michael Kilpatrick]

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

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 in general is that lowering the flow temperature risks legionella etc developing in the hot water cylinder.

I have the suspicion that Baxi use a common control PCB (Siemens made) for various models boilers.

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. Igave up waiting on the phone to Baxi after 40 mins.

Michael

[Theo]

I can't comment on your boiler specifics, but the legionella problem can be solved by running the immersion on a 60C thermostat once a week on a timer. You don't need to keep DHW at 60C all the time, only frequently enough to stop bugs breeding.

To save energy, you can be cunninger about it. Have two stats, one at whatever your normal DHW temperature is (let's say 50C), and one at 60C. When the timer fires, the first stat locks out the immersion unless the water is above 50C. Then the immersion is powered until the water rises about 60C, at which point it cuts off. You then just need to synchronise the immersion timer with the boiler programmer, so the boiler has had a chance to run the DHW up to 50C first.

We have a setup like this (the ASHP outputs 55C) and it takes 2-3kWh per week (it's separately metered).

Theo

[John Rumm]

ok that is handy... It makes life much simpler if the boiler understands the concept.

Yup a sensible system setup by the sounds of it, although probably not something accounted for in their control system... however that ought not stop you "rolling your own".

You need to create a system of split temperature operation; so that when it is reheating the DHW it does it at a higher flow temp, and yet runs the rads at whatever the weather copensated flow (WC) temp currently is.

(that also implies S Plan or W plan operation - you can't have Y Plan)

If you want to get posh then you can elect to run the DHW at a "normal" temp, and then say have a once weekly "sanitising" heat to a higher temperature. (something you get on posh control systems like that used by Vaillant)

Ok I can only make educated guesses here[1] since I am not familiar with that boiler or the weather compensator you are using. So keep in mind what follows may be complete nonsense! (have you got links to the WC manual?)

I would guess that the software in the boiler does not expect you to run a DHW zone off the CH "side", and hence will have no visibility of what the actual demand is coming from, and hence the need to ever override the WC set temp.

However it may be relatively easy to work around - depending on how the external temperature sensor works... Most use a NTC thermistor to read the temp and feed it to the boiler. At some point there may also be a way to adjust that mapping between external temp and required flow temp[2] (with older harder to heat houses needing a more "aggressive" ramping up of flow temp as the external temp falls.

If this is the way it works, then it makes it relatively easy to have the boiler believe the external temperature is whatever you fancy, and so indirectly control the flow temperature that way. Say leaving it in the tender care of the NTC when running the rads, but then relay switching the NTC resistance to "'kin cold" when you need to reheat the DHW - thus prompting the WC to run a high flow temp.

[1] Also assuming that the WC basically connects to the boiler and tells it what the external temperature is, and lets the boiler modulate the flow temp accordingly. (some systems designed for "dumb" boilers may use blending valves to do everything external to the boiler) [2] My Vaillant system understands "split temp operation" and also allows a user selectable mapping:

Basically you choose one of a number of preset heat curves, and that then dictates what flow temp it runs for a given external temp. So If I have mine set at 1.5, it runs low to mid 40s when it's 5 to 10 degrees out, but ramps up to 55 at zero, then mid 60s at -5.

[SH]

and if you happen to have ecoonomy 7 electricity, you can do the top up from 50 deg C to 60 Deg C during the night and save some money

[RJH]

It is - never heard of it in fact. Certainly something I'd look for if getting a new boiler.

I just fitted a weather compensation thingy to my Ideal Logic - and a very similar system. Only £25 and a simple fit.

Thing is, while it's relatively warm out, the boiler /seems/ to have trouble circulating water at anything less than about 55C, firing every couple of minutes, then pump overrunning. Why doesn't it just cut back on the burner and let it tick along?

[John Rumm]

How does it interface to the boiler, and what controls does it give you?

What is the modulation range on the boiler? You may find that the minimum output power on the boiler is higher than you ideally need on a mild day.

[RJH]

Just 2 wires to a terminal block in the boiler. Controls - the slope is set according to the CH temperature setting, which is controlled by a knob on the front of the boiler.

5 - 30kW. Can't imagine 5kW would heat much central heating, and my home is far from well insulated.

[John Rumm]

So it sounds like it is just a temperature sensor in a pretty box then?

Perhaps the curve you have selected keeps the flow temp up? If I set mine at 2.5, then it would still be demanding >50 even with an outside temp of over 10.

[Michael Kilpatrick]

Good point, I hadn't thought that through.

Recently I changed the timer/controller from a Hortsman 3-channel controller (which seemed to expire during a thunderstorm powercut) to a Timeguard TRT039N 4-channel unit - SO much better and a faster response when programming.

I bought a 4-way controller because I was considering using the 4th channel to wire up - via a relay - the towel radiators so they could run on a timer in the summer when the central heating doesn't come on to warm them up.

But I could instead use that spare 4th channel to control the immersion heater and set the timer to once a week in the morning after the heating/water has switched off. That has the advantage that on sunny mornings the immersion heating boost won't cost me anything.

That would mean I could then consider lowering the boiler flow temperatute.

Michael

[RJH]

I've got it set to 2.2 which is the highest it'll go without going out of the 'high efficiency' CH temperature setting control. And as you can see:

It should be asking for well over 50C (at 6C outside) - which it pretty much provides. The rooms/rads are warm enough. So it's either by design, or there's a fault with the modulation.

Or it could be because I've only got 4 (of 11) radiators fully open. Those 4 are doing a fair amount of work heating cold rooms, but it could be that . . . .

[Brian]

Interesting.

I wonder if you can comment / advise on a niggling problem I have.

Normally we heat the water via the gas CH - a typical indirect cylinder. We rarely have problems but have an immersion heater, just in case.

We tend only to use it if there is a problem with the CH or we have visitors and people are having showers in quick succession etc.

The problem is, it tends to trip its over heat protection. I?ve changed it

• at least once, tried adjusting the temperatures etc, but it still sometimes happens. Not every time an not obvious pattern.

• the sensor, not the heater.

We don?t run the HW ( the main tank) that hot when using only the CH. If we use the immersion heater only, the water is noticeably hotter.

[John Rumm]

Hard water area?

Sometimes if the immersion element is heavily scaled, it will cause it to run hotter, that might be enough to trip its overtemp stat.

Sounds like you have its stat set to a higher temp than the cylinder stat feeding the CH (or you have your CH flow temp set lower than the set point temp on the cylinder stat, so it never reaches target temp)