Weather Compensator

[docboardman]

Hi Everyone:

I've been modernising our CH system whilst renovating our house. Most (possibly all) of the CH pipework has been replaced. Also new rads everywhere. That was the easy bit.

The boiler has been replaced with a new condensing unit. I'm also looking at a programmable thermostat. Recently I've been told that I should have a weather compensator in the system. I believe this will regulate the boiler flow temperature to take into account outside air temp. (e.g. On a cool day the flow temp will be lower. On a cold day the flow temp will be hotter.) I think this is all meant to keep the boiler in the condensing range.

I've already looked at the boiler manufacturer's site, but they don't have a weather compensator. I've also done some web searches for them but these haven't been successful. Can anyone tell me where I can find a weather compensator? Also, does it work with a programmable thermostat or does it replace a programmable thermostat?

Cheers!

 

[htgeng]

The boilers I use have the compensator built in. In your case you need something like the BEM 5000. This has functionality you need and works with an outside sensor and sensor on the return pipe. The call for heat comes from the 5000. The room stat acts as a high limit stat to turn off all demand once that temp is reach.

They aren't a bad unit and are better than nothing!

 

[simond]

Grant do a nice WC package than can be fitted to most boilers.

I don't know why you didn't specify your boiler with WC inbuilt, it is not as cost effective to fit it afterwards, you need external electronic mixing valves etc.

 

[Onetap]

Boiler make and model?
Does it have a modulating burner and means to control it from an electronic signal (0-10V typically)? Probably not or they'd offer optional weather compensated fittings.

In which case you'd need an external 3-port mixing valve or injection mixing pump, weather compensation package and a second pump for the variable temperature system.

 

[Balenza]

Simond wrote

you need external electronic mixing valves

Onetap wrote

In which case you'd need an external 3-port mixing

Whats wrong with the BEM 5000 that was advocated earlier ?.

 

[megawatt]

Is there any empirical evidence about the effectiveness and savings from fitting a WC?

 

[htgeng]

Doc, If you use the Bem 5000, you don't need any other external mixers or pumps.

Basically the pump and burner come under direct control of the Bem. If the boiler has pump overrun then this can still control the pump too. When the system is up to temp(as determined by the outside sensor) then the Bem will detect this with the return sensor and kill the heat demand. The pump will continue to run and circulate the water until it cools sufficiently to signal the Bem to turn the burner back on.

Its a very basic system to give compensation but it works very well and I believe has been in existence since the early 80's (some of our older posters may confirm). I recall seeing an old version with a little piggybank symbol which was lit when the pump was on and burner off.

 

[Onetap]

Whats wrong with the BEM 5000 that was advocated earlier ?.

http://www.gotoplumbing.com/item_default.asp?catid=1358&accid=1041&itemid=12593&desc=2

It's not a weather compensator, it's a Boiler Energy Management device. It could only provide weather compensation by cycling the burner on and off at low boiler flow temperatures. This could only achieve an inaccurate saw-tooth type control of the flow temperature.

I suspect that the more frequent starts wouldn't do the gas valve any good. With a non-condensing boiler you'd need a back-end protection device to operate the boiler at low flow temperatures; this would take you back to a 3-port motorized valve then, so you wouldn't achieve much. A proper variable temperature (VT) system would inhibit the 3-port mixing valve opening (and so the hot water drawn from the boiler/CT system) if boiler return temperatures approached condensing.

Got all that?


It's (apparently) a cheap solution which, when you've bought one, you'll find it doesn't do what it says on the box.

And, yes, there is evidence about savings from weather compensation /outdoor reset. You can Google for it. It's most cost-effective with modulating condensing boilers since it maximizes the time the boiler operates at condensing temperatures. It also reduces the standing losses from any heating system. It's more about comfort though.

PS There is another problem with the BEM 5000; it isn't listed on the Danfoss website, so I think they've discontinued it; not surprising.

http://danfoss-randall.co.uk/Default.aspx

 

[Balenza]

Onetap wrote

It's not a weather compensator

Yet the link he gave states-

The unit is a weather compensator which during periods of heating demand varies the temperature of the water flowing to the radiators as a function of the outside temperature.

This could only achieve an inaccurate saw-tooth type control of the flow temperature

It also states - it will reduce unnecessary boiler cycling

so I think they've discontinued it; not surprising.

That may be true. But does that mean the OP has no choice but to fit expensive valves to acheive his weather compensation goal ?.

 

[htgeng]

That may be true. But does that mean the OP has no choice but to fit expensive valves to acheive his weather compensation goal ?.

Having trvs would help too. He can have a manual weather compensator - himself. Carefully adjust the flow temp as the outside temp dictates. Of course most people have a life, but I do know a few people who would do that.

I wouldn't knock the Bem as a solution and it does work. I have installed about 4 of them and would have heard back by now if they didnt't work as stated. It would be a shame if Danfoss had stopped making them! I did order one through PTS last year with no problem and Im pretty sure the warranty label was +2 years

 

[Onetap]

Onetap wrote

It's not a weather compensator

Yet the link he gave states-

The unit is a weather compensator .................

The full explanation is given above Balenza. Believe or disbelieve as you please.

Can you offer any other explanation as to how it operates an on/off burner at variable low flow temperatures?

They are meant to restrict firing at normal flow temperatures, I believe.

The fact that the literature says it is a weather compensator doesn't mean that it is. Electronic water conditioners are similarly often described as softeners; they aren't, as anyone who knows how proper sofeteners work will tell you. The gullible buy them thinking they're getting a cheap fix.

If the full specifications were still available from Danfoss, you could read them, but they aren't.

The problem is that I know a bit about VT systems and electronic control strategy and you don't like what I'm telling you, for some reason. That's your problem.

No-one has offered a different or a better explanation.

PS The valves and actuators aren't particularly expensive (in fact you could use a standard Sunvic mo-mo 3-port valve, if you knew how). . They are perceived as a high-end commodity and retailers and installers charge accordingly. BMS manufacturers are notorious for this, you can't get cheap BMS components, you have to buy a system from one of their nominated 'partners'.

 

[Onetap]

That may be true. But does that mean the OP has no choice but to fit expensive valves to acheive his weather compensation goal ?.

Having trvs would help too. He can have a manual weather compensator - himself. Carefully adjust the flow temp as the outside temp dictates. Of course most people have a life, but I do know a few people who would do that.

The outside air temperature varies during the day, so it isn't really practical. The other problem remains that, with a non-condensing boiler, a lower flow temperature will give a lower return temperature and you will get condensing conditions, corroding the boiler, for longer.

Another problem with simple weather compensated systems, where the flow temperature is firmly fixed to the outside air temperature, is this;

the low flow temperature is OK when the building has warmed up and has achieved steady state conditions. When it warming up from cold, you need a much higher temperature. Simple WC will give you a cold house in the mornings. I didn't see anything on the BEM literature that mentioned this.

 

[htgeng]

Another problem with simple weather compensated systems, where the flow temperature is firmly fixed to the outside air temperature, is this;

the low flow temperature is OK when the building has warmed up and has achieved steady state conditions. When it warming up from cold, you need a much higher temperature. Simple WC will give you a cold house in the mornings. I didn't see anything on the BEM literature that mentioned this.

The boilers own thermostat is turned up to maximum as per the instructions. I think there is some benefit to oversizing radiators, which I always do, to get a faster heat up time. Another recommendation is to time the heating to come on earlier. So if Mrs Smith wakes up at 8am then time to start heating at 6:30am.

Yes this doesn't work well for non-condensing, but Doc says he has just had a new one installed.

 

[Onetap]

The boilers own thermostat is turned up to maximum as per the instructions. I think there is some benefit to oversizing radiators, which I always do, to get a faster heat up time. Another recommendation is to time the heating to come on earlier. So if Mrs Smith wakes up at 8am then time to start heating at 6:30am.

Yes this doesn't work well for non-condensing, but Doc says he has just had a new one installed.

Yes, but the BEM device turns the burner on and off, so the boiler flow temperature remains relatively low, despite what the stat is set to.

The best method is a device with optimum start/stop (OSS) which varies the start time according to the weather. So Mrs Smith gets up at 8:00 to a warm house; the heating fires up at 6:30 on cold days, 7:30 on a mild day, or whatever is needed.

Docboardman didn't say that.

 

[Balenza]

Onetap wrote

so the boiler flow temperature remains relatively low

What about the "response rate" setting ?.
Does turning this up not raise the boiler flow temperature in line with low outside temperatures.
It must have a fixed switching differential through out these ranges I would have thought.