How to achieve a lower than 55 degree return temp on boiler?

spinningsheep

Hi guys, calling all you plumbers out there, I have a Baxi Duotec HE 28kw running 10 rads and a towel rail, 2 rads with no TRV's and the towel rail with no TRV either. All rads are on, so all radiators heat up. I am wondering, until recently I have set the flow temp on the boiler to 55 degrees so I know the the return is always less than 55, heating was fine. Now it is getting cooler, the temp is too low, the radiators are not as hot it seems. So, what temp would you think is good to run at so I am getting the required temp drop across all the rads, and to avoid the boiler "cycling". I know the system would have been balanced when installed but with it being a rented house I don't know what was done or how. I want efficiency but not at the expense of comfort. We are in an end of terraced with 4 bedrooms, loft insulated,fully double glazed but no cavity insulation as no cavity walls! The system was not changed when the boiler was replaced so the radiators are not "oversized" and I have read that a domestic system is designed to run on an 80 degree flow temp, but I don't think I would ever acheive a 25 degree temp drop on this system! Any thoughts, experience or advice much appreciated.

DVardysShadow

I always run boilers full temp. Let the TRV's sort it out. Control the rads without TRV's manually, but leave one on full. Ideally this should be in the same location as the room thermostat if there is one.

spinningsheep

Thanks for the response, but surely leaving it on full would mean higher gas bills as the boiler would not be condensing?

ziggyman99

There is no need to leave one rad on full, your boiler has an automatic bypass valve fitted. It's difficult to achieve a 55 deg return with a combi as the boiler is usually completely oversized for heating requirements. You can turn down the boiler temperature, as you said, but that would only work properly if your radiators are sized for a flow temp of 55 deg. Most systems are existing when new condensing boilers are installed so the radiators are sized for 75/80 flow temps. Some boilers, Vaillants for example, can be set so the boiler outputs less to the radiators.This will help to ensure long burns and reduced return temps for as long as possible. Also remember that your boiler is fully modulating and should reduce it's output as the return gets warmer.
I designed my own system with a heat store and a mixing valve on the return to the boiler. This ensures that the boiler stays in condensing mode right up to the moment the store is fully heated up. It also means no cycling. My boiler fires up then runs flat out for 15/20 mins then cuts out. It doesn't then fire until the heat store is nearly depleted. Great system but not without it's flaws - one being it's very complicated - not something I would install for a customer.

Cardew

Good question!

I have a non-condensing boiler at present, but inevitably at some time I will be forced to get a condensing boiler.

I have wondered how you can set up a CH system for a large number of rooms, but have it working efficiently(i.e. condensing) when only a few rooms are heated.

In my case it simply isn’t practical to continually vary the boiler water temperature setting as the boiler is in a boiler room attached to the garage.

Pincher

The heat loss is continuous. E.g. Outside temperature is 5"C, and it's 18"C inside the house, you might be losing heat at a rate of 5kW. So if your boiler is on a low flame that produces heat at 5kW, it never needs to turn off and on, i.e. cycling.

What you want is maximum heat when starting from a cold house, and then simmer when the house is nearly at 18 degrees. The problem is, if you have the good old on/off relay thermostat, you will get cycling whatever you do.

I won't extoll my system here, which I have done before, but you need a control system that is capable of optimising for condensation, one way or another. My system does a low flame simmer at around 44 to 48 degrees in steady state.

rogerblack

spinningsheep wrote: »

Hi guys, calling all you plumbers out there, I have a Baxi Duotec HE 28kw running 10 rads and a towel rail, 2 rads with no TRV's and the towel rail with no TRV either. All rads are on, so all radiators heat up.

Get an IR thermometer - http://www.ebay.co.uk/itm/Non-Contact-IR-Infrared-Digital-Thermometer-Laser-/130568475226 (no knowledge of this seller).

Use this to measure and compare all of the radiators return temperatures (you need to wrap a turn of any plastic tape around copper pipes you measure).

(you can do this by touch, but I find it easier with figures)

The best you can do is to equalise the return temps, by balancing the radiators.

If you're up for 'silly' solutions, a tiny PC fan, under each radiator will dramatically boost its output.