If it's any help we have several of these on our system. It's intended for UFH use but on our system they are used to limit the hot water temperature (to taps and shower) and control the return temperature to the boiler.

http://www.relianceworldwide.com/site/pdf/uk/heatguard28ufh.pdf

Its a wax capsule type. I think the one you mentioned is motorised?

Hi Jordi

There are several people who'll supply a Thermomatic K and I don't know how you determine which of them is better than the rest....is there a particular factor you're looking for?

Instead of the Thermomatic K I was reccomended (by others here) to use an Acaso blending valve. It does a similar job but some prefer the Acaso models to the TK. There is (as far as I know) only one supplier of the Acasos in the UK - if you want details whisper / email me. My Acaso is the CT (there are several models) and you simply preset the flow temp you want - the flow control does the rest. CT stands for constant temperature.
julian

Simplest solution I can come up with is:

a) Use a wax cartridge thermostatic mixer to control the flow temperature (mixes hot from the tank with return from the rads).

and

b) Use a Grundfos Alpha pump to control the return temperature. These pumps have a constant pressure mode that adjusts the speed as TRV open and close. Not sure how effective they are though.

As an alternative to b)..

c) Use a variable speed pump controlled by a temperature sensor on the return pipe. As the return heats up the pump slows down allowing the rads to cool the return.

I'm not a heating engineer though!

Hi Jordi,

LIke Julian, I used Acaso products. He's got the "CT", I've got the "Automix 20", essentially the same product, just working on commands from a different source. It's a simple three way valve bridging the flow and return pipes, before the pump; with a motorised head which rotates an internal vane. This is controlled by a temp. sensor on the flow pipe, downstream of the pump. I'm sure the "TK does the same job, but I can vouch for the Acaso product. Fit it and forget about rad temps. and with it any accumulator return problems. Ideally though you will need modulating pump as CW. already indicated.

Nice to hear from you Julian, How's the wood supply for the winter?

Mike

I have not read this thread fully but I hope this feedback is useful:
I think you need to get a Termomix. I have just installed one that is microprocessor controlled and it is brilliant. I replace the wax capsule type that was sold to me by the Baxi agent a few years ago which had become unpredictable in its temperature flow management.

Unfortunatly most of the termomix kit is sold with Uk instructions so your supplier will have to do a translation for you.

Thanks for all the information and suggestions. I have one comp0lication that the flow is also the vent and I cannot be put a valve that might close that off. I'l examine further the Thermomix from lkacaso, but at first glance I cannot see how I would be able to keep a flow below a temperature when returning to the tank, but I'll look further.

I have attched a layout of my proposed system using a zone valve which would cut the return from the rads and send the water around again.

I have looked at the Grundfos Alpha and Magna pumps, but they seem to work on constant pressure and depend on the therm valves closing down to regulate the flow. Under thermosyphonic flow the return from the rads is cold ,but onlythe top half of the rads is heated, which is fine for background heat alone.

Jordi
From an eartlier post
You do need a mixer for this. Do not do the “run the store lower temp option” as this will severely limit the buffered heat available. Unless you have greatly enlarged the radiator capacity don’t run them at 55 deg (so as to give a much cooler return into the store) this will also severely limit the heat output. With no mixer the heating pump will just horse the buffer water out of the tank. The other key point to consider is that when zones or radiators (with TRVs) close in, the hot water that is being taken from the tank will pretty much go round the pipes and come back to the tank- still hot – unless you are using a variable Grundfoss alpha pump. Although if just ciculating means you are not consuming heat, you are still forcing hot water into the bottom of the buffer so you will mix it up or invert it

This mixer should be across the heating flow and return, that is, in the supply line with a feed in from the return, it should be motorised with the sensor on the downstream supply flow pipe. You can do something similar with mechanical Danfoss valves but you have to calculate flow and temp ranges carefully so as to select the correct valve specification. The motorised unit is more tolerant in that it will adjust itself to chase your desired delivery temp. Use the ACASO unit as - no question about it. Beware that many other mixers for say domestic hot water or UFH do not have the high temp range, they temper down water from say 65 deg to 40 ish for domestic use, you need a valve that operates over say 90-50 degrees so that you can select radiator supply in the range 65-80 deg.

I would also consider a 3 port diverter in the return and operated with a pipe stat set around 40-45 ish deg. This should divert the radiator flow back round the rad system until it is cool enough to be returned to the buffer. If you have a buffer at 80 deg, and mix it down to 70 for radiator supply, then it would probably return from the rads at 60 deg. unless you can get the radiator system to drop 20+ deg then the return water is still quite warm to be going back into the tank. Therefore, you might have to run the tank through 2 complete cycles to use all the energy. Remember you have a device on the supply to the boiler (e.g. Laddomat) to raise the temp of the boiler feed. This will raise the temp to say 55 deg (so as to ensure the boiler exit temp is high enough). If you have filled the buffer with radiator return water at 60 ish deg then this is open fully and you are only really extracting energy over the upper range of temperature. Therefore, you could insert the diverter and recycle the radiator water till it falls to say 45 deg. Then at this point the diverter moves from isolating the buffer to feeding back to the buffer lower section. Hot water is drawn from the top section and mixed down to 70 deg (rad supply) this is done with some of the 45 ish water that is now in the rad circuit and coming back to the buffer. When the returning water rises in temp (>45 deg) the diverter operates again and locks out the buffer. So forth and so forth. This will ensure you have relatively cool tank with min turnover when you come to fire up the boiler.

The Laddomat is raising the temperature, not all devices are interchangeable so as to be able to temper down the temperature. The Laddomat and ACASO Thermovar use a mechanical valve cartridge similar to the old car radiator valves, but the ACASO based on the CT Auto mix it uses a motorised mixer (as a loading group) and works in either direction either as a loading valve into the boiler or for mixing down tempering as a valve from the store to the rads. The pump pushes round the system and pulls through the Auto mix. So you would have to split the loading group units between the pump and the mixer, and insert a 3 way flap valve (must be on the horizontal). Tee’d into this is the radiator supply fed via a 3 port diverter valve (i.e. a fairly ordinary central heating valve, but not a mid position valve – must be a diverter) with a pipe sensor set to say 45deg. You have to feed the recycled heating water back in between the mixer and the pump. This is because when you are on recycle the mixer is redundant. Therefore, the mixer only operates when pulling from the buffer tank.

Finishing with a cool buffer tank will also give a good heat sink if you have solar. No point if it’s full of warm or nearly hot water.
It goes with out saying that the radiator supply pump should be a Grundfoss alpha+ fully modulating pump. These can adjust themselves right down to a closed in dead head system. Aside from saving energy by only turning as fast as is needed, the reducing flow as TRVs/zones closes in again reduces the flow in and out of the buffer. Therefore, this is an absolute must.

Cheers
Mike up North