Hello
I have 2000 litre accumulator 35kw log batch boiler, the tank gets to 90/85 degrees top to bottom, It feeds the house via a Thermomatic K blender; so far only connected to heat a separate indirect DHW tank in the house and we are now looking at fitting heat distribution system which will be a combination of under floor and radiators.
Having calculated room heat losses, we are looking at radiator sizing to run at lower temperatures to extend the useful heat from the tank, I understand from previous discussions on this forum that a low return temp to the accumulator maintains stratification by reduces thermal mixing.
What is a reasonable flow and return temperature? How does 65° flow with 45° return sound and then size the radiators at Delta 35. Are there any issues to consider or should we be looking at higher or lower flow temps?
We have seen some radiators advertised in the sales literature as having a low temp flow suitable for renewable, heat pumps etc, they are centre filling and apparently distribute the heat better, anyone think they offer any advantages over the standard cheap and cheerful side fillers?

The optimum flow temp is logically related to the heat input needed to maintain a preset temperature..

What is needed is a compensated mixing valve, either sensing indoors or outdoors would be fit for prupose...

in your case you need one for the rads and one for the floor...operating on different slopes...


what is a k blender, any links?

Posted By: mikeajillCan't imagine why you need heating in Italy unless you are up a mountain,

Mikeajill, that's because you live in a country that doesn't have seasons!!! . Here I am at 1500ft and I need heating for 6 months of the year with 6 weeks or so below zero and I need cooooooooling for 3 months a year (for which I use a massive extractor fan during the night to suck cool night air in - last week 35 deg in the shade all pm). My solar panels provide all my non heating season DHW, save maybe one or two extra burns a year. Humidity is below 50% for 6 months and rarely above 70%.

Since I inherited the radiators I didn't have to think about sizing, I let the TRVs sort that out and v occasionally I up the flow temp a tiny bit. I do not mix hot with my return to help stratification as not needed with a v low return. I LOVE my Apha Adapt because when the number goes up I know the house is drawing more heat, I look at the temps up and down the tank and get a good idea how long before I next need to burn. Pic shows dials but there is a lot more insulation everywhere now!!

Yes I added corrosion inhibitor, costs a fair bit due to large volume but imagine the hassle cleaning out the tank, I also filter centrifugally before and after the radiators - as you can see in the pic. Don't try to understand the 3-way mixing 'solution' you can maybe also see.

Posted By: mikeajillWith an accumulator tank the temperature range over the heating cycle changes from 88 to 40deg

Yours might but mine drop to 25 deg happily when pump on 5 watts with significant stratification in the rads too. In hindsight I think probably I have the pump on Two 65% of the time or so not the other way around as I said in my first post.

I have a 2500 litre Accumulator fed by a 50 Kw log boiler serving a large house. There is an Automix on the flow and return. The boiler and accumulator is about 50 feet pipe run from the house half of which is heavily insulated within the garage and half runs underground. It has been in operation over 6 years and we are very pleased with it - it replaced a oil boiler. As such there is no attempt to resize the radiators, or change the DHW system or the UFH system in the conservatory.

i use an Arrex computerised wi-fi monitoring system for both room temperatures and monitoring key system points such as top and bottom of the accumulator, post pump and return and the DHW system . I also use a Honeywell CM Zone system on selected rooms and house areas to ensure we are only heating parts of the house we are using at any specific time. I cannot recommend both systems enough as they optimise the use of the Accumulators water supply and also give me a hour by hour historical record of what is happening in each room and the effect this is having on the system.

The most serious system problem I had was losing heat between the accumulator and the house - I insulated the pipes that are in the garage with further insulation (4 inches) provided more in the ducts surrounding the underground section but still lose about 10-15 degrees between the accumulator and the house. I would certainly do this differently if I were installing again, providing the accumulator closer too or if it was a new build , inside the house.

Over the years I tried several patterns of loading the boiler at different times of day - when I fitted the Arrex system I was astounded to see the heat loss on the external pipework, and the loss in the accumulator in the first half hour of use. This could be up to 20 degrees at the top and 15 degrees at the bottom depending on the load and the initial heat stored. I then noticed that the residual temperature fell much more slowly as the rooms came up to temperature and the CM zone system modulated the hot water drawn effectively cycling a much more stable water temperature through the house. I don't have an Alpha variable pump.

I then found that although the temperature in the system continued to fall over say 6-10 hours (depending on external temperature) to 40 degrees C the temperature in the rooms was maintained or continued to rise. This was because the residual heat in the rooms required much lower radiator temperature to maintain it and the CM system opened as heat was demanded.

So i developed a cycle where the accumulator started the day with a maximum heat charge of 90-95 degrees top and bottom if possible. When the radiators came on in the selected zones in the morning this would drop 20 degree due to the volume of cold water in the system and then stabilise - when the temperature in the Accumulator dropped to 40-45 degrees the boiler would be recharged to recharge the Accumulator to 90 degrees plus and maintain the temperature flow to the house. This would be required between one and three times in the late afternoon and evening depending on the external temperature and the originating charge in the Accumulator. The final charge may continue into the night when the load has been removed.

The two things that really surprised me were the external pipework losses and the way heat in the house was maintained whilst the radiator temperatures were reduced. I would invest in more insulation and more sophisticated controls and management systems. My flows are between 70 and 80 degrees first thing and between 40 and 50 degrees at the end of the cycle before refiring the boiler. Its obviously better to resize the radiators if you have the space but remember they will hold extra water to dilute the store when the system is switched on.

i hope this is helpful.

DaveS

Posted By: GotanewlifeI do not mix hot with my return to help stratification as not needed with a v low return.

I also suspect that maintenance of the stratification is vastly helped by the input manifold on the return which will allow the return water to enter the tank at its own temperature level