A bit o/t. Stratification in Thermal Stores

[Stereo]

This is sort of firewood related and I know there are some folk on here who know about such things. I have an Esse W23 woodfired range hooked up to a Gledhill Torrent 200l Thermal Store. It's a nice set up but I have a couple issues with it.

 

Firstly, as it's all gravity, if the tank is cool (gas boiler keeps it at min 50 degrees which is all we need for underfloor heating and DHW) and the stove is lit, it takes a long time to stop cool water coming back to the stove and makes the stove useless as a cooker. I understand I can sort this with something like a Laddomat.

 

Secondly, even after a good long burn, the store doesn't seem to stay hot very long. On a cold night I'll often hear the boiler cutting in a few hours after the fire has died down. I'm wondering if this is down to the store not being big enough, or if it's not working properly.

 

I've been reading up on stratification and can't quite get my head around why it is so important. The thing that bothers me about it is that the guy who installed the Esse and Store connected the flow from the Esse halfway up the store, not in the top. I understood that to achieve layering, the hot should go in at the top and sink down slowly. Isn't putting hot in halfway up going to cause churning of the water in the store?

 

Or am I just expecting too much of a 200l store to be able to keep a 3 bed barn conversion heated all night once fully charged? I would like to get to a point that I can load up the Esse before bed and not hear the gas all night but I can only get a couple hours of decent heat out of a single burn so I'm maybe just being unrealistic.

[difflock]

I would have thought 200l only suitable for domestic hot water supply, definately cannot physically store enough energy for space heating requirments.

The time I plumbed this house (with a 210l hot cyl btw) I noted that the top of the oil boiler driven heating coil was only half way up the tank, I reasoned the logic was the heat/hot water would rise, but that if for whatever reason the hot supply coming from the boiler/a boiler/another heat source ,like solar panels, was cooler, than the water already in the cylinder, it could not strip the heat away from the top of the cylinder.

cheers

marcus

[cornish wood burner]

The idea of stratification is to fill the top first, you then have water at the required temperature even if the tank is only 10% full. This will work on a direct system as water is fed in slowly at the top, however if you did the same with an indirect system you would only get say half a tank of hot water at best.

With your tank supplying dhw I imagine the temperature is set relatively low so that will limit the storage capacity of a small tank even more.

[Stereo]

DHW is supplied through a heat exchanger inside the store which is located at the top. I guess it's correct that if I feed water from my stove in at the top and it's cooler than what's there, that would be a bad thing. So I guess the flow going in halfway up is a good thing.

 

I'm a bit disappointed in it all really. Burning wood all winter in the stove doesn't seem to have lowered our gas usage much. For instance, we fired up the Esse at about 10 this morning and it's been burning steadily. I don't think the heating has been on. At about 1, the missus had a 5 minute shower and the gas boiler came on. Surely a 3 hour burn should give enough hot water for a 5 minute shower?

 

Maybe I need a Laddomat to make sure only hot water is sent to the tank and that the stove is always working at proper efficiency? I kind of get the feeling that nothing in my system is working the way it should.

 

DHW by the way is perfect. Mains pressure hot and much better than the gas combi we used before.

 

Anyone have any resources for calculating store size / boiler size for a given size of house?

[cornish wood burner]

If you are using a 200 Ltr tank the for every 10 deg you will store 2.3 kwh so from 50 to 70 store will hold 4.6 kWh not a lot!

[Tom D]

I have a rayburn, with no accumulator tank, however I have a 200l hot water tank and the rayburn can heat that in a few hours from stone cold. Something sounds wrong with your system to me. We have a rayburn, stove and a gas boiler linked to a neutraliser, the capacity of the neutraliser will only be around 20l so no real storage value.

 

Just a thought but I have noticed a massive difference in performance from good dry logs to those which are not so dry. I don't mean wringing wet either. Try burning bone dry softwood as this burns fast with a long flame, ideal for boilers, not so good for overnighting though. If you have any I'd burn bone dry conifer for a few days and see if this shows any difference in performance.

[Stereo]

Wood is dry as a bone. I've blamed that before but this year I have big stores of really well seasoned ash and dead standing elm. Seasoned outside and then barn stored for the late summer. Not much soft wood but I do have pallets and use that for quick heat.

 

I suppose I thought that the Esse would heat the house with gas backup but maybe it's the other way around. Maybe I should use a Laddomat so that the Esse does cooking / ambient heat and adds what is spare to the heat store?

 

Should have got a Rayburn. Bah. Needs more wood but we've got loads of that.

[cornish wood burner]

What sort of room temperatures are you running and where do you control it

What is the HW output of your stove and as already said dry wood?

Store temperature?

Sounds like you have the heating on at night?

Have you good insulation under the under floor heating

Have you checked for leaks ie constant topping up of the system?

No overflow running hot water to waste?

A 200 ltr tank is very small. It would be fine as a buffer while your stove comes up to temperature but as a store too small if you want to run heating from it.

You have a much bigger thermal store available to you if your heating is set in concrete.

Assuming this is the case can you turn the heating up by day when the fire is lit and turn it down a couple of degrees when it is out by night. This obviously depends on your controller.

[Alycidon]

As an Esse cooker dealer I (and Esse) consider it vital to install a Laddomat or the boiler control unit from Esse which does pretty much the same thing. Not fitting one does not allow the firebox to get hot enough to ignite the creosote given off by all woods, this then gets into the flue ways where it condenses and lines the cooker flue ways and the flue with tar which of course is combustible.

 

The W23 has from memory around 17kw max of heat to the water, its was replaced by the W35 with bigger boiler at least 5 years ago.

 

I am not a plumber so cant really comment on the plumbing aspects of the job, however 200L is not a big tank as a bulk energy store. The one I saw in the Esse training school recently linked up to a 990 CHQ cooker is I would think twice that size.

 

A

[Tom D]

One query: does the heat store heat to full temperature i.e. 80-90 deg? or is it tripping on the heating at 50deg for the underfloor? If it reaches full heat and uses a cold water mixing system for the underfloor then that should work but if you are getting up to 50 and then the heat is taken away to the floor then that could be the problem. boilers like the ease are designed to heat the water very hot, not just up to 50....