UFH buffer and DHW cylinder configuration

[MartC]

I'm looking into a new heating/hot water installation based upon a heat pump (~14kW) and ancillary input from a woodburner backboiler (~5kW). Heating will be underfloor throughout. I'll be pumping the woodburner circuit (vented with dump rad) since the cylinder/buffer will be on the ground floor. There seem to be a few common configurations up for consideration;

A) Heat pump and woodburner both direct to a sealed buffer vessel (~150l) connected to the UFH. A separate cylinder (~250l) for DHW which is fed indirect from only the heat pump (if unvented) or also the woodburner (if vented?). Diverter valves to switch heat input from buffer to cylinder.

B) Heat pump and woodburner direct to a vented store (~600l). UFH connected to lower half of store. Mains pressure DHW from large bottom to top internal coil (~9sqm). Diverter valves used to direct the heat pump and woodburner circuits to the top or bottom of the store as required for DHW or UFH.

C) Like B but with pumped loop through plate heat exchanger for DHW.

D) Combination of A and B. The DHW unvented cylinder is a tank (~300l) inside the top half of the sealed 'buffer' (~450l) and is heated by the buffer. The buffer is fed direct by both the heat pump and woodburner and their inputs can be diverted as for B. The UFH is connected direct to the buffer.

E) woodburner connected direct to its own vented store (~200l). Heat pump connected indirect to DHW cylinder (~300l) and also direct to UFH with diverter valves to switch input. The store is connected to the heat pump return via a plate heat exchanger.

My initial assessement is the two tank solution probably requires more utility room space, unless I can stack the pair. I'm attracted to the single tank solutions because I can get mains pressure hot water and still easily have the woodburner input to the DHW. Solution E) satisfies this requirement but I have reservations about passing hot (70degC) water through the heat pump, perhaps this is not an issue? Also there is no UFH buffer in this configuration. I'd rather a vented storage system, which B) satisfies, but if the tank-in-tank solution is technicaly superior then I'd be prepared to forgo this for D). I would appreciate any comments on the relative merits of these configurations to help make up my mind

Martin

[Tabby]

Some principles:

Heat pumps are efficient at low output temp, but efficiency drops off badly as output temp rises. With solid fuel there is only a slight efficiency penalty for high output temps. So whatever you do, you want

2 water holding zones, with the heatpump and solid fuel outputs separate. The hottest top layer from the heatpump can feed the solid fuel heated water zone.

Gravity circulation si considered essential with solid fuel, as the burner keeps going when power quits. A ground level hot tank will kill gravity circulation. In theory you might get this to work if you have some passive way to handle the excess heat, but thats not normally the case.

NT

[The Natural Philosopher]

+1 on this.

I would NOT feet 70C + water through a heat pump for sure.

Best use of heatpump is very closely spaced UFH. probably 6" spacing. That way you can really use the modest water temps efficiently. You will need DHW immersion anyway to top up the heatpump as it wont otherwise get hot enough.

You will need a step down mixer to reduce solid fuel temps to UFH ones..or a store..hmm

I'd avoid stores as much as possible.

What might work is to have a thermal store OR the DHW fed from solid fuel, and use that to preheat the heatpump. If switched to the DHW you can go up to 65C or so, but the store shouldn't go above 50C. If that is enough for the UFH the heatpump simply wont cut in.

But I would imagine your normal working practice will be heatpump and immersion for DHW *only* in summer, heatpump UFH/DHW alone in spring/autumn, and only when its bloody freezing light the stove. And that means your heatpump outflow will have to be well below freezing to get soil heat out. So its unlikely running it through a heat-ex post that will get it that hot !

[Tabby]

You a want the heatpump to work on the coldest water in the cylinder, so as to maximise the return per watt and per pound. So the feed to the heatpump wants to go from the very bottom of the store. Since it will deliver middling temp water, its output can return to the middle of the cylinder, and from there find its own stratified level.

The solid fuel heat exchanger will produce primarily hot water out, so its output can go to the top of the tank, or near. It will largely heat the medium warmth water from teh heat exchanger, or if that isnt being used, cold water, so its input can be from the middle of the tank.

This is going to be the bset setup for most of the time. Perhaps in future the feed and return points can be cpu controlled and the heat sources prioritised according to what's wanted, but we're not there today.

UFH would be fed from the middle of the tank or a bit lower.

You'd best include an immersion heater too, on occasion you'll be grateful for it, such as when unwell.

NT

[Onetap]

Strange; when you budgeted for all this kit, did you consider paying someone with the necessary expertise to design the system for you? Did you think you'd get expert advise on a DIY forum? I'm not concerned about what you do, but I can't follow the thought processes involved.

Are you employing a HETAS bloke to install the wood-burner and the flue? They generate scarey concentrations of CO.

[MartC]

Gravity circulation si considered essential with solid fuel, as the burner keeps going when power quits. A ground level hot tank will kill gravity circulation. In theory you might get this to work if you have some passive way to handle the excess heat, but thats not normally the case.

There will be a gravity fed dump rad on a valve that opens when the power fails.

[MartC]

DHW - I'm considering a Delonghi that gets to 65degC at a COP of 2, still better than using an immersion (I'll still have one of course), otherwise the Ecodan gets to 55degC at a similar COP. For UFH a spacing of 100 or 150 should keep it running much more efficiently when DHW is satisfied, I just need to tap into that lower temp water.

What's your reason for saying this?

It's going to be air source, but the running pattern is likely to be similar to as you describe. The stove is an extra 5kW when needed in the depths of winter, but when on in autumn/spring reduces the demand on the heat pump. What I have found is that using a stove for space heating just overheats the one room, I'm trying to tie it into heating the rest of the house too by putting it into the UFH, the split will be say 1kW into room and 4into water.

[Tim Lamb]

In message , MartC writes

snip..

Have you considered the impact of trying to get 4kW water and 1kW convection on smoke eater performance?

When your firebox is surrounded by water at something under 100deg.C claims to meet smoke regulations may be suspect.

Glossy brochures do not rush to give this information.

regards

[Tabby]

nf2$426$ [e-mail]...

I wonder how you propose to extract such a high percentage of the heat to the water. To achieve that you'd need a fine heat exchanger in the flue gases. The exchanger would quickly crud up with soot, and its output fall heavily. Also extracting so much heat would remove most of the updraught, another problem to solve. Do you have a cunning plan?

PS, just wondering if it might be possible with a direct contact exchanger tank mounted in the loft, ie the fumes travel over the top of the water. The water would then need ongoing filtering.

NT

[Dave Liquorice]

If you can have a gravity dump rad why can't you have a gravity tank?

If it hasn't jammed shut due to lack of use... IMHO the cooling of a solid fuel boiler need to be totally passive and fail safe.

[Onetap]

My previous question came to mind when I saw the considerable capital expenditure that will go/had gone into this project, but you have come to a DIY forum to solicit the opinions of NT. NT's enthusiasm gets the better of him, IHHO he is an expert on everything. On the frequent occasions that he strays into my area of expertise, I can vouch for the fact that he spouts some awful bollox. Worse than Drivel, but everyone knows to beware of Drivel's misinformation.

Utter bolleaux. Have you ever seen such a heat exchanger? No, you haven't. Why are you wasting everyone's time suggesting such clap- trap? A clue; flue gases from burning carbo-hydrates (all fossil & wood fuels) contain water vapour and carbon dioxide and monoxide. The gases also contain sulphur dioxide from the traces of sulphur in the fuel. Dissolve this in water, you get sulphurous acid. This is why the condense from condensing boilers is acidic and why non-condensing boilers run at condensing temperatures will suffer from rapid corrosion.

You're suggesting passing the flue gas over water? Can you anticipate a problem with that?

You'd need a gasifier for any high efficiency; the heat exchanger would otherwise suffer from tars condensing and solidifying on the heat exchange surface. Conventional stoves have the air flow upwards, i.e., the wrong way around, for maximum efficiency. Lots of info on gasifier combustion on the net.

[MartC]

My previous question came to mind when I saw the considerable capital expenditure that will go/had gone into this project, but you have come to a DIY forum to solicit the opinions of NT. NT's enthusiasm gets the better of him, IHHO he is an expert on everything. On the frequent occasions that he strays into my area of expertise, I can vouch for the fact that he spouts some awful bollox. Worse than Drivel, but everyone knows to beware of Drivel's misinformation.

I've spent nothing yet, but the property is gutted so it's a blank slate as far as heating is concerned. We're off mains gas hence the heat pump is a contender, there is an LPG tank already installed, no oil tank. I get free wood, and have been using the woodburner I inherited with the property (I did have a new liner installed by a HETAS guy who also sweeps it once a year as a requirement of my buildings insurance). This stove is too big and overheats the one room, I cannot get its heat to other parts of the house (except straight up the stairs!) with open door combinations. It's getting old and will be downsized when replaced so I was exploring the idea of the new one having a boiler and putting heat into the underfloor heating.

However, Tim and NT have both brought to my attention that I had clearly overestimated the water/room heat split, I've checked a few today and at best seen 3kW to each claimed for a 6kW stove, and less than this in most, so the idea is less attractive given that it adds such complexity to the wet installation. Do you have any other suggestions, ducting in this house would be difficult to conceal but is it an option worth considering?

Martin

[Tim Lamb]

In message , MartC writes

Given you have a free source of firewood, why not skip the direct heat to the room and have a fully enclosed *furnace* heating a thermal store by convection? Is there no opportunity for a cistern upstairs or in a cupboard to the side? You can pump water to your under floor system.

regards

[Tabby]

yawn. again.

Direct contact exchangers are common in industrial equipment, and can achieve higher efficiency than indirect types at lower cost.

Just curious as to whether it might be possible to make such technology functional at domestic use at low cost.

Thank you for stating the fairly obvious.

Issues, of course. Problems, depends how its done. I was wondering if perhaps acidity can be handled with either lime chips or by using only plastic, and an occasional flush. That would imply a plastic heat exchanger, which has its issues. Or occasional flushing might be enough on its own.

Yup. I asume of course the OP will stick with more conventional domestic haeting technology.

NT

[MartC]

Couldn't find much on these, just an american site, but it sounds a bit industrial to put in SWMBO's inglenook! The house layout is difficult, it would be a serious compromise to put the store upstairs. I'm coming to the conclusion that it's going to continue to be a dry stove in the living room and a heat pump direct to UFH and indirect to DHW cylinder. Currently investigating if I need a buffer for the UFH, it may be possible to skip this if I use a heat pump that can modulate it's output power.

Martin

[Tim Lamb]

In message , MartC writes

Umm... These things are always going to look industrial. They are also very expensive!

A flick through the adverts in a CLA magazine found

I suppose you could cheat! Much of the *heat to the room* comes from convection as air circulates over the steel or cast iron exterior surface. Insulation? There must be log burners with boiler functions which can be inset. Inglenook and SWMBO's ambitions may not help:-)

On the cistern location... my mother had a Rayburn solid fuel cooker in a bungalow. The hot cistern was in a cupboard to the side.

A log burner with no thermal store is not going to quickly warm up your house on a cold winter morning.

regards

[Onetap]

On May 23, 6:15=A0pm, "MartC" wrote: =A0Do you have any other suggestions, ducting in this house

I haven't read all the above, no time. You should start estimating the size of the store from the heat storage required by running the heat pump (GS or ASHP?) overnight on cheap rate electricity. I would not connect the wood burner directly to this store, it would take hours to reach a temperature at which there was some useful heat, during which time you'd get problems with tar deposits and acid condensation. You want to get it hot, as fast as possible, and then maybe extract some heat to the UFH and/or the thermal store.

You should stick with a conventional set-up for the woodburner, with a metal F&E cistern, heat dump radiator and gravity circulation to the HWS cylinder. This is achievable, even with the cylinder a short height above the boiler, if the installer can size gravity pipework.

The recommended volumes of wood storage is something that prospective users tend to under-estimate (think open-sided shed; huge shed) and your planned use may change after experience of using it. Flexible (?) flue liners are not regarded as a permanent solution (see HETAS guide on-line) and are only suitable for thoroughly seasoned wood. Damper wood will produce more condensation, shorten the life and invalidate the guarantee.

A batch wood-burner is an uncontrolled heat source and the manufacturers' recommendations are that they should NOT be used to heat an unvented water cylinder. It can be done, if the installer knows how, but the cylinder cannot then be used as part of the thermal store for the wood burner since the obligatory 2-port zone valve shuts when the cylinder gets hot.

I think I'd go for a 2 coil cylinder and keep the two primary systems separate.The wood burner will boil in service and there will be a lot of corrosion particles in the water, compared to a sealed & inhibited system.

You could extract heat from the woodburner into the UFH or the thermal store, through a neutralizer cylinder or low loss header, but the cost of the controls will be large compared to the 3-5 kW heat available. I'd do this at a later date, once you've used the system, if you still think it worthwhile. You need a device to monitor and control the return temperature to prevent it falling too low whilst doing so.

[Onetap]

Yes, but they're gasifiers, efficient and some types are designed for use with a sealed system. If you have room for an separate plantroom, they are practical. There were plans for Renewable Heat Incentives, to pay the owner per kW hr of wood-heat produced; I don't know what the state of these are now, whether they were scrapped with the cut-backs. However, the payback was such that people were buying them as a more attractive investment, given that there's minimal interest available elsewhere for their money.

[Onetap]

PS Forgot. RHIs should also be available for the heat pump installation, providing you use an accredited installer and approved equipment and providing George hasn't scrapped them. You'll probably find the installers will want to sell you a wood burner as well.

[Onetap]

Still going, it seems.